1.
Melting
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Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the energy of the solid increases, typically by the application of heat or pressure. At the melting point, the ordering of ions or molecules in the solid breaks down to an ordered state. Substances in the state generally have reduced viscosity as the temperature increases. An exception to this principle is the element sulfur, whose viscosity increases to a point due to polymerization, some organic compounds melt through mesophases, states of partial order between solid and liquid. Melting is therefore classified as a phase transition. Melting occurs when the Gibbs free energy of the liquid becomes lower than the solid for that material, the temperature at which this occurs is dependent on the ambient pressure. Low-temperature helium is the only exception to the general rule. Helium-3 has an enthalpy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative enthalpy of fusion below 0.8 K. This means that, at appropriate constant pressures, heat must be removed from these substances in order to melt them, among the theoretical criteria for melting, the Lindemann and of Born criteria are those most frequently used as a basis to analyse the melting conditions. The Lindemann melting criterion is supported by experimental data both for crystalline materials and for transitions in amorphous materials. The Born criterion is based on rigidity catastrophe caused by the elastic shear modulus. Under a standard set of conditions, the point of a substance is a characteristic property. The melting point is equal to the freezing point. However, under carefully created conditions, supercooling or superheating past the melting or freezing point can occur, water on a very clean glass surface will often supercool several degrees below the freezing point without freezing. Fine emulsions of pure water have been cooled to −38 degrees Celsius without nucleation to form ice, nucleation occurs due to fluctuations in the properties of the material. If the material is still there is often nothing to trigger this change. Thermodynamically, the liquid is in the metastable state with respect to the crystalline phase
2.
Freezing
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Freezing, or solidification, is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point. For most substances, the melting and freezing points are the temperature, however. For example, agar displays a hysteresis in its melting point and it melts at 85 °C and solidifies from 32 °C to 40 °C. Most liquids freeze by crystallization, formation of crystalline solid from the uniform liquid, because of the latent heat of fusion, the freezing is greatly slowed down and the temperature will not drop anymore once the freezing starts but will continue dropping once it finishes. Crystallization consists of two events, nucleation and crystal growth. Nucleation is the step wherein the molecules start to gather into clusters, on the scale, arranging in a defined. The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size, in spite of the second law of thermodynamics, crystallization of pure liquids usually begins at a lower temperature than the melting point, due to high activation energy of homogeneous nucleation. The creation of a nucleus implies the formation of an interface at the boundaries of the new phase, some energy is expended to form this interface, based on the surface energy of each phase. If a hypothetical nucleus is too small, the energy that would be released by forming its volume is not enough to create its surface, Freezing does not start until the temperature is low enough to provide enough energy to form stable nuclei. Under high pressure water will super cool to as low as −70 °C before freezing, Freezing is almost always an exothermic process, meaning that as liquid changes into solid, heat and pressure is released. This is often seen as counter-intuitive, since the temperature of the material does not rise during freezing, but this can be understood, since heat must be continually removed from the freezing liquid or the freezing process will stop. The energy released upon freezing is a latent heat, and is known as the enthalpy of fusion and is exactly the same as the required to melt the same amount of the solid. Low-temperature helium is the only exception to the general rule. Helium-3 has an enthalpy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative enthalpy of fusion below 0.8 K. This means that, at appropriate constant pressures, heat must be added to these substances in order to freeze them, certain materials, such as glass and glycerol, may harden without crystallizing, these are called amorphous solids. Amorphous materials as well as some polymers do not have a freezing point, instead, there is a gradual change in their viscoelastic properties over a range of temperatures. Such materials are characterized by a transition that occurs at a glass transition temperature. Because vitrification is a process, it does not qualify as freezing
3.
Crystallization
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Crystallization is the process where a solid forms where the atoms or molecules are highly organized in a structure known as a crystal. Some of the ways which crystals form are through precipitating from a solution, crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs. In chemical engineering crystallization occurs in a crystallizer, crystallization is therefore related to precipitation, although the result is not amorphous or disordered, but a crystal. The crystallization process consists of two events, nucleation and crystal growth which are driven by thermodynamic properties as well as chemical properties. These stable clusters constitute the nuclei, therefore, the clusters need to reach a critical size in order to become stable nuclei. Such critical size is dictated by different factors. The crystal growth is the subsequent size increase of the nuclei that succeed in achieving the critical cluster size, crystal growth is a dynamic process occurring in equilibrium where solute molecules or atoms precipitate out of solution, and dissolve back into solution. Supersaturation is one of the forces of crystallization, as the solubility of a species is an equilibrium process quantified by Ksp. Depending upon the conditions, either nucleation or growth may be predominant over the other, many compounds have the ability to crystallize with some having different crystal structures, a phenomenon called polymorphism. Each polymorph is in fact a different thermodynamic solid state and crystal polymorphs of the compound exhibit different physical properties, such as dissolution rate, shape, melting point. For this reason, polymorphism is of importance in industrial manufacture of crystalline products. Additionally, crystal phases can sometimes be interconverted by varying factors such as temperature, there are many examples of natural process that involve crystallization. Geological time scale process examples include, Natural crystal formation, Stalactite/stalagmite, usual time scale process examples include, Snow flakes formation, Honey crystallization. Crystal formation can be divided into two types, where the first type of crystals are composed of a cation and anion, also known as a salt, the second type of crystals are composed of uncharged species, for example menthol. The formation of a solution does not guarantee crystal formation. A typical laboratory technique for crystal formation is to dissolve the solid in a solution in which it is partially soluble, the hot mixture is then filtered to remove any insoluble impurities. The filtrate is allowed to slowly cool, crystals that form are then filtered and washed with a solvent in which they are not soluble, but is miscible with the mother liquor. The process is repeated to increase the purity in a technique known as recrystallization
4.
State of matter
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In physics, a state of matter is one of the distinct forms that matter takes on. Four states of matter are observable in everyday life, solid, liquid, gas, some other states are believed to be possible but remain theoretical for now. For a complete list of all states of matter, see the list of states of matter. Historically, the distinction is based on qualitative differences in properties. Matter in the state maintains a fixed volume and shape, with component particles close together. Matter in the state maintains a fixed volume, but has a variable shape that adapts to fit its container. Its particles are close together but move freely. Matter in the state has both variable volume and shape, adapting both to fit its container. Its particles are close together nor fixed in place. Matter in the state has variable volume and shape, but as well as neutral atoms, it contains a significant number of ions and electrons. Plasma is the most common form of matter in the universe. The term phase is used as a synonym for state of matter. In a solid the particles are packed together. The forces between particles are strong so that the particles move freely but can only vibrate. As a result, a solid has a stable, definite shape, solids can only change their shape by force, as when broken or cut. In crystalline solids, the particles are packed in a regularly ordered, there are various different crystal structures, and the same substance can have more than one structure. For example, iron has a cubic structure at temperatures below 912 °C. Ice has fifteen known crystal structures, or fifteen solid phases, glasses and other non-crystalline, amorphous solids without long-range order are not thermal equilibrium ground states, therefore they are described below as nonclassical states of matter
5.
Solid
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Solid is one of the four fundamental states of matter. It is characterized by structural rigidity and resistance to changes of shape or volume, unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does. The atoms in a solid are tightly bound to other, either in a regular geometric lattice or irregularly. The branch of physics deals with solids is called solid-state physics. Materials science is concerned with the physical and chemical properties of solids. Solid-state chemistry is concerned with the synthesis of novel materials, as well as the science of identification. The atoms, molecules or ions which make up solids may be arranged in a repeating pattern. Materials whose constituents are arranged in a regular pattern are known as crystals, in some cases, the regular ordering can continue unbroken over a large scale, for example diamonds, where each diamond is a single crystal. Almost all common metals, and many ceramics, are polycrystalline, in other materials, there is no long-range order in the position of the atoms. These solids are known as amorphous solids, examples include polystyrene, whether a solid is crystalline or amorphous depends on the material involved, and the conditions in which it was formed. Solids which are formed by slow cooling will tend to be crystalline, likewise, the specific crystal structure adopted by a crystalline solid depends on the material involved and on how it was formed. While many common objects, such as an ice cube or a coin, are chemically identical throughout, for example, a typical rock is an aggregate of several different minerals and mineraloids, with no specific chemical composition. Wood is an organic material consisting primarily of cellulose fibers embedded in a matrix of organic lignin. In materials science, composites of more than one constituent material can be designed to have desired properties, the forces between the atoms in a solid can take a variety of forms. For example, a crystal of sodium chloride is made up of sodium and chlorine. In diamond or silicon, the atoms share electrons and form covalent bonds, in metals, electrons are shared in metallic bonding. Some solids, particularly most organic compounds, are together with van der Waals forces resulting from the polarization of the electronic charge cloud on each molecule. The dissimilarities between the types of solid result from the differences between their bonding, metals typically are strong, dense, and good conductors of both electricity and heat
6.
Liquid
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A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a constant volume independent of pressure. As such, it is one of the four states of matter. A liquid is made up of tiny vibrating particles of matter, such as atoms, water is, by far, the most common liquid on Earth. Like a gas, a liquid is able to flow and take the shape of a container, most liquids resist compression, although others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, a distinctive property of the liquid state is surface tension, leading to wetting phenomena. The density of a liquid is usually close to that of a solid, therefore, liquid and solid are both termed condensed matter. On the other hand, as liquids and gases share the ability to flow, although liquid water is abundant on Earth, this state of matter is actually the least common in the known universe, because liquids require a relatively narrow temperature/pressure range to exist. Most known matter in the universe is in form as interstellar clouds or in plasma form within stars. Liquid is one of the four states of matter, with the others being solid, gas. Unlike a solid, the molecules in a liquid have a greater freedom to move. The forces that bind the molecules together in a solid are only temporary in a liquid, a liquid, like a gas, displays the properties of a fluid. A liquid can flow, assume the shape of a container, if liquid is placed in a bag, it can be squeezed into any shape. These properties make a suitable for applications such as hydraulics. Liquid particles are bound firmly but not rigidly and they are able to move around one another freely, resulting in a limited degree of particle mobility. As the temperature increases, the vibrations of the molecules causes distances between the molecules to increase. When a liquid reaches its point, the cohesive forces that bind the molecules closely together break. If the temperature is decreased, the distances between the molecules become smaller, only two elements are liquid at standard conditions for temperature and pressure, mercury and bromine. Four more elements have melting points slightly above room temperature, francium, caesium, gallium and rubidium, metal alloys that are liquid at room temperature include NaK, a sodium-potassium metal alloy, galinstan, a fusible alloy liquid, and some amalgams
7.
