Ice is water frozen into a solid state. Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaque bluish-white color. In the Solar System, ice is abundant and occurs from as close to the Sun as Mercury to as far away as the Oort cloud objects. Beyond the Solar System, it occurs as interstellar ice, it is abundant on Earth's surface – in the polar regions and above the snow line – and, as a common form of precipitation and deposition, plays a key role in Earth's water cycle and climate. It occurs as frost, icicles or ice spikes. Ice molecules can exhibit more different phases that depend on temperature and pressure; when water is cooled up to three different types of amorphous ice can form depending on the history of its pressure and temperature. When cooled correlated proton tunneling occurs below −253.15 °C giving rise to macroscopic quantum phenomena. All the ice on Earth's surface and in its atmosphere is of a hexagonal crystalline structure denoted as ice Ih with minute traces of cubic ice denoted as ice Ic.
The most common phase transition to ice Ih occurs when liquid water is cooled below 0 °C at standard atmospheric pressure. It may be deposited directly by water vapor, as happens in the formation of frost; the transition from ice to water is melting and from ice directly to water vapor is sublimation. Ice is used in a variety including cooling, winter sports and ice sculpture; as a occurring crystalline inorganic solid with an ordered structure, ice is considered to be a mineral. It possesses a regular crystalline structure based on the molecule of water, which consists of a single oxygen atom covalently bonded to two hydrogen atoms, or H–O–H. However, many of the physical properties of water and ice are controlled by the formation of hydrogen bonds between adjacent oxygen and hydrogen atoms. An unusual property of ice frozen at atmospheric pressure is that the solid is 8.3% less dense than liquid water. The density of ice is 0.9167–0.9168 g/cm3 at 0 °C and standard atmospheric pressure, whereas water has a density of 0.9998–0.999863 g/cm3 at the same temperature and pressure.
Liquid water is densest 1.00 g/cm3, at 4 °C and becomes less dense as the water molecules begin to form the hexagonal crystals of ice as the freezing point is reached. This is due to hydrogen bonding dominating the intermolecular forces, which results in a packing of molecules less compact in the solid. Density of ice increases with decreasing temperature and has a value of 0.9340 g/cm3 at −180 °C. When water freezes, it increases in volume; the effect of expansion during freezing can be dramatic, ice expansion is a basic cause of freeze-thaw weathering of rock in nature and damage to building foundations and roadways from frost heaving. It is a common cause of the flooding of houses when water pipes burst due to the pressure of expanding water when it freezes; the result of this process is that ice floats on liquid water, an important feature in Earth's biosphere. It has been argued that without this property, natural bodies of water would freeze, in some cases permanently, from the bottom up, resulting in a loss of bottom-dependent animal and plant life in fresh and sea water.
Sufficiently thin ice sheets allow light to pass through while protecting the underside from short-term weather extremes such as wind chill. This creates a sheltered environment for algal colonies; when sea water freezes, the ice is riddled with brine-filled channels which sustain sympagic organisms such as bacteria, algae and annelids, which in turn provide food for animals such as krill and specialised fish like the bald notothen, fed upon in turn by larger animals such as emperor penguins and minke whales. When ice melts, it absorbs as much energy as it would take to heat an equivalent mass of water by 80 °C. During the melting process, the temperature remains constant at 0 °C. While melting, any energy added breaks the hydrogen bonds between ice molecules. Energy becomes available to increase the thermal energy only after enough hydrogen bonds are broken that the ice can be considered liquid water; the amount of energy consumed in breaking hydrogen bonds in the transition from ice to water is known as the heat of fusion.
