Calcium is a chemical element with symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air, its physical and chemical properties are most similar to its heavier homologues strontium and barium. It is the fifth most abundant element in Earth's crust and the third most abundant metal, after iron and aluminium; the most common calcium compound on Earth is calcium carbonate, found in limestone and the fossilised remnants of early sea life. The name derives from Latin calx "lime", obtained from heating limestone; some calcium compounds were known to the ancients, though their chemistry was unknown until the seventeenth century. Pure calcium was isolated in 1808 via electrolysis of its oxide by Humphry Davy, who named the element. Calcium compounds are used in many industries: in foods and pharmaceuticals for calcium supplementation, in the paper industry as bleaches, as components in cement and electrical insulators, in the manufacture of soaps.
On the other hand, the metal in pure form has few applications due to its high reactivity. Calcium is the fifth-most abundant element in the human body; as electrolytes, calcium ions play a vital role in the physiological and biochemical processes of organisms and cells: in signal transduction pathways where they act as a second messenger. Calcium ions outside cells are important for maintaining the potential difference across excitable cell membranes as well as proper bone formation. Calcium is a ductile silvery metal whose properties are similar to the heavier elements in its group, strontium and radium. A calcium atom has twenty electrons, arranged in the electron configuration 4s2. Like the other elements placed in group 2 of the periodic table, calcium has two valence electrons in the outermost s-orbital, which are easily lost in chemical reactions to form a dipositive ion with the stable electron configuration of a noble gas, in this case argon. Hence, calcium is always divalent in its compounds, which are ionic.
Hypothetical univalent salts of calcium would be stable with respect to their elements, but not to disproportionation to the divalent salts and calcium metal, because the enthalpy of formation of MX2 is much higher than those of the hypothetical MX. This occurs because of the much greater lattice energy afforded by the more charged Ca2+ cation compared to the hypothetical Ca+ cation. Calcium, strontium and radium are always considered to be alkaline earth metals. Beryllium and magnesium are different from the other members of the group in their physical and chemical behaviour: they behave more like aluminium and zinc and have some of the weaker metallic character of the post-transition metals, why the traditional definition of the term "alkaline earth metal" excludes them; this classification is obsolete in English-language sources, but is still used in other countries such as Japan. As a result, comparisons with strontium and barium are more germane to calcium chemistry than comparisons with magnesium.
Calcium metal melts at 842 °C and boils at 1494 °C. It crystallises in the face-centered cubic arrangement like strontium, its density of 1.55 g/cm3 is the lowest in its group. Calcium can be cut with a knife with effort. While calcium is a poorer conductor of electricity than copper or aluminium by volume, it is a better conductor by mass than both due to its low density. While calcium is infeasible as a conductor for most terrestrial applications as it reacts with atmospheric oxygen, its use as such in space has been considered; the chemistry of calcium is that of a typical heavy alkaline earth metal. For example, calcium spontaneously reacts with water more than magnesium and less than strontium to produce calcium hydroxide and hydrogen gas, it reacts with the oxygen and nitrogen in the air to form a mixture of calcium oxide and calcium nitride. When finely divided, it spontaneously burns in air to produce the nitride. In bulk, calcium is less reactive: it forms a hydration coating in moist air, but below 30% relative humidity it may be stored indefinitely at room temperature.
Besides the simple oxide CaO, the peroxide CaO2 can be made by direct oxidation of calcium metal under a high pressure of oxygen, there is some evidence for a yellow superoxide Ca2. Calcium hydroxide, Ca2, is a strong base, though it is not as strong as the hydroxides of strontium, barium or the alkali metals. All four dihalides of calcium are known. Calcium carbonate and calcium sulfate are abundant minerals. Like strontium and barium, as well as the alkali metals and the divalent lanthanides europium and ytterbium, calcium metal dissolves directly in liquid ammonia to give a dark blue solution. Due to the large size of the Ca2+ ion, high coordination numbers are common, up to 24 in some intermetallic compounds such as CaZn13. Calcium is complexed by oxygen chelates such as EDTA and polyphosphates, which are useful in an
Properties of water
Water is a polar inorganic compound, at room temperature a tasteless and odorless liquid, nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "universal solvent" and the "solvent of life", it is the most abundant substance on Earth and the only common substance to exist as a solid and gas on Earth's surface. It is the third most abundant molecule in the universe. Water molecules form hydrogen bonds with each other and are polar; this polarity allows it to dissociate ions in salts and bond to other polar substances such as alcohols and acids, thus dissolving them. Its hydrogen bonding causes its many unique properties, such as having a solid form less dense than its liquid form, a high boiling point of 100 °C for its molar mass, a high heat capacity. Water is amphoteric, meaning that it can exhibit properties of an acid or a base, depending on the pH of the solution that it is in. Related to its amphoteric character, it undergoes self-ionization.