Supercooling
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Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid. A liquid crossing its standard freezing point will crystalize in the presence of a crystal or nucleus around which a crystal structure can form creating a solid. Lacking any such nuclei, the phase can be maintained all the way down to the temperature at which crystal homogeneous nucleation occurs. Homogeneous nucleation can occur above the transition temperature, but if homogeneous nucleation has not occurred above that temperature. Water normally freezes at 273.15 K, but it can be supercooled at standard pressure down to its homogeneous nucleation at almost 224.8 K. If water is cooled at a rate on the order of 106 K/s, the crystal nucleation can be avoided and water becomes a glass — that is and its glass transition temperature is much colder and harder to determine, but studies estimate it at about 136 K. Glassy water can be heated up to approximately 150 K without nucleation occurring, in the range of temperatures between 231 K and 150 K, experiments find only crystal ice. Droplets of supercooled water often exist in stratiform and cumulus clouds, Freezing rain is also caused by supercooled droplets. The process opposite to supercooling, the melting of a solid above the point, is much more difficult. It is possible, at a pressure, to superheat a liquid above its boiling point without it becoming gaseous. Supercooling is often confused with freezing-point depression, supercooling is the cooling of a liquid below its freezing point without it becoming solid. Constitutional supercooling, which occurs during solidification, is due to compositional solid changes, when solidifying a liquid, the interface is often unstable, and the velocity of the solid–liquid interface must be small in order to avoid constitutional supercooling. Supercooled zones are observed when the temperature gradient at the interface is larger than the temperature gradient. Therefore, the thermal gradient necessary to create a stable solid front is as expressed below. ∂ T ∂ x < m C0 v k D For more information, the authors are confusing supercooling with freezing-point depression. The paragraphs above warn about this confusion, in order to survive extreme low temperatures in certain environments, some animals undergo forms of supercooling that allow them to remain unfrozen and avoid cell damage and death. The winter flounder is one fish that utilizes these proteins to survive in its frigid environment. Noncolligative proteins are secreted by the liver into the bloodstream, other animals use colligative antifreezes, which increases the concentration of solutes in their bodily fluids, thus lowering their freezing point
8.
Carboxylic acid
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A carboxylic acid /ˌkɑːrbɒkˈsɪlɪk/ is an organic compound that contains a carboxyl group. The general formula of an acid is R–COOH, with R referring to the rest of the molecule. Carboxylic acids occur widely and include the amino acids and acetic acid, salts and esters of carboxylic acids are called carboxylates. When a carboxyl group is deprotonated, its conjugate base forms a carboxylate anion, carboxylate ions are resonance-stabilized, and this increased stability makes carboxylic acids more acidic than alcohols. Carboxylic acids can be seen as reduced or alkylated forms of the Lewis acid carbon dioxide, carboxylic acids are commonly identified using their trivial names, and usually have the suffix -ic acid. IUPAC-recommended names also exist, in system, carboxylic acids have an -oic acid suffix. For example, butyric acid is butanoic acid by IUPAC guidelines, the -oic acid nomenclature detail is based on the name of the previously-known chemical benzoic acid. Alternately, it can be named as a carboxy or carboxylic acid substituent on another parent structure, for example, 2-carboxyfuran. The carboxylate anion of an acid is usually named with the suffix -ate, in keeping with the general pattern of -ic acid and -ate for a conjugate acid and its conjugate base. For example, the base of acetic acid is acetate. The radical •COOH has only a fleeting existence. The acid dissociation constant of •COOH has been measured using electron paramagnetic resonance spectroscopy, the carboxyl group tends to dimerise to form oxalic acid. Because they are both hydrogen-bond acceptors and hydrogen-bond donors, they participate in hydrogen bonding. Together the hydroxyl and carbonyl group forms the functional group carboxyl, carboxylic acids usually exist as dimeric pairs in nonpolar media due to their tendency to self-associate. Smaller carboxylic acids are soluble in water, whereas higher carboxylic acids are less due to the increasing hydrophobic nature of the alkyl chain. These longer chain acids tend to be soluble in less-polar solvents such as ethers. Carboxylic acids tend to have higher boiling points than water, not only because of their surface area. Carboxylic acids tend to evaporate or boil as these dimers, for boiling to occur, either the dimer bonds must be broken or the entire dimer arrangement must be vaporised, both of which increase the enthalpy of vaporization requirements significantly
9.
Mercury (element)
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Mercury is a chemical element with symbol Hg and atomic number 80. It is commonly known as quicksilver and was formerly named hydrargyrum, Mercury occurs in deposits throughout the world mostly as cinnabar. The red pigment vermilion is obtained by grinding natural cinnabar or synthetic mercuric sulfide, likewise, mechanical pressure gauges and electronic strain gauge sensors have replaced mercury sphygmomanometers. Mercury remains in use in research applications and in amalgam for dental restoration in some locales. It is used in fluorescent lighting, electricity passed through mercury vapor in a fluorescent lamp produces short-wave ultraviolet light which then causes the phosphor in the tube to fluoresce, making visible light. Mercury poisoning can result from exposure to water-soluble forms of mercury, Mercury is a heavy, silvery-white liquid metal. Compared to other metals, it is a conductor of heat. It has a point of −38.83 °C and a boiling point of 356.73 °C. Upon freezing, the volume of mercury decreases by 3. 59%, the coefficient of volume expansion is 181.59 × 10−6 at 0 °C,181.71 × 10−6 at 20 °C and 182.50 × 10−6 at 100 °C. Solid mercury is malleable and ductile and can be cut with a knife, because this configuration strongly resists removal of an electron, mercury behaves similarly to noble gases, which form weak bonds and hence melt at low temperatures. The stability of the 6s shell is due to the presence of a filled 4f shell, an f shell poorly screens the nuclear charge that increases the attractive Coulomb interaction of the 6s shell and the nucleus. Like silver, mercury reacts with hydrogen sulfide. Mercury reacts with solid sulfur flakes, which are used in mercury spill kits to absorb mercury, Mercury dissolves many other metals such as gold and silver to form amalgams. Iron is an exception, and iron flasks have traditionally used to trade mercury. Several other first row transition metals with the exception of manganese, copper, other elements that do not readily form amalgams with mercury include platinum. Sodium amalgam is a reducing agent in organic synthesis, and is also used in high-pressure sodium lamps. Mercury readily combines with aluminium to form a mercury-aluminium amalgam when the two pure metals come into contact, since the amalgam destroys the aluminium oxide layer which protects metallic aluminium from oxidizing in-depth, even small amounts of mercury can seriously corrode aluminium. For this reason, mercury is not allowed aboard an aircraft under most circumstances because of the risk of it forming an amalgam with exposed aluminium parts in the aircraft, Mercury embrittlement is the most common type of liquid metal embrittlement
10.
Kelvin
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The kelvin is a unit of measure for temperature based upon an absolute scale. It is one of the seven units in the International System of Units and is assigned the unit symbol K. The kelvin is defined as the fraction 1⁄273.16 of the temperature of the triple point of water. In other words, it is defined such that the point of water is exactly 273.16 K. The Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist William Lord Kelvin, unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or typeset as a degree. The kelvin is the unit of temperature measurement in the physical sciences, but is often used in conjunction with the Celsius degree. The definition implies that absolute zero is equivalent to −273.15 °C, Kelvin calculated that absolute zero was equivalent to −273 °C on the air thermometers of the time. This absolute scale is known today as the Kelvin thermodynamic temperature scale, when spelled out or spoken, the unit is pluralised using the same grammatical rules as for other SI units such as the volt or ohm. When reference is made to the Kelvin scale, the word kelvin—which is normally a noun—functions adjectivally to modify the noun scale and is capitalized, as with most other SI unit symbols there is a space between the numeric value and the kelvin symbol. Before the 13th CGPM in 1967–1968, the unit kelvin was called a degree and it was distinguished from the other scales with either the adjective suffix Kelvin or with absolute and its symbol was °K. The latter term, which was the official name from 1948 until 1954, was ambiguous since it could also be interpreted as referring to the Rankine scale. Before the 13th CGPM, the form was degrees absolute. The 13th CGPM changed the name to simply kelvin. Its measured value was 7002273160280000000♠0.01028 °C with an uncertainty of 60 µK, the use of SI prefixed forms of the degree Celsius to express a temperature interval has not been widely adopted. In 2005 the CIPM embarked on a program to redefine the kelvin using a more experimentally rigorous methodology, the current definition as of 2016 is unsatisfactory for temperatures below 20 K and above 7003130000000000000♠1300 K. In particular, the committee proposed redefining the kelvin such that Boltzmanns constant takes the exact value 6977138065049999999♠1. 3806505×10−23 J/K, from a scientific point of view, this will link temperature to the rest of SI and result in a stable definition that is independent of any particular substance. From a practical point of view, the redefinition will pass unnoticed, the kelvin is often used in the measure of the colour temperature of light sources. Colour temperature is based upon the principle that a black body radiator emits light whose colour depends on the temperature of the radiator, black bodies with temperatures below about 7003400000000000000♠4000 K appear reddish, whereas those above about 7003750000000000000♠7500 K appear bluish
11.
Celsius
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Celsius, also known as centigrade, is a metric scale and unit of measurement for temperature. As an SI derived unit, it is used by most countries in the world and it is named after the Swedish astronomer Anders Celsius, who developed a similar temperature scale. The degree Celsius can refer to a temperature on the Celsius scale as well as a unit to indicate a temperature interval. Before being renamed to honour Anders Celsius in 1948, the unit was called centigrade, from the Latin centum, which means 100, and gradus, which means steps. The scale is based on 0° for the point of water. This scale is widely taught in schools today, by international agreement the unit degree Celsius and the Celsius scale are currently defined by two different temperatures, absolute zero, and the triple point of VSMOW. This definition also precisely relates the Celsius scale to the Kelvin scale, absolute zero, the lowest temperature possible, is defined as being precisely 0 K and −273.15 °C. The temperature of the point of water is defined as precisely 273.16 K at 611.657 pascals pressure. This definition fixes the magnitude of both the degree Celsius and the kelvin as precisely 1 part in 273.16 of the difference between absolute zero and the point of water. Thus, it sets the magnitude of one degree Celsius and that of one kelvin as exactly the same, additionally, it establishes the difference between the two scales null points as being precisely 273.15 degrees. In his paper Observations of two persistent degrees on a thermometer, he recounted his experiments showing that the point of ice is essentially unaffected by pressure. He also determined with precision how the boiling point of water varied as a function of atmospheric pressure. He proposed that the point of his temperature scale, being the boiling point. This pressure is known as one standard atmosphere, the BIPMs 10th General Conference on Weights and Measures later defined one standard atmosphere to equal precisely 1013250dynes per square centimetre. On 19 May 1743 he published the design of a mercury thermometer, in 1744, coincident with the death of Anders Celsius, the Swedish botanist Carolus Linnaeus reversed Celsiuss scale. In it, Linnaeus recounted the temperatures inside the orangery at the University of Uppsala Botanical Garden, since the 19th century, the scientific and thermometry communities worldwide referred to this scale as the centigrade scale. Temperatures on the scale were often reported simply as degrees or. More properly, what was defined as centigrade then would now be hectograde.2 gradians, for scientific use, Celsius is the term usually used, with centigrade otherwise continuing to be in common but decreasing use, especially in informal contexts in English-speaking countries
12.