As with water, ice absorbs light at the red end of the spectrum preferentially as the result of an overtone of an oxygen–hydrogen bond stretch. Compared with water, this absorption is shifted toward lower energies. Thus, ice appears blue, with a greener tint than liquid water. Since absorption is cumulative, the color effect intensifies with increasing thickness or if internal reflections cause the light to take a longer path through the ice. Other colors can appear in the presence of light absorbing impurities, where the impurity is dictating the color rather than the ice itself. For instance, icebergs containing impurities can appear grey or green. Ice may be any one of the 18 known solid crystalline phases of water, or in an amorphous solid state at various densities. Most liquids under increased pressure freeze at higher temperatures because the pressure helps to hold the molecules together. However, the strong hydrogen bonds in water make it different: For some pressures higher than 1 atm, water freezes at a temperature below
A downburst is a strong ground-level wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the point of contact at ground level. Producing damaging winds, it may be confused with a tornado, where high-velocity winds circle a central area, air moves inward and upward. Downbursts are created by an area of rain-cooled air that, after reaching ground level, spreads out in all directions producing strong winds. Dry downbursts are associated with thunderstorms with little rain, while wet downbursts are created by thunderstorms with high amounts of rainfall. Microbursts and macrobursts are downbursts at small and larger scales, respectively. Another variety, the heat burst, is created by vertical currents on the backside of old outflow boundaries and squall lines where rainfall is lacking. Heat bursts generate higher temperatures due to the lack of rain-cooled air in their formation. Downbursts create vertical wind shear or microbursts, dangerous to aviation.
A downburst is created by a column of sinking air that after hitting ground level, spreads out in all directions and is capable of producing damaging straight-line winds of over 240 km/h producing damage similar to, but distinguishable from, that caused by tornadoes. This is because the physical properties of a downburst are different from those of a tornado. Downburst damage will radiate from a central point as the descending column spreads out when hitting the surface, whereas tornado damage tends towards convergent damage consistent with rotating winds. To differentiate between tornado damage and damage from a downburst, the term straight-line winds is applied to damage from microbursts. Downbursts are strong downdrafts from thunderstorms. Downbursts in air, precipitation free or contains virga are known as dry downbursts. Most downbursts are less than 4 km in extent: these are called microbursts. Downbursts larger than 4 km in extent are sometimes called macrobursts. Downbursts can occur over large areas.
In the extreme case, a derecho can cover a huge area more than 320 km wide and over 1,600 km long, lasting up to 12 hours or more, is associated with some of the most intense straight-line winds, but the generative process is somewhat different from that of most downbursts. Straight-line winds are strong winds that can produce damage, demonstrating a lack of the rotational damage pattern associated with tornadoes. Straight-line winds are common with the gust front of a thunderstorm or originate with a downburst from a thunderstorm; these events can cause considerable damage in the absence of a tornado. The winds can gust to 210 km/h and winds of 93 km/h or more can last for more than twenty minutes. In the United States, such straight-line wind events are most common during the spring when instability is highest and weather fronts cross the country. Straight-line wind events in the form of derechos can take place throughout the eastern half of the U. S. Straight-line winds may be damaging to marine interests.
Small ships and sailboats are at risk from this meteorological phenomenon. The formation of a downburst starts with hail or large raindrops falling through drier air. Hailstones melt and raindrops evaporate, pulling latent heat from surrounding air and cooling it considerably. Cooler air has a higher density than the warmer air around it, so it sinks to the ground; as the cold air hits the ground it spreads out and a mesoscale front can be observed as a gust front. Areas under and adjacent to the downburst are the areas which receive the highest winds and rainfall, if any is present; because the rain-cooled air is descending from the middle troposphere, a significant drop in temperatures is noticed. Due to interaction with the ground, the downburst loses strength as it fans out and forms the distinctive "curl shape", seen at the periphery of the microburst. Downbursts last only a few minutes and dissipate, except in the case of squall lines and derecho events. However, despite their short lifespan, microbursts are a serious hazard to aviation and property and can result in substantial damage to the area.