The product of the activities, or the concentrations of H+ and OH− is a constant, so their respective concentrations are inversely proportional to each other. Water is the chemical substance with chemical formula H2O. Water is a odorless liquid at ambient temperature and pressure. Liquid water has weak absorption bands at wavelengths of around 750 nm which cause it to appear to have a blue colour; this can be observed in a water-filled bath or wash-basin whose lining is white. Large ice crystals, as in glaciers appear blue. Unlike other analogous hydrides of the oxygen family, water is a liquid under standard conditions due to hydrogen bonding; the molecules of water are moving in relation to each other, the hydrogen bonds are continually breaking and reforming at timescales faster than 200 femtoseconds. However, these bonds are strong enough to create many of the peculiar properties of water, some of which make it integral to life. Within the Earth's atmosphere and surface, the liquid phase is the most common and is the form, denoted by the word "water".
The solid phase of water is known as ice and takes the structure of hard, amalgamated crystals, such as ice cubes, or loosely accumulated granular crystals, like snow. Aside from common hexagonal crystalline ice, other crystalline and amorphous phases of ice are known; the gaseous phase of water is known as water vapor. Visible steam and clouds are formed from minute droplets of water suspended in the air. Water forms a supercritical fluid; the critical temperature is 647 K and the critical pressure is 22.064 MPa. In nature this only occurs in hostile conditions. A example of occurring supercritical water is in the hottest parts of deep water hydrothermal vents, in which water is heated to the critical temperature by volcanic plumes and the critical pressure is caused by the weight of the ocean at the extreme depths where the vents are located; this pressure is reached at a depth of about 2200 meters: much less than the mean depth of the ocean. Water has a high specific heat capacity of 4.1814 J/ at 25 °C – the second highest among all the heteroatomic species, as well as a high heat of vaporization, both of which are a result of the extensive hydrogen bonding between its molecules.
These two unusual properties allow water to moderate Earth's climate by buffering large fluctuations in temperature. Most of the additional energy stored in the climate system since 1970 has accumulated in the oceans; the specific enthalpy of fusion of water is 333.55 kJ/kg at 0 °C: the same amount of energy is required to melt ice as to warm ice from −160 °C up to its melting point or to heat the same amount of water by about 80 °C. Of common substances, only that of ammonia is higher; this property confers resistance to melting on the ice of glaciers and drift ice. Before and since the advent of mechanical refrigeration, ice was and still is in common use for retarding food spoilage; the specific heat capacity of ice at −10 °C is 2.03 J/ and the heat capacity of steam at 100 °C is 2.08 J/. The density of water is about 1 gram per cubic centimetre: this relationship was used to define the gram; the density varies with temperature, but not linearly: as the temperature increases, the density rises to a peak at 3.98 °C and decreases.
This unusual negative thermal expansion below 4 °C is observed in molten silica. Regular, hexagonal ice is less dense than liquid water—upon freezing, the density of water decreases by about 9%. Other substances that expand on freezing are silicon, gallium (melting point of 303 K |, germanium and bismuth. Pure silicon has a negative coefficient of thermal expansion for temperatures between about 18 and 120 kelvins; these effects are due to the reduction of thermal motion with cooling, which allows water molecules to form more hydrogen bonds that prevent the molecules from coming close to each other. While below 4 °C the breakage of hydrogen bonds due to freezing allows water molecules to pack closer despite the increase in the thermal motion, above 4 °C water expands as the temperature increases. Water near the boiling point is ab
Hydrogen is a chemical element with symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the Universe, constituting 75% of all baryonic mass. Non-remnant stars are composed of hydrogen in the plasma state; the most common isotope of hydrogen, termed protium, has no neutrons. The universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, tasteless, non-toxic, nonmetallic combustible diatomic gas with the molecular formula H2. Since hydrogen forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays a important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge when it is known as a hydride, or as a positively charged species denoted by the symbol H+.