Fahrenheit
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Fahrenheit is a temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit, after whom the scale is named. It uses the degree Fahrenheit as the unit, several accounts of how he originally defined his scale exist. The lower defining point,0 °F, was established as the temperature of a solution of brine made from parts of ice. Further limits were established as the point of ice and his best estimate of the average human body temperature. All other countries in the world now use the Celsius scale, defined since 1954 by absolute zero being −273.15 °C, on the Fahrenheit scale, the freezing point of water is 32 degrees Fahrenheit and the boiling point is 212 °F. This puts the boiling and freezing points of water exactly 180 degrees apart, therefore, a degree on the Fahrenheit scale is 1⁄180 of the interval between the freezing point and the boiling point. On the Celsius scale, the freezing and boiling points of water are 100 degrees apart, a temperature interval of 1 °F is equal to an interval of 5⁄9 degrees Celsius. The Fahrenheit and Celsius scales intersect at −40°, absolute zero is −273.15 °C or −459.67 °F. For an exact conversion, the formulas can be applied. Again, f is the value in Fahrenheit and c the value in Celsius, f °Fahrenheit to c °Celsius, C °Celsius to f °Fahrenheit, −40 = f. Fahrenheit proposed his temperature scale in 1724, basing it on two points of temperature. In his initial scale, the point is determined by placing the thermometer in a mixture of ice, water. This is a mixture which stabilizes its temperature automatically, that stable temperature was defined as 0 °F. The second point,96 degrees, was approximately the human bodys temperature, in any case, the definition of the Fahrenheit scale has changed since. According to a letter Fahrenheit wrote to his friend Herman Boerhaave, his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømers scale, brine freezes at zero, water freezes and melts at 7.5 degrees, body temperature is 22.5, Fahrenheit multiplied each value by four in order to eliminate fractions and increase the granularity of the scale. Fahrenheit observed that water boils at about 212 degrees using this scale, under this system, the Fahrenheit scale is redefined slightly so that the freezing point of water is exactly 32 °F, and the boiling point is exactly 212 °F or 180 degrees higher. It is for this reason that human body temperature is approximately 98° on the revised scale
13.
Agar
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Agar or agar-agar is a jelly-like substance, equivalent to vegan gelatin, obtained from algae. It was discovered in the late 1650s or early 1660s, by Mino Tarōzaemon in Japan, where it is called kanten. Agar is derived from the polysaccharide agarose, which forms the structure in the cell walls of certain species of algae. These algae are known as agarophytes and belong to the Rhodophyta phylum, Agar is actually the resulting mixture of two components, the linear polysaccharide agarose, and a heterogeneous mixture of smaller molecules called agaropectin. Throughout history into modern times, agar has been used as an ingredient in desserts throughout Asia. The gelling agent in agar is an unbranched polysaccharide obtained from the walls of some species of red algae, primarily from the genera Gelidium. For commercial purposes, it is derived primarily from Gelidium amansii, in chemical terms, agar is a polymer made up of subunits of the sugar galactose. Agar was discovered in Japan in 1658 by Mino Tarōzaemon, an innkeeper who was said to have discarded surplus seaweed soup, over the following centuries, agar became a common gelling agent in several Southeast Asian cuisines. Agar was first subjected to analysis in 1859 by the French chemist Anselme Payen. Beginning in the late 19th century, agar began to be used heavily as a medium for growing various microbes. Agar was first described for use in microbiology in 1882 by the German microbiologist Walther Hesse, Agar quickly supplanted gelatin as the base of microbiological media, due to its higher melting temperature, allowing microbes to be grown at higher temperatures without the media liquefying. With its newfound use in microbiology, agar production quickly increased and this production centered on Japan which produced most of the worlds agar until World War II. However, with the outbreak of World War II many nations were forced to establish domestic agar industries in order to continue microbiological research, around the time of World War II, approximately 2,500 tons of agar were produced annually. By the mid-1970s, production worldwide had increased dramatically to approximately 10,000 tons each year, since then, production of agar has fluctuated due to unstable and sometimes over-utilized seaweed populations. The word agar comes from agar-agar, the Malay name for red algae from which the jelly is produced and it is also known as Kanten, Japanese isinglass, Ceylon moss or Jaffna moss. Gracilaria lichenoides is specifically referred to as agal-agal or Ceylon agar, Agar consists of a mixture of two polysaccharides, agarose and agaropectin, with agarose making up about 70% of the mixture. Agarose is a polymer, made up of repeating units of agarobiose. Agar exhibits hysteresis, melting at 85 °C and solidifying from 32–40 °C and this property lends a suitable balance between easy melting and good gel stability at relatively high temperatures
14.
Hysteresis
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Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a magnetic field. Plots of a component of the moment often form a loop or hysteresis curve. This history dependence is the basis of memory in a disk drive. Hysteresis occurs in ferromagnetic and ferroelectric materials, as well as in the deformation of rubber bands and shape-memory alloys, in natural systems it is often associated with irreversible thermodynamic change such as phase transitions and with internal friction, and dissipation is a common side effect. Hysteresis can be found in physics, chemistry, engineering, biology and economics and it is incorporated in many artificial systems, for example, in thermostats and Schmitt triggers, it prevents unwanted frequent switching. Hysteresis can be a lag between an input and an output that disappears if the input is varied more slowly, this is known as rate-dependent hysteresis. However, phenomena such as the magnetic hysteresis loops are mainly rate-independent, systems with hysteresis are nonlinear, and can be mathematically challenging to model. The term hysteresis is derived from ὑστέρησις, an ancient Greek word meaning deficiency or lagging behind and it was coined around 1890 by Sir James Alfred Ewing to describe the behaviour of magnetic materials. Some early work on describing hysteresis in mechanical systems was performed by James Clerk Maxwell, subsequently, hysteretic models have received significant attention in the works of Ferenc Preisach, Louis Néel and D. H. Everett in connection with magnetism and absorption. A more formal theory of systems with hysteresis was developed in the 1970s by a group of Russian mathematicians led by Mark Krasnoselskii. One type of hysteresis is a lag between input and output. A simple example is a sinusoidal input X that results in a sinusoidal output Y and this kind of hysteresis is often referred to as rate-dependent hysteresis. If the input is reduced to zero, the continues to respond for a finite time. This constitutes a memory of the past, but a limited one because it disappears as the output decays to zero, the phase lag depends on the frequency of the input, and goes to zero as the frequency decreases. When rate-dependent hysteresis is due to effects like friction, it is associated with power loss. Systems with rate-independent hysteresis have a persistent memory of the past that remains after the transients have died out, the future development of such a system depends on the history of states visited, but does not fade as the events recede into the past. If an input variable X cycles from X0 to X1 and back again, the values of Y depend on the path of values that X passes through but not on the speed at which it traverses the path
15.
Nucleation
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Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-assembly or self-organization. Nucleation is typically defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears, nucleation is often found to be very sensitive to impurities in the system. Because of this, it is important to distinguish between heterogeneous nucleation and homogeneous nucleation. Heterogeneous nucleation occurs at sites on surfaces in the system. Homogeneous nucleation occurs away from a surface, nucleation is usually a stochastic process, so even in two identical systems nucleation will occur at different times. This behaviour is similar to radioactive decay, a common mechanism is illustrated in the animation to the right. This shows nucleation of a new phase in an existing phase and this nucleus of the red phase then grows and converts the system to this phase. The standard theory that describes this behaviour for the nucleation of a new phase is called classical nucleation theory. This is seen for example in the nucleation of ice in supercooled water droplets. The decay rate of the exponential gives the nucleation rate, classical nucleation theory is a widely used approximate theory for estimating these rates, and how they vary with variables such as temperature. It correctly predicts that the time you have to wait for nucleation decreases extremely rapidly when supersaturated and it is not just new phases such as liquids and crystals that form via nucleation followed by growth. The self-assembly process that forms objects like the amyloid aggregates associated with Alzheimers disease also starts with nucleation, energy consuming self-organising systems such as the microtubules in cells also show nucleation and growth. Clouds form when wet air cools and many small water droplets nucleate from the supersaturated air, the amount of water vapor that air can carry decreases with lower temperatures. The excess vapor begins to nucleate and to small water droplets which form a cloud. Nucleation of the droplets of water is heterogeneous, occurring on particles referred to as cloud condensation nuclei. Cloud seeding is the process of adding artificial condensation nuclei to quicken the formation of clouds, nucleation is the first step in crystallisation, so it determines if a crystal can form. Frequently crystals do not form even when they are thermodynamically the favored state, for example, small droplets of very pure water can remain liquid down to below -30 °C although ice is the stable state below 0 °C. Bubbles of carbon dioxide nucleate shortly after the pressure is released from a container of carbonated liquid, nucleation often occurs more easily at a pre-existing interface, as happens on boiling chips and on string used to make rock candy
16.
Water
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Water is a transparent and nearly colorless chemical substance that is the main constituent of Earths streams, lakes, and oceans, and the fluids of most living organisms. Its chemical formula is H2O, meaning that its molecule contains one oxygen, Water strictly refers to the liquid state of that substance, that prevails at standard ambient temperature and pressure, but it often refers also to its solid state or its gaseous state. It also occurs in nature as snow, glaciers, ice packs and icebergs, clouds, fog, dew, aquifers, Water covers 71% of the Earths surface. It is vital for all forms of life. Only 2. 5% of this water is freshwater, and 98. 8% of that water is in ice and groundwater. Less than 0. 3% of all freshwater is in rivers, lakes, and the atmosphere, a greater quantity of water is found in the earths interior. Water on Earth moves continually through the cycle of evaporation and transpiration, condensation, precipitation. Evaporation and transpiration contribute to the precipitation over land, large amounts of water are also chemically combined or adsorbed in hydrated minerals. Safe drinking water is essential to humans and other even though it provides no calories or organic nutrients. There is a correlation between access to safe water and gross domestic product per capita. However, some observers have estimated that by 2025 more than half of the population will be facing water-based vulnerability. A report, issued in November 2009, suggests that by 2030, in developing regions of the world. Water plays an important role in the world economy, approximately 70% of the freshwater used by humans goes to agriculture. Fishing in salt and fresh water bodies is a source of food for many parts of the world. Much of long-distance trade of commodities and manufactured products is transported by boats through seas, rivers, lakes, large quantities of water, ice, and steam are used for cooling and heating, in industry and homes. Water is an excellent solvent for a variety of chemical substances, as such it is widely used in industrial processes. Water is also central to many sports and other forms of entertainment, such as swimming, pleasure boating, boat racing, surfing, sport fishing, Water is a liquid at the temperatures and pressures that are most adequate for life. Specifically, at atmospheric pressure of 1 bar, water is a liquid between the temperatures of 273.15 K and 373.15 K
17.