A special, much rarer, kind of downburst is a heat burst, which results from precipitation-evaporated air compressionally heating as it descends from high altitude on the backside of a dying squall line or outflow boundary. Heat bursts are chiefly a nocturnal occurrence, can produce winds over 160 km/h, are characterized by exceptionally dry air, can raise the surface temperature to 38 °C or more, sometimes persist for several hours. Downbursts microbursts, are exceedingly dangerous to aircraft which are taking off or landing due to the strong vertical wind shear caused by these events. A number of fatal crashes have been attributed to downbursts. Bow echo Convective storm detection Haboob Line echo wave pattern List of derecho events List of microbursts Mesovortex National Weather Service. "Downbursts". National Weather Service Forecast Office Columbia, SC. 5 May 2010. 4 December 2010. Http://www.erh.noaa.gov/cae/svrwx/downburst.htm Fujita, T. T.. "Tornadoes and Downbursts in the Context of Generalized Planetary Scales".
Journal of the Atmospheric Sciences, 38. Fujita, T. T.. "The Downburst and macroburst". SMRP Research Paper 210, 122 pp. Wilson, James W. and Roger M. Wak
Sublimation (phase transition)
Sublimation is the transition of a substance directly from the solid to the gas phase, without passing through the intermediate liquid phase. Sublimation is an endothermic process that occurs at temperatures and pressures below a substance's triple point in its phase diagram, which corresponds to the lowest pressure at which the substance can exist as a liquid; the reverse process of sublimation is deposition or desublimation, in which a substance passes directly from a gas to a solid phase. Sublimation has been used as a generic term to describe a solid-to-gas transition followed by a gas-to-solid transition. While a transition from liquid to gas is described as evaporation if it occurs below the boiling point of the liquid, as boiling if it occurs at the boiling point, there is no such distinction within the solid-to-gas transition, always described as sublimation. 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 partial pressure of the substance, not the total pressure of the entire system. So, all solids that possess an appreciable vapour pressure at a certain temperature can sublime in air. For some substances, such as carbon and arsenic, sublimation is much easier than evaporation from the melt, because the pressure of their triple point is high, it is difficult to obtain them as liquids; the term sublimation refers to a physical change of state and is not used to describe the transformation of a solid to a gas in a chemical reaction. For example, the dissociation on heating of solid ammonium chloride into hydrogen chloride and ammonia is not sublimation but a chemical reaction; the combustion of candles, containing paraffin wax, to carbon dioxide and water vapor is not sublimation but a chemical reaction with oxygen. Sublimation is caused by the absorption of heat which provides enough energy for some molecules to overcome the attractive forces of their neighbors and escape into the vapor phase.
Since the process requires additional energy, it is an endothermic change. The enthalpy of sublimation can be calculated by adding the enthalpy of fusion and the enthalpy of vaporization. Solid carbon dioxide sublimes everywhere along the line below the triple point (e.g. at the temperature of −78.5 °C at atmospheric pressure, 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 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 cold spell is caused by sunshine acting directly on the upper layers of the snow. Ablation is a process that includes erosive wear of glacier ice. Naphthalene, an organic compound found in pesticides such as mothballs, sublimes because it is made of non-polar molecules that are held together only by van der Waals intermolecular forces.
Naphthalene is a solid that sublimes at standard atmospheric temperature with the sublimation point at around 80 °C or 176 °F. At low temperature, its vapour pressure is high enough, 1 mmHg at 53 °C, to make the solid form of naphthalene evaporate into gas. On cool surfaces, the naphthalene vapours will solidify to form needle-like crystals. Iodine produces fumes on gentle heating, it is possible to obtain liquid iodine at atmospheric pressure by controlling the temperature at just above the melting point of iodine. In forensic science, iodine vapor can reveal latent fingerprints on paper. Arsenic can sublime at high temperatures. Cadmium and zinc are not suitable materials for use in vacuum because they sublimate much more than other common materials. Sublimation is a technique used by chemists to purify compounds. A solid is placed in a sublimation apparatus and heated under vacuum. Under this reduced pressure, the solid volatilizes and condenses as a purified compound on a cooled surface, leaving a non-volatile residue of impurities behind.