The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex. As the only neutral atom for which the Schrödinger equation can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics. Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, that it produces water when burned, the property for which it was named: in Greek, hydrogen means "water-former". Industrial production is from steam reforming natural gas, less from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production for the fertilizer market. Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks.
Hydrogen gas is flammable and will burn in air at a wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol: 2 H2 + O2 → 2 H2O + 572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74% and with chlorine at 5–95%; the explosive reactions may be triggered by heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C. Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine, compared to the visible plume of a Space Shuttle Solid Rocket Booster, which uses an ammonium perchlorate composite; the detection of a burning hydrogen leak may require a flame detector. Hydrogen flames in other conditions are blue; the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin.
H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are potentially dangerous acids; the ground state energy level of the electron in a hydrogen atom is −13.6 eV, equivalent to an ultraviolet photon of 91 nm wavelength. The energy levels of hydrogen can be calculated accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity; because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, therefore only certain allowed energies. A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation, Dirac equation or the Feynman path integral formulation to calculate the probability density of the electron around the proton.
The most complicated treatments allow for the small effects of special relativity and vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all—illustrating how the "planetary orbit" differs from electron motion. There exist two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei. In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1. At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form known as the "normal form"; the equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an excited state and has a higher energy
Venezuela the Bolivarian Republic of Venezuela, is a country on the northern coast of South America, consisting of a continental landmass and a large number of small islands and islets in the Caribbean Sea. The capital and largest urban agglomeration is the city of Caracas, it has a territorial extension of 916,445 km2. The continental territory is bordered on the north by the Caribbean Sea and the Atlantic Ocean, on the west by Colombia, Brazil on the south and Tobago to the north-east and on the east by Guyana. With this last country, the Venezuelan government maintains a claim for Guayana Esequiba over an area of 159,542 km2. For its maritime areas, it exercises sovereignty over 71,295 km2 of territorial waters, 22,224 km2 in its contiguous zone, 471,507 km2 of the Caribbean Sea and the Atlantic Ocean under the concept of exclusive economic zone, 99,889 km2 of continental shelf; this marine area borders those of 13 states. The country has high biodiversity and is ranked seventh in the world's list of nations with the most number of species.
There are habitats ranging from the Andes Mountains in the west to the Amazon basin rain-forest in the south via extensive llanos plains, the Caribbean coast and the Orinoco River Delta in the east. The territory now known as Venezuela was colonized by Spain in 1522 amid resistance from indigenous peoples. In 1811, it became one of the first Spanish-American territories to declare independence, not securely established until 1821, when Venezuela was a department of the federal republic of Gran Colombia, it gained full independence as a country in 1830. During the 19th century, Venezuela suffered political turmoil and autocracy, remaining dominated by regional caudillos until the mid-20th century. Since 1958, the country has had a series of democratic governments. Economic shocks in the 1980s and 1990s led to several political crises, including the deadly Caracazo riots of 1989, two attempted coups in 1992, the impeachment of President Carlos Andrés Pérez for embezzlement of public funds in 1993.
A collapse in confidence in the existing parties saw the 1998 election of former coup-involved career officer Hugo Chávez and the launch of the Bolivarian Revolution. The revolution began with a 1999 Constituent Assembly, where a new Constitution of Venezuela was written; this new constitution changed the name of the country to Bolivarian Republic of Venezuela. The sovereign state is a federal presidential republic consisting of 23 states, the Capital District, federal dependencies. Venezuela claims all Guyanese territory west of the Essequibo River, a 159,500-square-kilometre tract dubbed Guayana Esequiba or the Zona en Reclamación. Venezuela is among the most urbanized countries in Latin America. Oil was discovered in the early 20th century, today, Venezuela has the world's largest known oil reserves and has been one of the world's leading exporters of oil; the country was an underdeveloped exporter of agricultural commodities such as coffee and cocoa, but oil came to dominate exports and government revenues.
The 1980s oil glut led to a long-running economic crisis. Inflation peaked at 100% in 1996 and poverty rates rose to 66% in 1995 as per capita GDP fell to the same level as 1963, down a third from its 1978 peak; the recovery of oil prices in the early 2000s gave. The Venezuelan government under Hugo Chávez established populist social welfare policies that boosted the Venezuelan economy and increased social spending, temporarily reducing economic inequality and poverty in the early years of the regime. However, such populist policies became inadequate, causing the nation's collapse as their excesses—including a uniquely extreme fossil fuel subsidy—are blamed for destabilizing the nation's economy; the destabilized economy led to a crisis in Bolivarian Venezuela, resulting in hyperinflation, an economic depression, shortages of basic goods and drastic increases in unemployment, disease, child mortality and crime. These factors have precipitated the Venezuelan Migrant Crisis where more than three million people have fled the country.