Chemical element
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A chemical element or element is a species of atoms having the same number of protons in their atomic nuclei. There are 118 elements that have identified, of which the first 94 occur naturally on Earth with the remaining 24 being synthetic elements. There are 80 elements that have at least one stable isotope and 38 that have exclusively radioactive isotopes, iron is the most abundant element making up Earth, while oxygen is the most common element in the Earths crust. Chemical elements constitute all of the matter of the universe. The two lightest elements, hydrogen and helium, were formed in the Big Bang and are the most common elements in the universe. The next three elements were formed mostly by cosmic ray spallation, and are rarer than those that follow. Formation of elements with from 6 to 26 protons occurred and continues to occur in main sequence stars via stellar nucleosynthesis, the high abundance of oxygen, silicon, and iron on Earth reflects their common production in such stars. The term element is used for atoms with a number of protons as well as for a pure chemical substance consisting of a single element. A single element can form multiple substances differing in their structure, when different elements are chemically combined, with the atoms held together by chemical bonds, they form chemical compounds. Only a minority of elements are found uncombined as relatively pure minerals, among the more common of such native elements are copper, silver, gold, carbon, and sulfur. All but a few of the most inert elements, such as gases and noble metals, are usually found on Earth in chemically combined form. While about 32 of the elements occur on Earth in native uncombined forms. For example, atmospheric air is primarily a mixture of nitrogen, oxygen, and argon, the history of the discovery and use of the elements began with primitive human societies that found native elements like carbon, sulfur, copper and gold. Later civilizations extracted elemental copper, tin, lead and iron from their ores by smelting, using charcoal, alchemists and chemists subsequently identified many more, almost all of the naturally occurring elements were known by 1900. Save for unstable radioactive elements with short half-lives, all of the elements are available industrially, almost all other elements found in nature were made by various natural methods of nucleosynthesis. On Earth, small amounts of new atoms are produced in nucleogenic reactions, or in cosmogenic processes. Of the 94 naturally occurring elements, those with atomic numbers 1 through 82 each have at least one stable isotope, Isotopes considered stable are those for which no radioactive decay has yet been observed. Elements with atomic numbers 83 through 94 are unstable to the point that radioactive decay of all isotopes can be detected, the very heaviest elements undergo radioactive decay with half-lives so short that they are not found in nature and must be synthesized
18.
Tungsten
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Tungsten, also known as wolfram, is a chemical element with symbol W and atomic number 74. The word tungsten comes from the Swedish language tung sten, which translates to heavy stone. Its name in Swedish is volfram, however, in order to distinguish it from scheelite, a hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth almost exclusively in chemical compounds. It was identified as a new element in 1781, and first isolated as a metal in 1783 and its important ores include wolframite and scheelite. The free element is remarkable for its robustness, especially the fact that it has the highest melting point of all the elements discovered, melting at 3,422 °C,6,192 °F. Its high density is 19.3 times that of water, comparable to that of uranium and gold, polycrystalline tungsten is an intrinsically brittle and hard material, making it difficult to work. However, pure single-crystalline tungsten is more ductile, and can be cut with a hard-steel hacksaw, tungstens many alloys have numerous applications, including incandescent light bulb filaments, X-ray tubes, electrodes in TIG welding, superalloys, and radiation shielding. Tungstens hardness and high density give it military applications in penetrating projectiles, tungsten compounds are also often used as industrial catalysts. Tungsten is the metal from the third transition series that is known to occur in biomolecules. It is the heaviest element known to be essential to any living organism, tungsten interferes with molybdenum and copper metabolism and is somewhat toxic to animal life. In its raw form, tungsten is a hard metal that is often brittle. If made very pure, tungsten retains its hardness, and becomes malleable enough that it can be worked easily and it is worked by forging, drawing, or extruding. Tungsten objects are commonly formed by sintering. Of all metals in pure form, tungsten has the highest melting point, lowest vapor pressure, although carbon remains solid at higher temperatures than tungsten, carbon sublimes, rather than melts, so tungsten is considered to have a higher melting point. Tungsten has the lowest coefficient of expansion of any pure metal. The low thermal expansion and high melting point and tensile strength of tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d electrons, alloying small quantities of tungsten with steel greatly increases its toughness. Tungsten exists in two crystalline forms, α and β. The former has a cubic structure and is the more stable form
19.
Carbon
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Carbon is a chemical element with symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds, three isotopes occur naturally, 12C and 13C being stable, while 14C is a radioactive isotope, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity, Carbon is the 15th most abundant element in the Earths crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is the second most abundant element in the body by mass after oxygen. The atoms of carbon can bond together in different ways, termed allotropes of carbon, the best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form, for example, graphite is opaque and black while diamond is highly transparent. Graphite is soft enough to form a streak on paper, while diamond is the hardest naturally occurring material known, graphite is a good electrical conductor while diamond has a low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials, all carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen, the most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and transition metal carbonyl complexes. The largest sources of carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil. For this reason, carbon has often referred to as the king of the elements. The allotropes of carbon graphite, one of the softest known substances, and diamond. It bonds readily with other small atoms including other carbon atoms, Carbon is known to form almost ten million different compounds, a large majority of all chemical compounds. Carbon also has the highest sublimation point of all elements, although thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements such as iron and copper that are weaker reducing agents at room temperature. Carbon is the element, with a ground-state electron configuration of 1s22s22p2. Its first four ionisation energies,1086.5,2352.6,4620.5 and 6222.7 kJ/mol, are higher than those of the heavier group 14 elements. Carbons covalent radii are normally taken as 77.2 pm,66.7 pm and 60.3 pm, although these may vary depending on coordination number, in general, covalent radius decreases with lower coordination number and higher bond order. Carbon compounds form the basis of all life on Earth
20.
Sublimation (phase transition)
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Sublimation is the phase transition of a substance directly from the solid to the gas phase without passing through the intermediate liquid phase. Sublimation is a process that occurs at temperatures and pressures below a substances triple point in its phase diagram. The reverse process of sublimation is deposition or desublimation, in which a substance directly from a gas to a solid phase. Sublimation has also used as a generic term to describe a solid-to-gas transition followed by a gas-to-solid transition. At normal pressures, most chemical compounds and elements possess three different states at different temperatures, in these cases, the transition from the solid to the gaseous state requires an intermediate liquid state. The pressure referred to is the pressure of the substance. So, all solids that possess an appreciable vapor pressure at a temperature usually can sublimate in air. The term sublimation refers to a change of state and is not used to describe transformation of a solid to a gas in a chemical reaction. For example, the dissociation on heating of ammonium chloride into hydrogen chloride and ammonia is not sublimation. Similarly the combustion of candles, containing paraffin wax, to carbon dioxide and water vapor is not sublimation, sublimation requires additional energy and is an endothermic change. The enthalpy of sublimation can be calculated by adding the enthalpy of fusion, solid carbon dioxide sublimes everywhere along the line below the triple point, whereas its melting into liquid CO2 can occur only along the line at pressures and temperatures above the triple point. Snow and ice sublime, although more slowly, at temperatures below the freezing/melting point temperature line at 0 °C for most pressures, in freeze-drying, the material to be dehydrated is frozen and its water is allowed to sublime under reduced pressure or vacuum. The loss of snow from a snowfield during a spell is often caused by sunshine acting directly on the upper layers of the snow. Ablation is a process that includes sublimation and erosive wear of glacier ice, naphthalene, an organic compound commonly found in pesticide such as mothball also sublimes. It sublimes easily because it is made of molecules and has van der Waals intermolecular forces. Naphthalene is a solid that sublimes at atmospheric temperature with the sublimation point at around 80˚C or 176˚F. At low temperature, its pressure is high enough,1 mmHg at 53˚C. On the cool surface, the sublimated vapour will be solidified to form a needle-like crystal, iodine produces fumes on gentle heating
21.
Pascal (unit)
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The pascal is the SI derived unit of pressure used to quantify internal pressure, stress, Youngs modulus and ultimate tensile strength. It is defined as one newton per square meter and it is named after the French polymath Blaise Pascal. Common multiple units of the pascal are the hectopascal which is equal to one millibar, the unit of measurement called standard atmosphere is defined as 101,325 Pa and approximates to the average pressure at sea-level at the latitude 45° N. Meteorological reports typically state atmospheric pressure in hectopascals, the unit is named after Blaise Pascal, noted for his contributions to hydrodynamics and hydrostatics, and experiments with a barometer. The name pascal was adopted for the SI unit newton per square metre by the 14th General Conference on Weights, one pascal is the pressure exerted by a force of magnitude one newton perpendicularly upon an area of one square metre. The unit of measurement called atmosphere or standard atmosphere is 101325 Pa and this value is often used as a reference pressure and specified as such in some national and international standards, such as ISO2787, ISO2533 and ISO5024. In contrast, IUPAC recommends the use of 100 kPa as a standard pressure when reporting the properties of substances, geophysicists use the gigapascal in measuring or calculating tectonic stresses and pressures within the Earth. Medical elastography measures tissue stiffness non-invasively with ultrasound or magnetic resonance imaging, in materials science and engineering, the pascal measures the stiffness, tensile strength and compressive strength of materials. In engineering use, because the pascal represents a small quantity. The pascal is also equivalent to the SI unit of energy density and this applies not only to the thermodynamics of pressurised gases, but also to the energy density of electric, magnetic, and gravitational fields. In measurements of sound pressure, or loudness of sound, one pascal is equal to 94 decibels SPL, the quietest sound a human can hear, known as the threshold of hearing, is 0 dB SPL, or 20 µPa. The airtightness of buildings is measured at 50 Pa, the units of atmospheric pressure commonly used in meteorology were formerly the bar, which was close to the average air pressure on Earth, and the millibar. Since the introduction of SI units, meteorologists generally measure pressures in hectopascals unit, exceptions include Canada and Portugal, which use kilopascals. In many other fields of science, the SI is preferred, many countries also use the millibar or hectopascal to give aviation altimeter settings. In practically all fields, the kilopascal is used instead. Centimetre of water Metric prefix Orders of magnitude Pascals law
22.