Once heating ceases and the vacuum is removed, the purified compound may be collected from the cooling surface. For higher purification efficiencies, a temperature gradient is applied, which allows for the separation of different fractions. Typical setups use an evacuated glass tube, heated in a controlled manner; the material flow is from the hot end, where the initial material is placed, to the cold end, connected to a pump stand. By controlling temperatures along the length of the tube, the operator can control the zones of re-condensation, with volatile compounds being pumped out of the system moderately volatile compounds re-condensing along the tube according to their different volatilities, non-volatile compounds remaining in the hot end. Vacuum sublimation of this type is the method of choice for purification of organic compounds for use in the organic electronics industry, where high purities are needed to satisfy the standards for consumer electronics and other applications. In ancient alchemy, a protoscience that contributed to the development of modern chemistry and medicine, alchemists developed a structure of basic laboratory techniques, theory and experimental methods.
Sublimation was used to refer to the process in which a
Aviation, or air transport, refers to the activities surrounding mechanical flight and the aircraft industry. Aircraft includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as balloons and airships. Aviation began in the 18th century with the development of the hot air balloon, an apparatus capable of atmospheric displacement through buoyancy; some of the most significant advancements in aviation technology came with the controlled gliding flying of Otto Lilienthal in 1896. Since that time, aviation has been technologically revolutionized by the introduction of the jet which permitted a major form of transport throughout the world; the word aviation was coined by the French writer and former naval officer Gabriel La Landelle in 1863. He derived the term from the verb avier, itself derived from the Latin word avis and the suffix -ation. There are early legends of human flight such as the stories of Icarus in Greek myth and Jamshid and Shah Kay Kāvus in Persian myth.
Somewhat more credible claims of short-distance human flights appear, such as the flying automaton of Archytas of Tarentum, the winged flights of Abbas ibn Firnas, Eilmer of Malmesbury, the hot-air Passarola of Bartholomeu Lourenço de Gusmão. The modern age of aviation began with the first untethered human lighter-than-air flight on November 21, 1783, of a hot air balloon designed by the Montgolfier brothers; the practicality of balloons was limited. It was recognized that a steerable, or dirigible, balloon was required. Jean-Pierre Blanchard flew the first human-powered dirigible in 1784 and crossed the English Channel in one in 1785. Rigid airships became the first aircraft to transport passengers and cargo over great distances; the best known aircraft of this type were manufactured by the German Zeppelin company. The most successful Zeppelin was the Graf Zeppelin, it flew over one million miles, including an around-the-world flight in August 1929. However, the dominance of the Zeppelins over the airplanes of that period, which had a range of only a few hundred miles, was diminishing as airplane design advanced.
The "Golden Age" of the airships ended on May 6, 1937 when the Hindenburg caught fire, killing 36 people. The cause of the Hindenburg accident was blamed on the use of hydrogen instead of helium as the lift gas. An internal investigation by the manufacturer revealed that the coating used in the material covering the frame was flammable and allowed static electricity to build up in the airship. Changes to the coating formulation reduced the risk of further Hindenburg type accidents. Although there have been periodic initiatives to revive their use, airships have seen only niche application since that time. In 1799, Sir George Cayley set forth the concept of the modern airplane as a fixed-wing flying machine with separate systems for lift and control. Early dirigible developments included machine-powered propulsion, rigid frames and improved speed and maneuverability There are many competing claims for the earliest powered, heavier-than-air flight; the first recorded powered flight was carried out by Clément Ader on October 9, 1890 in his bat-winged self-propelled fixed-wing aircraft, the Ader Éole.
It was the first manned, heavier-than-air flight of a significant distance but insignificant altitude from level ground. Seven years on 14 October 1897, Ader's Avion III was tested without success in front of two officials from the French War ministry; the report on the trials was not publicized until 1910. In November 1906 Ader claimed to have made a successful flight on 14 October 1897, achieving an "uninterrupted flight" of around 300 metres. Although believed at the time, these claims were discredited; the Wright brothers made the first successful powered and sustained airplane flight on December 17, 1903, a feat made possible by their invention of three-axis control. Only a decade at the start of World War I, heavier-than-air powered aircraft had become practical for reconnaissance, artillery spotting, attacks against ground positions. Aircraft began to transport people and cargo as designs grew more reliable; the Wright brothers took aloft the first passenger, Charles Furnas, one of their mechanics, on May 14, 1908.