By 2017, Venezuela was declared to be in default regarding debt payments by credit rating agencies. In 2018, the country's economic policies led to extreme hyperinflation, with estimates expecting an inflation rate of 1,370,000% by the end of the year. Venezuela is a charter member of the UN, OAS, UNASUR, ALBA, Mercosur, LAIA and OEI. According to the most popular and accepted version, in 1499, an expedition led by Alonso de Ojeda visited the Venezuelan coast; the stilt houses in the area of Lake Maracaibo reminded the Italian navigator, Amerigo Vespucci, of the city of Venice, Italy, so he named the region Veneziola, or "Little Venice". The Spanish version of Veneziola is Venezuela. Martín Fernández de Enciso, a member of the Vespucci and Ojeda crew, gave a different account. In his work Summa de geografía, he states that the crew found indigenous people who called themselves the Veneciuela. Thus, the name "Venezuela" may have evolved from the native word; the official name was Estado de Venezuela, República de Venezuela, Estados Unidos de Venezuela, a
Pharmacolite is an uncommon calcium arsenate mineral with formula CaHAsO4·2. It occurs as soft, white clusters of fibrous crystals and encrustations which crystallize in the monoclinic system, it is the arsenate analogue of the phosphate brushite. Pharmacolite was first described in 1800 for an occurrence in the Sophia Mine in the Böckelsbach Valley of Wittichen, Black Forest, Baden-Württemberg, Germany; the name is from the Greek φάρμακον, alluding to its poisonous arsenic content. It forms by secondary processes from primary arsenic minerals, it is associated with picropharmacolite, hornesite and rosslerite
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance; the reference substance for liquids is nearly always water at its densest. Nonetheless, the temperature and pressure must be specified for the reference. Pressure is nearly always 1 atm. Temperatures for both sample and reference vary from industry to industry. In British beer brewing, the practice for specific gravity as specified above is to multiply it by 1,000. Specific gravity is used in industry as a simple means of obtaining information about the concentration of solutions of various materials such as brines, antifreeze coolants, sugar solutions and acids. Being a ratio of densities, specific gravity is a dimensionless quantity; the reason for the specific gravity being dimensionless is to provide a global consistency between the U. S. and Metric Systems, since various units for density may be used such as pounds per cubic feet or grams per cubic centimeter, etc.
Specific gravity varies with pressure. Substances with a specific gravity of 1 are neutrally buoyant in water; those with SG greater than 1 are denser than water and will, disregarding surface tension effects, sink in it. Those with an SG less than 1 will float on it. In scientific work, the relationship of mass to volume is expressed directly in terms of the density of the substance under study, it is in industry where specific gravity finds wide application for historical reasons. True specific gravity can be expressed mathematically as: S G true = ρ sample ρ H 2 O where ρsample is the density of the sample and ρH2O is the density of water; the apparent specific gravity is the ratio of the weights of equal volumes of sample and water in air: S G apparent = W A, sample W A, H 2 O where WA,sample represents the weight of the sample measured in air and WA,H2O the weight of water measured in air. It can be shown that true specific gravity can be computed from different properties: S G true = ρ sample ρ H 2 O = m sample V m H 2 O V = m sample m H 2 O g g = W V, sample W V, H 2 O where g is the local acceleration due to gravity, V is the volume of the sample and of water, ρsample is the density of the sample, ρH2O is the density of water and WV represents a weight obtained in vacuum.
The density of water varies with pressure as does the density of the sample. So it is necessary to specify the temperatures and pressures at which the densities or weights were determined, it is nearly always the case. But as specific gravity refers to incompressible aqueous solutions or other incompressible substances, variations in density caused by pressure are neglected at least where apparent specific gravity is being measured. For true specific gravity calculations, air pressure must be considered. Temperatures are specified by the notation, with Ts representing the temperature at which the sample's density was determined and Tr the temperature at which the reference density is specified. For example, SG would be understood to mean that the density of the sample was determined at 20 °C and of the water at 4 °C. Taking into account different sample and reference temperatures, we note that, while SGH2O = 1.000000, it is the case that SGH2O = 0.998203⁄0.999840 = 0.998363. Here, temperature is being specified using the current ITS-90 scale and the densities used here and in the rest of this article are based on that scale.