Refractory
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A refractory material is a material that retains its strength at high temperatures. Refractory materials are used in linings for furnaces, kilns, incinerators and they are also used to make crucibles and moulds for casting glass and metals and for surfacing flame deflector systems for rocket launch structures. Today, the iron- and steel-industry uses approximately 70% of all refractories produced, refractory materials must be chemically and physically stable at high temperatures. Depending on the environment, they must be resistant to thermal shock, be chemically inert, and/or have specific ranges of thermal conductivity. The oxides of aluminium, silicon and magnesium are the most important materials used in the manufacturing of refractories, another oxide usually found in refractories is the oxide of calcium. Fire clays are also used in the manufacture of refractories. Refractories must be according to the conditions they face. Some applications require special refractory materials, zirconia is used when the material must withstand extremely high temperatures. Silicon carbide and carbon are two other materials used in some very severe temperature conditions, but they cannot be used in contact with oxygen, as they will oxidize. Binary compounds such as tungsten carbide or boron nitride can be very refractory, hafnium carbide is the most refractory binary compound known, with a melting point of 3890 °C. The ternary compound tantalum hafnium carbide has one of the highest melting points of all known compounds, Refractories can be classified on the basis of chemical composition, method of manufacture, physical form or according to their applications, fusion temperature. Acidic refractories consist of acidic materials like alumina and silica. They are generally not attacked or affected by acidic materials, and they include substances such as silica, alumina, and fire clay brick refractories. Notable reagents that can attack both alumina and silica are hydrofluoric acid, phosphoric acid, and fluorinated gases, at high temperatures, acidic refractories may also react with limes and basic oxides. These are used in areas where slags and atmosphere are either acidic or basic and are stable to both acids and bases. The main raw materials belong to, but are not confined to, common examples of these materials are alumina, chromia and carbon. These are used in areas where slags and atmosphere are basic, they are stable to alkaline materials, the main raw materials belong to the RO group to which magnesia is a very common example. Other examples include dolomite and chrome-magnesia, for the first half of the twentieth century, the steel making process used artificial periclase as a lining material for the furnace
23.
Helium
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Helium is a chemical element with symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and its boiling point is the lowest among all the elements. Its abundance is similar to figure in the Sun and in Jupiter. This is due to the high nuclear binding energy of helium-4 with respect to the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of nuclear fusion and radioactive decay. Most helium in the universe is helium-4, and is believed to have formed during the Big Bang. Large amounts of new helium are being created by fusion of hydrogen in stars. Helium is named for the Greek god of the Sun, Helios and it was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer, Janssen observed during the solar eclipse of 1868 while Lockyer observed from Britain. Lockyer was the first to propose that the line was due to a new element, the formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite. In 1903, large reserves of helium were found in gas fields in parts of the United States. Liquid helium is used in cryogenics, particularly in the cooling of superconducting magnets, a well-known but minor use is as a lifting gas in balloons and airships. As with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre, on Earth it is relatively rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the radioactive decay of heavy radioactive elements. Previously, terrestrial helium—a non-renewable resource, because once released into the atmosphere it readily escapes into space—was thought to be in short supply. The first evidence of helium was observed on August 18,1868, the line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India. This line was assumed to be sodium. He concluded that it was caused by an element in the Sun unknown on Earth, Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος
24.
Absolute zero
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Absolute zero is the lower limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reaches its minimum value, taken as 0. The corresponding Kelvin and Rankine temperature scales set their zero points at absolute zero by definition, in the quantum-mechanical description, matter at absolute zero is in its ground state, the point of lowest internal energy. And a system at absolute zero still possesses quantum mechanical zero-point energy, the kinetic energy of the ground state cannot be removed. Scientists and technologists routinely achieve temperatures close to zero, where matter exhibits quantum effects such as superconductivity and superfluidity. At temperatures near 0 K, nearly all molecular motion ceases and ΔS =0 for any adiabatic process, in such a circumstance, pure substances can form perfect crystals as T →0. Max Plancks strong form of the law of thermodynamics states the entropy of a perfect crystal vanishes at absolute zero. The Nernst postulate identifies the isotherm T =0 as coincident with the adiabat S =0, although other isotherms, as no two adiabats intersect, no other adiabat can intersect the T =0 isotherm. Consequently no adiabatic process initiated at nonzero temperature can lead to zero temperature, a perfect crystal is one in which the internal lattice structure extends uninterrupted in all directions. The perfect order can be represented by translational symmetry along three axes, every lattice element of the structure is in its proper place, whether it is a single atom or a molecular grouping. For substances that exist in two crystalline forms, such as diamond and graphite for carbon, there is a kind of chemical degeneracy. The question remains whether both can have zero entropy at T =0 even though each is perfectly ordered, using the Debye model, the specific heat and entropy of a pure crystal are proportional to T3, while the enthalpy and chemical potential are proportional to T4. These quantities drop toward their T =0 limiting values and approach with zero slopes, for the specific heats at least, the limiting value itself is definitely zero, as borne out by experiments to below 10 K. Even the less detailed Einstein model shows this curious drop in specific heats, in fact, all specific heats vanish at absolute zero, not just those of crystals. Likewise for the coefficient of thermal expansion, maxwells relations show that various other quantities also vanish. Since the relation between changes in Gibbs free energy, the enthalpy and the entropy is Δ G = Δ H − T Δ S thus, as T decreases, ΔG and ΔH approach each other. Experimentally, it is found that all spontaneous processes result in a decrease in G as they proceed toward equilibrium, if ΔS and/or T are small, the condition ΔG <0 may imply that ΔH <0, which would indicate an exothermic reaction. However, this is not required, endothermic reactions can proceed spontaneously if the TΔS term is large enough, moreover, the slopes of the derivatives of ΔG and ΔH converge and are equal to zero at T =0. e. An actual process is the most exothermic one, one model that estimates the properties of an electron gas at absolute zero in metals is the Fermi gas
25.
Melting point apparatus
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A melting point apparatus is a scientific instrument used to determine the melting point of a substance. Some types of melting point apparatuses include the Thiele tube, Fisher-Johns apparatus, Gallenkamp melting point apparatus, while the outward designs of apparatuses can vary greatly most apparatuses use a sample loaded into a sealed capillary that is then placed in the apparatuses. The sample is heated, either by a heating block or an oil bath. The operator or the records the temperature range starting with the initial phase change temperature. The temperature range that is determined can then be averaged to gain the point of the sample being examined. Apparatuses usually have a panel that allows the starting and final temperatures. Some machines have several channels which permit more than one sample to be tested at a time, the control panel might have buttons which allow the start and end of the melting point range to be recorded. A Thiele tube is an instrument that is filled with oil that is heated by using an open flame. The sample is placed in the opening in a capillary tube alongside a mercury thermometer, by using different oils different temperature ranges can be reached and used to determine melting points. The Thiele tube may also be used to determine boiling points, by using a liquid sample instead of a solid sample
26.
Laboratory
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A laboratory is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratories used for scientific research take many forms because of the requirements of specialists in the various fields of science. A physics laboratory might contain a particle accelerator or vacuum chamber, a chemist or biologist might use a wet laboratory, while a psychologists laboratory might be a room with one-way mirrors and hidden cameras in which to observe behavior. In some laboratories, such as commonly used by computer scientists. Scientists in other fields will use other types of laboratories. Engineers use laboratories as well to design, build, and test technological devices, scientific laboratories can be found as research and learning spaces in schools and universities, industry, government, or military facilities, and even aboard ships and spacecraft. Early instances of laboratories recorded in English involved alchemy and the preparation of medicines, larger or more sophisticated equipment is generally called a scientific instrument. Both laboratory equipment and scientific instruments are increasingly being designed and shared using open hardware principles, open source labs use open source scientific hardware. The title of laboratory is used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories. In many labs, though, hazards are present, in laboratories where dangerous conditions might exist, safety precautions are important. Rules exist to minimize the risk, and safety equipment is used to protect the lab user from injury or to assist in responding to an emergency. This standard is referred to as the Laboratory Standard. Under this standard, a laboratory is required to produce a Chemical Hygiene Plan which addresses the specific hazards found in its location, the CHP must be reviewed annually. Many schools and businesses employ safety, health, and environmental specialists, such as a Chemical Hygiene Officer to develop, manage, and evaluate their CHP. Additionally, third party review is used to provide an objective outside view which provides a fresh look at areas. An important element of such audits is the review of regulatory compliance, training is critical to the ongoing safe operation of the laboratory facility. Educators, staff and management must be engaged in working to reduce the likelihood of accidents, injuries, efforts are made to ensure laboratory safety videos are both relevant and engaging. The dictionary definition of laboratory at Wiktionary Media related to Laboratory at Wikimedia Commons Nobel Laureates Interactive 360° Laboratories
27.
Kofler bench
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A Kofler bench or Kofler hot-stage microscope is a metal strip with a temperature gradient. Any substance can be placed on a section of the strip revealing its thermal behaviour at the temperature at that point and this melting-point apparatus for use with a microscope was developed by the Austrian pharmacognosist Ludwig Kofler and his wife Adelheid Kofler. In 1936, the Koflers and Mayrhofer published their Mikroskopische Methoden in der Mikrochemie, Kofler, his wife Adelheid, and their colleague, Maria Kuhnert-Brandstätter, investigated numerous organic molecules, and published some 250 papers describing their work. Thermomicroscopy, incepted by Ludwig and Adelheid Kofler and developed further by Maria Kuhnert-Brandstätter and Walter C, mcCrone is a technique for studying the phases of solid drug substances. The evolution of hot-stage microscopy to aid solid-state characterizations of pharmaceutical solids, development of Pharmacognosis at the University of Innsbruck
28.