During the 1920s and 1930s great progress was made in the field of aviation, including the first transatlantic flight of Alcock and Brown in 1919, Charles Lindbergh's solo transatlantic flight in 1927, Charles Kingsford Smith's transpacific flight the following year. One of the most successful designs of this period was the Douglas DC-3, which became the first airliner to be profitable carrying passengers starting the modern era of passenger airline service. By the beginning of World War II, many towns and cities had built airports, there were numerous qualified pilots available; the war brought many innovations to aviation, including the first jet aircraft and the first liquid-fueled rockets. After World War II in North America, there was a boom in general aviation, both private and commercial, as thousands of pilots were released from military service and many inexpensive war-surplus transport and training aircraft became available. Manufacturers such as Cessna and Beechcraft expanded production to provide light aircraft for the new middle-class market.
A light pillar is an atmospheric optical phenomenon in which a vertical beam of light appears to extend above and/or below a light source. The effect is created by the reflection of light from tiny ice crystals that are suspended in the atmosphere or that comprise high-altitude clouds. If the light comes from the Sun, the phenomenon is called solar pillar. Light pillars can be caused by the Moon or terrestrial sources, such as streetlights. Since they are caused by the interaction of light with ice crystals, light pillars belong to the family of halos; the crystals responsible for light pillars consist of flat, hexagonal plates, which tend to orient themselves more or less horizontally as they fall through the air. Each flake acts as a tiny mirror which reflects light sources which are directly above or below it, the presence of flakes at a spread of altitudes causes the reflection to be elongated vertically into a column; the larger and more numerous the crystals, the more pronounced. More column-shaped crystals can cause light pillars as well.
In cold weather, the ice crystals can be suspended near the ground, in which case they are referred to as diamond dust. Unlike a light beam, a light pillar is not physically located below the light source, its appearance of a vertical line is an optical illusion, resulting from the collective reflection off the ice crystals, only those of which that appear to lie in a vertical line direct the light rays towards the observer. Crepuscular rays Diamond dust Halo Light beam Sun dog Crown flash False sunrise False sunset Pillars. Atmospheric Optics. Explanations and many images. Light Pillars: An Introduction to Sun Pillars and Related Phenomena; the Weather Doctor's Weather Eyes. Another nice explanation, all on one page Fabulous frozen frames – Sydney Morning Herald. November 1, 2006 A Sun Pillar Over North Carolina. NASA Astronomy Picture of the Day, 15 December 2008
A nimbostratus cloud or nimbostratus is a low, gray dark, nearly uniform cloud that produces continuous rain, snow, or sleet and no lightning or thunder. Although it is a low-based cloud, it forms most in the middle level of the troposphere and spreads vertically into the low and high levels. Nimbostratus produces precipitation over a wide area. Nimbo - is from the Latin word nimbus. Downward-growing nimbostratus can have the same vertical extent as most large upward-growing cumulus, but its horizontal extent tends to be greater. Nimbostratus has a diffuse cloud base found anywhere from near surface in the low levels to about 3,000 m in the middle level of the troposphere. Although dark at its base, it appears illuminated from within to a surface observer. Nimbostratus has a thickness of about 2000 to 4000 m. Though found worldwide, nimbostratus occurs more in the middle latitudes, it is coded CM2 on the SYNOP report. Nimbostratus occurs along a warm front or occluded front where the rising warm air mass creates nimbostratus along with shallower stratus clouds producing less rain, these clouds being preceded by higher-level clouds such as cirrostratus and altostratus.