On the previous IPTS-68 scale, the densities at 20 °C and 4 °C are 0.9982071 and 0.9999720 respective
In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how fast light propagates through the material. It is defined as n = c v, where c is the speed of light in vacuum and v is the phase velocity of light in the medium. For example, the refractive index of water is 1.333, meaning that light travels 1.333 times as fast in vacuum as in water. The refractive index determines how much the path of light is bent, or refracted, when entering a material; this is described by Snell's law of refraction, n1 sinθ1 = n2 sinθ2, where θ1 and θ2 are the angles of incidence and refraction of a ray crossing the interface between two media with refractive indices n1 and n2. The refractive indices determine the amount of light, reflected when reaching the interface, as well as the critical angle for total internal reflection and Brewster's angle; the refractive index can be seen as the factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values: the speed of light in a medium is v = c/n, the wavelength in that medium is λ = λ0/n, where λ0 is the wavelength of that light in vacuum.
This implies that vacuum has a refractive index of 1, that the frequency of the wave is not affected by the refractive index. As a result, the energy of the photon, therefore the perceived color of the refracted light to a human eye which depends on photon energy, is not affected by the refraction or the refractive index of the medium. While the refractive index affects wavelength, it depends on photon frequency and energy so the resulting difference in the bending angle causes white light to split into its constituent colors; this is called dispersion. It can be observed in prisms and rainbows, chromatic aberration in lenses. Light propagation in absorbing materials can be described using a complex-valued refractive index; the imaginary part handles the attenuation, while the real part accounts for refraction. The concept of refractive index applies within the full electromagnetic spectrum, from X-rays to radio waves, it can be applied to wave phenomena such as sound. In this case the speed of sound is used instead of that of light, a reference medium other than vacuum must be chosen.
The refractive index n of an optical medium is defined as the ratio of the speed of light in vacuum, c = 299792458 m/s, the phase velocity v of light in the medium, n = c v. The phase velocity is the speed at which the crests or the phase of the wave moves, which may be different from the group velocity, the speed at which the pulse of light or the envelope of the wave moves; the definition above is sometimes referred to as the absolute refractive index or the absolute index of refraction to distinguish it from definitions where the speed of light in other reference media than vacuum is used. Air at a standardized pressure and temperature has been common as a reference medium. Thomas Young was the person who first used, invented, the name "index of refraction", in 1807. At the same time he changed this value of refractive power into a single number, instead of the traditional ratio of two numbers; the ratio had the disadvantage of different appearances. Newton, who called it the "proportion of the sines of incidence and refraction", wrote it as a ratio of two numbers, like "529 to 396".
Hauksbee, who called it the "ratio of refraction", wrote it as a ratio with a fixed numerator, like "10000 to 7451.9". Hutton wrote it as a ratio with a fixed denominator, like 1.3358 to 1. Young did not use a symbol for the index of refraction, in 1807. In the next years, others started using different symbols: n, m, µ; the symbol n prevailed. For visible light most transparent media have refractive indices between 1 and 2. A few examples are given in the adjacent table; these values are measured at the yellow doublet D-line of sodium, with a wavelength of 589 nanometers, as is conventionally done. Gases at atmospheric pressure have refractive indices close to 1 because of their low density. All solids and liquids have refractive indices above 1.3, with aerogel as the clear exception. Aerogel is a low density solid that can be produced with refractive index in the range from 1.002 to 1.265. Moissanite lies at the other end of the range with a refractive index as high as 2.65. Most plastics have refractive indices in the range from 1.3 to 1.7, but some high-refractive-index polymers can have values as high as 1.76.
For infrared light refractive indices can be higher. Germanium is transparent in the wavelength region from 2 to 14 µm and has a refractive index of about 4. A type of new materials, called topological insulator, was found holding higher refractive index of up to 6 in near to mid infrared frequency range. Moreover, topological insulator material are transparent; these excellent properties make them a type of significant materials for infrared optics. According to the theory of relativity, no information can travel faster than the speed of light in vacuum, but this does not mean that the refractive index cannot be lower than 1; the refractive index measures the phase velocity of light. The phase velocity is the speed at which the crests of the wave move and can be faster than the speed of light in vacuum, thereby give a refractive index below 1; this can occur close to resonance frequencies, for absorbing media, in plasmas, for X-rays. In the X-ray regime the refractive indices are