Differential scanning calorimetry
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Both the sample and reference are maintained at nearly the same temperature throughout the experiment. Generally, the program for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time. The reference sample should have a heat capacity over the range of temperatures to be scanned. The technique was developed by E. S. Watson and M. J. ONeill in 1962, and introduced commercially at the 1963 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. The first adiabatic differential scanning calorimeter that could be used in biochemistry was developed by P. L. Privalov and D. R. Monaselidze in 1964 at Institute of Physics in Tbilisi, Georgia. The term DSC was coined to describe this instrument, which measures energy directly, whether less or more heat must flow to the sample depends on whether the process is exothermic or endothermic. For example, as a solid sample melts to a liquid and this is due to the absorption of heat by the sample as it undergoes the endothermic phase transition from solid to liquid. Likewise, as the sample undergoes exothermic processes less heat is required to raise the sample temperature, by observing the difference in heat flow between the sample and reference, differential scanning calorimeters are able to measure the amount of heat absorbed or released during such transitions. DSC may also be used to more subtle physical changes. It is widely used in settings as a quality control instrument due to its applicability in evaluating sample purity. An alternative technique, which shares much in common with DSC, is differential thermal analysis, in this technique it is the heat flow to the sample and reference that remains the same rather than the temperature. When the sample and reference are heated identically, phase changes, both DSC and DTA provide similar information. The result of a DSC experiment is a curve of heat flux versus temperature or versus time, there are two different conventions, exothermic reactions in the sample shown with a positive or negative peak, depending on the kind of technology used in the experiment. This curve can be used to calculate enthalpies of transitions and this is done by integrating the peak corresponding to a given transition. The calorimetric constant will vary from instrument to instrument, and can be determined by analyzing a sample with known enthalpies of transition. Differential scanning calorimetry can be used to measure a number of properties of a sample. Using this technique it is possible to observe fusion and crystallization events as well as glass transition temperatures Tg, DSC can also be used to study oxidation, as well as other chemical reactions. Glass transitions may occur as the temperature of a solid is increased
29.
Enthalpy of fusion
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This energy includes the contribution required to make room for any associated change in volume by displacing its environment against ambient pressure. The temperature at which the transition occurs is the melting point. By convention, the pressure is assumed to be 1 atm unless otherwise specified, the enthalpy of fusion is a latent heat, because during melting the introduction of heat cannot be observed as a temperature change, as the temperature remains constant during the process. The latent heat of fusion is the change of any amount of substance when it melts. The liquid phase has an internal energy than the solid phase. When liquid water is cooled, its temperature falls steadily until it drops just below the line of freezing point at 0 °C, the temperature then remains constant at the freezing point while the water crystallizes. Once the water is frozen, its temperature continues to fall. The enthalpy of fusion is almost always a quantity, helium is the only known exception. Helium-3 has an enthalpy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative enthalpy of fusion below 0.77 K. This means that, at appropriate constant pressures, these substances freeze with the addition of heat, in the case of 4He, this pressure range is between 24.992 and 25.00 atm. These values are mostly from the CRC Handbook of Chemistry and Physics, the conversion between cal/g and J/g in the above table uses the thermochemical calorie =4.184 joules rather than the International Steam Table calorie =4.1868 joules. 1) To heat 1 kg of water from 283.15 K to 303.15 K requires 83.6 kJ, however, to melt ice also requires energy. This error can be reduced when a heat capacity parameter is taken into account. H. Freeman and Company, p.236, ISBN 0-7167-7355-4 Ott, bevan, Boerio-Goates, Juliana, Chemical Thermodynamics, Advanced Applications, Academic Press, ISBN 0-12-530985-6
30.
Thiele tube
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The Thiele tube, named after the German chemist Johannes Thiele, is a laboratory glassware designed to contain and heat an oil bath. Such a setup is used in the determination of the melting point of a substance. The apparatus itself resembles a glass test tube with an attached handle, oil is poured into the tube, and then the handle is heated, either by a small flame or some other heating element. The shape of the Thiele tube allows for formation of convection currents in the oil when it is heated and these currents maintain a fairly uniform temperature distribution throughout the oil in the tube. The side arm of the tube is designed to generate these convection currents, the sample, packed in a capillary tube, is attached to the thermometer, and held by means of a rubber band or a small slice of rubber tubing. The Thiele tube is heated using a microburner with a small flame. A sample in a capillary, attached to a thermometer with a rubber band, is immersed in the tube. Heating is commenced, and the ranges at which the sample melts can then be observed. During heating, the point at which melting is observed and the constant is the melting point of the sample. A more modern method uses dedicated equipment, known as a melting point apparatus, a slow heating rate at the melting point is needed in order to get an accurate measurement. Record the temperature on the thermometer when the sample starts to melt, a sample in a fusion tube is attached to a thermometer with a rubber band, and immersed in the tube. A sealed capillary, open end pointing down, is placed in the fusion tube, the Thiele tube is heated, dissolved gases evolve from the sample first. Once the sample starts to boil, heating is stopped, the temperature at which the liquid sample is sucked into the sealed capillary is the boiling point of the sample
31.
Pyrometer
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A pyrometer is a type of remote-sensing thermometer used to measure the temperature of a surface. Various forms of pyrometers have historically existed, the word pyrometer comes from the Greek word for fire, πυρ, and meter, meaning to measure. The word pyrometer was originally coined to denote a device capable of measuring the temperature of an object by its incandescence, modern pyrometers or infrared thermometers also measure the temperature of cooler objects, down to room temperature, by detecting their infrared radiation flux. A modern pyrometer has a system and a detector. The optical system focuses the radiation onto the detector. These are most commonly overcome by using thin filament pyrometry or soot pyrometry, both techniques involve small solids in contact with hot gases. Later examples used the expansion of a metal bar, the first disappearing filament pyrometer was built by L. Holborn and F. Kurlbaum in 1901. This device had an electrical filament between an observers eye and an incandescent object. The current through the filament was adjusted until it was of the colour as the object. The temperature returned by the vanishing filament pyrometer and others of its kind, with greater use of brightness pyrometers, it became obvious that problems existed with relying on knowledge of the value of emissivity. Emissivity was found to change, often drastically, with surface roughness, bulk and surface composition, to get around these difficulties, the ratio or two-color pyrometer was developed. This solution assumes that the emissivity is the same at both wavelengths and cancels out in the division and this is known as the gray body assumption. Ratio pyrometers are essentially two brightness pyrometers in a single instrument, the operational principles of the ratio pyrometers were developed in the 1920s and 1930s, and they were commercially available in 1939. As the ratio pyrometer came into use, it was determined that many materials, of which metals are an example. For these materials, the emissivity does not cancel out and the measurement is in error. The amount of error depends on the emissivities and the wavelengths where the measurements are taken, two-color ratio pyrometers cannot measure whether a material’s emissivity is wavelength dependent. Pyrometers are suited especially to the measurement of moving objects or any surfaces that can not be reached or can not be touched, temperature is a fundamental parameter in metallurgical furnace operations. Reliable and continuous measurement of the temperature is essential for effective control of the operation
32.
Planck's law
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Plancks law describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature T. The law is named after Max Planck, who proposed it in 1900 and it is a pioneering result of modern physics and quantum theory. The spectral radiance of a body, Bν, describes the amount of energy it gives off as radiation of different frequencies. It is measured in terms of the power emitted per unit area of the body, the spectral radiance can also be measured per unit wavelength λ instead of per unit frequency. In this case, it is given by B λ =2 h c 2 λ51 e h c λ k B T −1. The law may also be expressed in terms, such as the number of photons emitted at a certain wavelength. The SI units of Bν are W·sr−1·m−2·Hz−1, while those of Bλ are W·sr−1·m−3, in the limit of low frequencies, Plancks law tends to the Rayleigh–Jeans law, while in the limit of high frequencies it tends to the Wien approximation. Every physical body spontaneously and continuously emits electromagnetic radiation, near thermodynamic equilibrium, the emitted radiation is nearly described by Plancks law. Because of its dependence on temperature, Planck radiation is said to be thermal radiation, the higher the temperature of a body the more radiation it emits at every wavelength. Planck radiation has an intensity at a specific wavelength that depends on the temperature. For example, at room temperature, a body emits thermal radiation that is mostly infrared, at higher temperatures the amount of infrared radiation increases and can be felt as heat, and the body glows visibly red. At even higher temperatures, a body is bright yellow or blue-white and emits significant amounts of short wavelength radiation, including ultraviolet. The surface of the sun emits large amounts of infrared and ultraviolet radiation, its emission is peaked in the visible spectrum. Planck radiation is the greatest amount of radiation that any body at thermal equilibrium can emit from its surface, the passage of radiation across an interface between media can be characterized by the emissivity of the interface, usually denoted by the symbol ε. It is in general dependent on chemical composition and physical structure, on temperature, on the wavelength, on the angle of passage, the emissivity of a natural interface is always between ε =0 and 1. A body that interfaces with another medium which both has ε =1 and absorbs all the incident upon it, is said to be a black body. At equilibrium, the radiation inside this enclosure follows Plancks law, just as the Maxwell–Boltzmann distribution is the unique maximum entropy energy distribution for a gas of material particles at thermal equilibrium, so is Plancks distribution for a gas of photons. If the photon gas is not Planckian, the law of thermodynamics guarantees that interactions will cause the photon energy distribution to change
33.
Emissivity
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The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation and it may include both visible radiation and infrared radiation, which is not visible to human eyes, the thermal radiation from very hot objects is easily visible to the eye. Quantitatively, emissivity is the ratio of the radiation from a surface to the radiation from an ideal black surface at the same temperature as given by the Stefan–Boltzmann law. The ratio varies from 0 to 1, emissivities are important in several contexts, insulated windows. – Warm surfaces are usually cooled directly by air, but they also cool themselves by emitting thermal radiation and this second cooling mechanism is important for simple glass windows, which have emissivities close to the maximum possible value of 1.0. Low-E windows with transparent low emissivity coatings emit less radiation than ordinary windows. In winter, these coatings can halve the rate at which a window loses heat compared to a glass window. – Similarly, solar heat collectors lose heat by emitting thermal radiation, advanced solar collectors incorporate selective surfaces that have very low emissivities. These collectors waste very little of the energy through emission of thermal radiation. – The planets are solar thermal collectors on a large scale, the temperature of a planets surface is determined by the balance between the heat absorbed by the planet from sunlight, heat emitted from its core, and thermal radiation emitted back into space. Emissivity of a planet is determined by the nature of its surface, – Pyrometers and infrared cameras are instruments used to measure the temperature of an object by using its thermal radiation, no actual contact with the object is needed. The calibration of these involves the emissivity of the surface thats being measured. Emissivities ε can be measured using simple devices such as Leslies cube in conjunction with a radiation detector such as a thermopile or a bolometer. The apparatus compares the thermal radiation from a surface to be tested with the radiation from a nearly ideal. The detectors are essentially black absorbers with very sensitive thermometers that record the temperature rise when exposed to thermal radiation. For measuring room temperature emissivities, the detectors must absorb thermal radiation completely at infrared wavelengths near 10×10−6 meters, visible light has a wavelength range of about 0.4 to 0. 7×10−6 meters from violet to deep red. Emissivity measurements for many surfaces are compiled in many handbooks and texts, some of these are listed in the following table. Notes, These emissivities are the total hemispherical emissivities from the surfaces, the values of the emissivities apply to materials that are optically thick
34.