When an altostratus cloud thickens and descends into lower altitudes, it will become nimbostratus. Nimbostratus, unlike cumulonimbus, is not associated with thunderstorms, however at an unusually unstable warm front caused as a result of the advancing warm air being hot and unstable, cumulonimbus clouds may be embedded within the usual nimbostratus. Lightning from an embedded cumulonimbus cloud may interact with the nimbostratus but only in the immediate area around it. In this situation with lightning and rain occurring it would be hard to tell which type of cloud was producing the rain from the ground, however cumulonimbus tend to produce larger droplets and more intense downpours; the occurrence of cumulonimbus and nimbostratus together is uncommon, only nimbostratus is found at a warm front. Nimbostratus is a sign of an approaching warm or occluded front producing steady moderate precipitation, as opposed to the shorter period of heavier precipitation released by a cold-frontal cumulonimbus cloud.
Precipitation may last depending on the speed of the frontal system. A nimbostratus virga cloud is the same as normal nimbostratus, but the rain or snow falls as virga which doesn't reach the ground. Stratus or stratocumulus replace the nimbostratus after the passage of the warm or occluded front. Under Luke Howard's first systematized study of clouds, carried out in France in 1802, three general cloud forms were established based on appearance and characteristics of formation: cirriform and stratiform; these were further divided into lower types depending on altitude. In addition to these three main types, Howard added two names to designate multiple cloud types joined together: cumulostratus, a blending of cumulus clouds and stratus layers, nimbus, a complex blending of cirriform and stratiform clouds with sufficient vertical development to produce significant precipitation. In the 20th century, an IMC commission for the study of clouds put forward a refined and more restricted definition of the genus nimbus reclassifying it as a stratiform cloud type.
It was renamed nimbostratus, published with the new name in the 1932 edition of the International Atlas of Clouds and of States of the Sky. This left cumulonimbus as the only nimbiform type. Species and varieties: Nimbostratus is thick and featureless, so this genus type is not subdivided into species or varieties. Precipitation-based supplementary features: Nimbostratus is a major precipitation cloud and produces the virga or praecipitatio features; the latter can achieve heavy intensity due to the cloud's vertical depth. Accessory cloud: Nimbostratus pannus is an accessory cloud of nimbostratus that forms as a ragged layer in precipitation below the main cloud deck. Pannus is coded CL7. Genitus mother clouds: This genus type can form from cumulus and cumulonimbus. Mutatus mother clouds: Nimbostratus can form due to the complete transformation of altocumulus and stratocumulus. Multi-level nimbostratus is physically related to other stratiform genus-types by way of being non-convective in nature.
However, the other sheet-like clouds each occupy only one or two levels at the same time. Stratus clouds form from near ground level to 2,000 metres at all latitudes. In the middle level are the altostratus clouds that form from 2,000 metres to 7,000 metres in polar areas, 7,000 metres in temperate areas, 7,600 metres in tropical areas. Although altostratus forms in the middle level of the troposphere, strong frontal lift can push it into the lower part of the high-level; the main high-level stratiform cloud is cirrostratus, composed of ice crystals that produce halo effects around the sun. Cirrostratus forms at altitudes of 3,000 to 7,600 metres in high latitudes, 5,000 to 12,000 metres in temperate latitudes, 6,100 to 18,000 metres in low, tropical latitudes. Of the non-stratiform clouds and cumulus congestus are the most related to nimbostratus because of their vertical extent and ability to produce moderate to heavy precipitation; the remaining cumuliform and stratocumuliform clouds have the least in common with nimbostratus.
National Science Digital Library - Nimbostratus Nim
Sulfuric acid known as vitriol, is a mineral acid composed of the elements sulfur and hydrogen, with molecular formula H2SO4. It is a colorless and syrupy liquid, soluble in water, in a reaction, exothermic, its corrosiveness can be ascribed to its strong acidic nature, and, if at a high concentration, its dehydrating and oxidizing properties. It is hygroscopic absorbing water vapor from the air. Upon contact, sulfuric acid can cause severe chemical burns and secondary thermal burns. Sulfuric acid is a important commodity chemical, a nation's sulfuric acid production is a good indicator of its industrial strength, it is produced with different methods, such as contact process, wet sulfuric acid process, lead chamber process and some other methods. Sulfuric acid is a key substance in the chemical industry, it is most used in fertilizer manufacture, but is important in mineral processing, oil refining, wastewater processing, chemical synthesis. It has a wide range of end applications including in domestic acidic drain cleaners, as an electrolyte in lead-acid batteries, in various cleaning agents.
Although nearly 100% sulfuric acid can be made, the subsequent loss of SO3 at the boiling point brings the concentration to 98.3% acid. The 98.3% grade is more stable in storage, is the usual form of what is described as "concentrated sulfuric acid". Other concentrations are used for different purposes; some common concentrations are: "Chamber acid" and "tower acid" were the two concentrations of sulfuric acid produced by the lead chamber process, chamber acid being the acid produced in the lead chamber itself and tower acid being the acid recovered from the bottom of the Glover tower. They are now obsolete as commercial concentrations of sulfuric acid, although they may be prepared in the laboratory from concentrated sulfuric acid if needed. In particular, "10M" sulfuric acid is prepared by adding 98% sulfuric acid to an equal volume of water, with good stirring: the temperature of the mixture can rise to 80 °C or higher. Sulfuric acid reacts with its anhydride, SO3, to form H2S2O7, called pyrosulfuric acid, fuming sulfuric acid, Disulfuric acid or oleum or, less Nordhausen acid.
Concentrations of oleum are either expressed in terms of % SO3 or as % H2SO4. Pure H2S2O7 is a solid with melting point of 36 °C. Pure sulfuric acid has a vapor pressure of <0.001 mmHg at 25 °C and 1 mmHg at 145.8 °C, 98% sulfuric acid has a <1 mmHg vapor pressure at 40 °C. Pure sulfuric acid is a viscous clear liquid, like oil, this explains the old name of the acid. Commercial sulfuric acid is sold in several different purity grades. Technical grade H2SO4 is impure and colored, but is suitable for making fertilizer. Pure grades, such as United States Pharmacopeia grade, are used for making pharmaceuticals and dyestuffs. Analytical grades are available. Nine hydrates are known, but four of them were confirmed to be tetrahydrate and octahydrate. Anhydrous H2SO4 is a polar liquid, having a dielectric constant of around 100, it has a high electrical conductivity, caused by dissociation through protonating itself, a process known as autoprotolysis. 2 H2SO4 ⇌ H3SO+4 + HSO−4The equilibrium constant for the autoprotolysis is Kap = = 2.7×10−4The comparable equilibrium constant for water, Kw is 10−14, a factor of 1010 smaller.
In spite of the viscosity of the acid, the effective conductivities of the H3SO+4 and HSO−4 ions are high due to an intramolecular proton-switch mechanism, making sulfuric acid a good conductor of electricity. It is an excellent solvent for many reactions; because the hydration reaction of sulfuric acid is exothermic, dilution should always be performed by adding the acid to the water rather than the water to the acid. Because the reaction is in an equilibrium that favors the rapid protonation of water, addition of acid to the water ensures that the acid is the limiting reagent; this reaction is best thought of as the formation of hydronium ions: H2SO4 + H2O → H3O+ + HSO−4 Ka1 = 2.4×106 HSO−4 + H2O → H3O+ + SO2−4 Ka2 = 1.0×10−2 HSO−4 is the bisulfate anion and SO2−4 is the sulfate anion. Ka1 and Ka2 are the acid dissociation constants; because the hydration of sulfuric acid is thermodynamically favorable and the affinity of it for water is sufficiently strong, sulfuric acid is an excellent dehydrating agent.
Concentrated sulfuric acid has a powerful dehydrating property, removing water from other chemical compounds including sugar and other carbohydrates and producing carbon and steam. In the laboratory, this is demonstrated by mixing table sugar into sulfuric acid; the sugar changes from white to dark brown and to black as carbon is formed. A rigid column of black, porous carbon will emerge as well; the carbon will smell of caramel due to the heat generated. C 12 H 22 O 11 ⏞ sucrose → H 2 SO 4 12 C + 11 H 2