Latent heat
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Latent heat is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process. An example is latent heat of fusion for a change, melting. The term was introduced around 1762 by British chemist Joseph Black and it is derived from the Latin latere. Black used the term in the context of calorimetry where a transfer caused a volume change while the thermodynamic systems temperature was constant. In contrast to latent heat, sensible heat involves a transfer that results in a temperature change of the system. The terms ″sensible heat″ and ″latent heat″ are specific forms of energy, ″Sensible heat″ is a bodys internal energy that may be ″sensed″ or felt. ″Latent heat″ is internal energy concerning the phase of a material, both sensible and latent heats are observed in many processes of transport of energy in nature. The original usage of the term, as introduced by Black, was applied to systems that were held at constant temperature. Such usage referred to latent heat of expansion and several other related latent heats and these latent heats are defined independently of the conceptual framework of thermodynamics. Two common forms of latent heat are latent heat of fusion and these names describe the direction of energy flow when changing from one phase to the next, from solid to liquid, and liquid to gas. In both cases the change is endothermic, meaning that the system absorbs energy, for example, when water evaporates, energy is required for the water molecules to overcome the forces of attraction between them, the transition from water to vapor requires an input of energy. If the vapor condenses to a liquid on a surface. The large value of the enthalpy of condensation of vapor is the reason that steam is a far more effective heating medium than boiling water. It is an important component of Earths surface energy budget, latent heat flux has been commonly measured with the Bowen ratio technique, or more recently since the mid-1900s by the Jonathan Beaver method. The English word latent comes from Latin latēns, meaning lying hidden, intensive properties are material characteristics and are not dependent on the size or extent of the sample. Commonly quoted and tabulated in the literature are the specific latent heat of fusion, the following table shows the specific latent heats and change of phase temperatures of some common fluids and gases. Bowen ratio Eddy covariance flux Sublimation Specific heat capacity Enthalpy of fusion Enthalpy of vaporization
35.
Gibbs free energy
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Just as in mechanics, where the decrease in potential energy is defined as maximum useful work that can be performed, similarly different potentials have different meanings. The Gibbs energy is also the potential that is minimized when a system reaches chemical equilibrium at constant pressure and temperature. Its derivative with respect to the coordinate of the system vanishes at the equilibrium point. As such, a reduction in G is a condition for the spontaneity of processes at constant pressure and temperature. The Gibbs free energy, originally called available energy, was developed in the 1870s by the American scientist Josiah Willard Gibbs. The initial state of the body, according to Gibbs, is supposed to be such that the body can be made to pass from it to states of dissipated energy by reversible processes. In his 1876 magnum opus On the Equilibrium of Heterogeneous Substances, according to the second law of thermodynamics, for systems reacting at STP, there is a general natural tendency to achieve a minimum of the Gibbs free energy. A quantitative measure of the favorability of a reaction at constant temperature and pressure is the change ΔG in Gibbs free energy that is caused by the reaction. As a necessary condition for the reaction to occur at constant temperature and pressure, ΔG must be smaller than the non-PV work, ΔG equals the maximum amount of non-PV work that can be performed as a result of the chemical reaction for the case of reversible process. The equation can be seen from the perspective of the system taken together with its surroundings. First assume that the reaction at constant temperature and pressure is the only one that is occurring. Then the entropy released or absorbed by the system equals the entropy that the environment must absorb or release, the reaction will only be allowed if the total entropy change of the universe is zero or positive. This is reflected in a negative ΔG, and the reaction is called exergonic, if we couple reactions, then an otherwise endergonic chemical reaction can be made to happen. In traditional use, the term free was included in Gibbs free energy to mean available in the form of useful work, the characterization becomes more precise if we add the qualification that it is the energy available for non-volume work. However, a number of books and journal articles do not include the attachment free. This is the result of a 1988 IUPAC meeting to set unified terminologies for the scientific community. This standard, however, has not yet been universally adopted. Further, Gibbs stated, In this description, as used by Gibbs, ε refers to the energy of the body, η refers to the entropy of the body
36.
Enthalpy
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Enthalpy /ˈɛnθəlpi/ is a measurement of energy in a thermodynamic system. It is the thermodynamic quantity equivalent to the heat content of a system. It is equal to the energy of the system plus the product of pressure. Enthalpy is defined as a function that depends only on the prevailing equilibrium state identified by the systems internal energy, pressure. The unit of measurement for enthalpy in the International System of Units is the joule, but other historical, conventional units are still in use, such as the British thermal unit and the calorie. At constant pressure, the enthalpy change equals the energy transferred from the environment through heating or work other than expansion work, the total enthalpy, H, of a system cannot be measured directly. The same situation exists in classical mechanics, only a change or difference in energy carries physical meaning. Enthalpy itself is a potential, so in order to measure the enthalpy of a system, we must refer to a defined reference point, therefore what we measure is the change in enthalpy. The ΔH is a change in endothermic reactions, and negative in heat-releasing exothermic processes. For processes under constant pressure, ΔH is equal to the change in the energy of the system. This means that the change in enthalpy under such conditions is the heat absorbed by the material through a reaction or by external heat transfer. Enthalpies for chemical substances at constant pressure assume standard state, most commonly 1 bar pressure, standard state does not, strictly speaking, specify a temperature, but expressions for enthalpy generally reference the standard heat of formation at 25 °C. Enthalpy of ideal gases and incompressible solids and liquids does not depend on pressure, unlike entropy, real materials at common temperatures and pressures usually closely approximate this behavior, which greatly simplifies enthalpy calculation and use in practical designs and analyses. The word enthalpy stems from the Ancient Greek verb enthalpein, which means to warm in and it combines the Classical Greek prefix ἐν- en-, meaning to put into, and the verb θάλπειν thalpein, meaning to heat. The word enthalpy is often attributed to Benoît Paul Émile Clapeyron. This misconception was popularized by the 1927 publication of The Mollier Steam Tables, however, neither the concept, the word, nor the symbol for enthalpy existed until well after Clapeyrons death. The earliest writings to contain the concept of enthalpy did not appear until 1875, however, Gibbs did not use the word enthalpy in his writings. The actual word first appears in the literature in a 1909 publication by J. P. Dalton
37.
Entropy
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In statistical thermodynamics, entropy is a measure of the number of microscopic configurations Ω that a thermodynamic system can have when in a state as specified by some macroscopic variables. Formally, S = k B ln Ω, for example, gas in a container with known volume, pressure, and temperature could have an enormous number of possible configurations of the collection of individual gas molecules. Each instantaneous configuration of the gas may be regarded as random, Entropy may be understood as a measure of disorder within a macroscopic system. The second law of thermodynamics states that an isolated systems entropy never decreases, such systems spontaneously evolve towards thermodynamic equilibrium, the state with maximum entropy. Non-isolated systems may lose entropy, provided their environments entropy increases by at least that amount, since entropy is a function of the state of the system, a change in entropy of a system is determined by its initial and final states. This applies whether the process is reversible or irreversible, however, irreversible processes increase the combined entropy of the system and its environment. The above definition is called the macroscopic definition of entropy because it can be used without regard to any microscopic description of the contents of a system. The concept of entropy has found to be generally useful and has several other formulations. Entropy was discovered when it was noticed to be a quantity that behaves as a function of state and it has the dimension of energy divided by temperature, which has a unit of joules per kelvin in the International System of Units. But the entropy of a substance is usually given as an intensive property—either entropy per unit mass or entropy per unit amount of substance. In statistical mechanics this reflects that the state of a system is generally non-degenerate. Understanding the role of entropy in various processes requires an understanding of how. It is often said that entropy is an expression of the disorder, or randomness of a system, the second law is now often seen as an expression of the fundamental postulate of statistical mechanics through the modern definition of entropy. In other words, in any natural process there exists an inherent tendency towards the dissipation of useful energy and he made the analogy with that of how water falls in a water wheel. This was an insight into the second law of thermodynamics. g. Clausius described entropy as the transformation-content, i. e. dissipative energy use and this was in contrast to earlier views, based on the theories of Isaac Newton, that heat was an indestructible particle that had mass. Later, scientists such as Ludwig Boltzmann, Josiah Willard Gibbs, henceforth, the essential problem in statistical thermodynamics, i. e. according to Erwin Schrödinger, has been to determine the distribution of a given amount of energy E over N identical systems. Carathéodory linked entropy with a definition of irreversibility, in terms of trajectories
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Temperature
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A temperature is an objective comparative measurement of hot or cold. It is measured by a thermometer, several scales and units exist for measuring temperature, the most common being Celsius, Fahrenheit, and, especially in science, Kelvin. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, the kinetic theory offers a valuable but limited account of the behavior of the materials of macroscopic bodies, especially of fluids. Temperature is important in all fields of science including physics, geology, chemistry, atmospheric sciences, medicine. The Celsius scale is used for temperature measurements in most of the world. Because of the 100 degree interval, it is called a centigrade scale.15, the United States commonly uses the Fahrenheit scale, on which water freezes at 32°F and boils at 212°F at sea-level atmospheric pressure. Many scientific measurements use the Kelvin temperature scale, named in honor of the Scottish physicist who first defined it and it is a thermodynamic or absolute temperature scale. Its zero point, 0K, is defined to coincide with the coldest physically-possible temperature and its degrees are defined through thermodynamics. The temperature of zero occurs at 0K = −273. 15°C. For historical reasons, the triple point temperature of water is fixed at 273.16 units of the measurement increment, Temperature is one of the principal quantities in the study of thermodynamics. There is a variety of kinds of temperature scale and it may be convenient to classify them as empirically and theoretically based. Empirical temperature scales are historically older, while theoretically based scales arose in the middle of the nineteenth century, empirically based temperature scales rely directly on measurements of simple physical properties of materials. For example, the length of a column of mercury, confined in a capillary tube, is dependent largely on temperature. Such scales are only within convenient ranges of temperature. For example, above the point of mercury, a mercury-in-glass thermometer is impracticable. A material is of no use as a thermometer near one of its phase-change temperatures, in spite of these restrictions, most generally used practical thermometers are of the empirically based kind. Especially, it was used for calorimetry, which contributed greatly to the discovery of thermodynamics, nevertheless, empirical thermometry has serious drawbacks when judged as a basis for theoretical physics. Theoretically based temperature scales are based directly on theoretical arguments, especially those of thermodynamics, kinetic theory and they rely on theoretical properties of idealized devices and materials
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Pressure
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Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the relative to the ambient pressure. Various units are used to express pressure, Pressure may also be expressed in terms of standard atmospheric pressure, the atmosphere is equal to this pressure and the torr is defined as 1⁄760 of this. Manometric units such as the centimetre of water, millimetre of mercury, Pressure is the amount of force acting per unit area. The symbol for it is p or P, the IUPAC recommendation for pressure is a lower-case p. However, upper-case P is widely used. The usage of P vs p depends upon the field in one is working, on the nearby presence of other symbols for quantities such as power and momentum. Mathematically, p = F A where, p is the pressure, F is the normal force and it relates the vector surface element with the normal force acting on it. It is incorrect to say the pressure is directed in such or such direction, the pressure, as a scalar, has no direction. The force given by the relationship to the quantity has a direction. If we change the orientation of the element, the direction of the normal force changes accordingly. Pressure is distributed to solid boundaries or across arbitrary sections of normal to these boundaries or sections at every point. It is a parameter in thermodynamics, and it is conjugate to volume. The SI unit for pressure is the pascal, equal to one newton per square metre and this name for the unit was added in 1971, before that, pressure in SI was expressed simply in newtons per square metre. Other units of pressure, such as pounds per square inch, the CGS unit of pressure is the barye, equal to 1 dyn·cm−2 or 0.1 Pa. Pressure is sometimes expressed in grams-force or kilograms-force per square centimetre, but using the names kilogram, gram, kilogram-force, or gram-force as units of force is expressly forbidden in SI. The technical atmosphere is 1 kgf/cm2, since a system under pressure has potential to perform work on its surroundings, pressure is a measure of potential energy stored per unit volume. It is therefore related to density and may be expressed in units such as joules per cubic metre. Similar pressures are given in kilopascals in most other fields, where the prefix is rarely used
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Boiling point
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The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the environmental pressure. A liquid in a vacuum has a lower boiling point than when that liquid is at atmospheric pressure. A liquid at high pressure has a boiling point than when that liquid is at atmospheric pressure. For a given pressure, different liquids boil at different temperatures, for example, water boils at 100 °C at sea level, but at 93.4 °C at 2,000 metres altitude. The normal boiling point of a liquid is the case in which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level,1 atmosphere. At that temperature, the pressure of the liquid becomes sufficient to overcome atmospheric pressure. The standard boiling point has been defined by IUPAC since 1982 as the temperature at which boiling occurs under a pressure of 1 bar, the heat of vaporization is the energy required to transform a given quantity of a substance from a liquid into a gas at a given pressure. Liquids may change to a vapor at temperatures below their boiling points through the process of evaporation, evaporation is a surface phenomenon in which molecules located near the liquids edge, not contained by enough liquid pressure on that side, escape into the surroundings as vapor. On the other hand, boiling is a process in which molecules anywhere in the liquid escape, a saturated liquid contains as much thermal energy as it can without boiling. The saturation temperature is the temperature for a corresponding saturation pressure at which a liquid boils into its vapor phase, the liquid can be said to be saturated with thermal energy. Any addition of energy results in a phase transition. If the pressure in a system remains constant, a vapor at saturation temperature will begin to condense into its liquid phase as thermal energy is removed, similarly, a liquid at saturation temperature and pressure will boil into its vapor phase as additional thermal energy is applied. The boiling point corresponds to the temperature at which the pressure of the liquid equals the surrounding environmental pressure. Thus, the point is dependent on the pressure. Boiling points may be published with respect to the NIST, USA standard pressure of 101.325 kPa, at higher elevations, where the atmospheric pressure is much lower, the boiling point is also lower. The boiling point increases with increased pressure up to the critical point, the boiling point cannot be increased beyond the critical point. Likewise, the point decreases with decreasing pressure until the triple point is reached
41.
Glass
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Glass is a non-crystalline amorphous solid that is often transparent and has widespread practical, technological, and decorative usage in, for example, window panes, tableware, and optoelectronics. The most familiar, and historically the oldest, types of glass are silicate glasses based on the chemical compound silica, the primary constituent of sand. The term glass, in usage, is often used to refer only to this type of material. Many applications of silicate glasses derive from their optical transparency, giving rise to their use as window panes. Glass can be coloured by adding metallic salts, and can also be painted and printed with vitreous enamels and these qualities have led to the extensive use of glass in the manufacture of art objects and in particular, stained glass windows. Although brittle, silicate glass is extremely durable, and many examples of glass fragments exist from early glass-making cultures, because glass can be formed or moulded into any shape, it has been traditionally used for vessels, bowls, vases, bottles, jars and drinking glasses. In its most solid forms it has also used for paperweights, marbles. Some objects historically were so commonly made of glass that they are simply called by the name of the material, such as drinking glasses. Porcelains and many polymer thermoplastics familiar from everyday use are glasses and these sorts of glasses can be made of quite different kinds of materials than silica, metallic alloys, ionic melts, aqueous solutions, molecular liquids, and polymers. For many applications, like glass bottles or eyewear, polymer glasses are a lighter alternative than traditional glass, silica is a common fundamental constituent of glass. In nature, vitrification of quartz occurs when lightning strikes sand, forming hollow, fused quartz is a glass made from chemically-pure SiO2. It has excellent resistance to shock, being able to survive immersion in water while red hot. However, its high melting-temperature and viscosity make it difficult to work with, normally, other substances are added to simplify processing. One is sodium carbonate, which lowers the transition temperature. The soda makes the glass water-soluble, which is undesirable, so lime, some magnesium oxide. The resulting glass contains about 70 to 74% silica by weight and is called a soda-lime glass, soda-lime glasses account for about 90% of manufactured glass. Most common glass contains other ingredients to change its properties, lead glass or flint glass is more brilliant because the increased refractive index causes noticeably more specular reflection and increased optical dispersion. Adding barium also increases the refractive index, iron can be incorporated into glass to absorb infrared energy, for example in heat absorbing filters for movie projectors, while cerium oxide can be used for glass that absorbs UV wavelengths
42.
Glass transition
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An amorphous solid that exhibits a glass transition is called a glass. The reverse transition, achieved by supercooling a viscous liquid into the state, is called vitrification. The glass-transition temperature Tg of a material characterizes the range of temperatures over which this transition occurs. It is always lower than the temperature, Tm, of the crystalline state of the material. Hard plastics like polystyrene and poly are used well below their glass transition temperatures, such conventions include a constant cooling rate and a viscosity threshold of 1012 Pa·s, among others. The question of whether some phase transition underlies the glass transition is a matter of continuing research, the glass transition of a liquid to a solid-like state may occur with either cooling or compression. The transition comprises a smooth increase in the viscosity of a material by as much as 17 orders of magnitude without any pronounced change in material structure, the consequence of this dramatic increase is a glass exhibiting solid-like mechanical properties on the timescale of practical observation. In many materials that undergo a freezing transition, rapid cooling will avoid this phase transition. Other materials, such as polymers, lack a well defined crystalline state and easily form glasses. The tendency for a material to form a glass while quenched is called glass forming ability and this ability depends on the composition of the material and can be predicted by the rigidity theory. Below the transition temperature range, the structure does not relax in accordance with the cooling rate used. The expansion coefficient for the state is roughly equivalent to that of the crystalline solid. If slower cooling rates are used, the time for structural relaxation to occur may result in a higher density glass product. Similarly, by annealing the glass structure in time approaches an equilibrium density corresponding to the liquid at this same temperature. Tg is located at the intersection between the curve for the glassy state and the supercooled liquid. The configuration of the glass in this range changes slowly with time towards the equilibrium structure. The principle of the minimization of the Gibbs free energy provides the driving force necessary for the eventual change. It should be noted here that at higher temperatures than Tg
43.
Viscous liquid
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The mechanical properties of glass-forming liquids depend primarily on the viscosity. Therefore, the following working points are defined in terms of viscosity, in the opposite case of clearly non-Arrhenius behaviour the liquid is called fragile. This classification has no relation with the common usage of the word fragility to mean brittleness. Amorphous materials are classified accordingly to the deviation from Arrhenius type behaviour of their viscosities as either strong when QH-QL<QL or fragile when QH-QL≥QL, the fragility of amorphous materials is numerically characterized by the Doremus’ fragility ratio RD=QH/QL. Strong melts are those with <1, whereas fragile melts are those with ≥1, fragility is related to materials bond breaking processes caused by thermal fluctuations. Bond breaking modifies the properties of a material so that the higher the concentration of broken bonds termed configurons the lower the viscosity. The microscopic dynamics at low to moderate viscosities is addressed by a theory, developed by Wolfgang Götze. This theory describes a slowing down of structural relaxation on cooling towards a critical temperature Tc, Götze, W, Complex Dynamics of glass forming liquids. Zarzycki, J, Les Verres et létat vitreux
44.
Freezing-point depression
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Freezing-point depression is the process in which adding a solute to a solvent decreases the freezing point of the solvent. Examples include salt in water, alcohol in water, or the mixing of two such as impurities in a finely powdered drug. In the last case, the compound is the solute. The resulting solution or solid-solid mixture has a freezing point than the pure solvent or solid. This phenomenon is what causes sea water, to liquid at temperatures below 0 °C. The phenomenon of freezing-point depression has many practical uses, the radiator fluid in an automobile is a mixture of water and ethylene glycol. As a result of freezing-point depression, radiators do not freeze in winter, road salting takes advantage of this effect to lower the freezing point of the ice it is placed on. Lowering the freezing point allows the ice to melt at lower temperatures, preventing the accumulation of dangerous. Commonly used sodium chloride can depress the freezing point of water to about −21 °C, if the road surface temperature is lower NaCl becomes ineffective and other salts are used, such as calcium chloride, magnesium chloride or a mixture of many. These salts are somewhat aggressive to metals, especially iron, so in airports safer media such as formate, potassium formate, sodium acetate. Freezing-point depression is used by organisms that live in extreme cold. Such creatures have evolved means through which they can produce high concentration of compounds such as sorbitol and glycerol. In other animals, such as the spring peeper frog, the molality is increased temporarily as a reaction to cold temperatures. In the case of the frog, freezing temperatures trigger a large scale breakdown of glycogen in the frogs liver. With the formula below, freezing-point depression can be used to measure the degree of dissociation or the mass of the solute. This kind of measurement is called cryoscopy and relies on measurement of the freezing point. The degree of dissociation is measured by determining the van t Hoff factor i by first determining mB, in this case, the molar mass of the solute must be known. The molar mass of a solute is determined by comparing mB with the amount of solute dissolved, in this case, i must be known, and the procedure is primarily useful for organic compounds using a nonpolar solvent
