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Hydrazine

Hydrazine

Properties
General
The structure formula of Hydrazine.
Name	Hydrazine
Chemical formula	N₂H₄
Appearance	Colourless liquid
Physical
Formula weight	32.0 amu
Melting point	274 K (1 °C)
Boiling point	387 K (114 °C)
Density	1.01g/ml
Solubility	very soluble
Thermochemistry
Δ_fH⁰_gas	95.35 kJ/mol
Δ_fH⁰_liquid	50.63 kJ/mol
Δ_fH⁰_solid	37.63 kJ/mol
S⁰_{gas, 1 bar}	238.66 J/mol·K
S⁰_{liquid, 1 bar}	121.52 J/mol·K
S⁰_solid	? J/mol·K
Safety
Ingestion	Extremely Toxic, possibly carcinogenic
Inhalation	Very dangerous—extremely destructive to the upper respiratory tract
Skin	Can cause severe burns, can be absorbed into bloodstream
Eyes	Can cause permanent damage
More info	[http://www.hhmi.org/research/labsafe/lcss/lcss46.html Hazardous Chemical Database]
LD₅₀	as low as 25mg/kg
SI units were used where possible. Unless otherwise stated, standard conditions were used. Disclaimer and references

Hydrazine is a chemical compound with formula N₂H₄ used as a rocket fuel. Hydrazine is a liquid with weak basic properties similar to ammonia. Due to the alpha effect the nucleophilicity is much stronger than that of ammonia, which makes it more reactive. It can be made by oxidizing ammonia with sodium hypochlorite (the Raschig process). It is a monopropellant rocket fuel. Hydrazine derivatives 1,1-dimethylhydrazine and 1,2-dimethylhydrazine, in which two of the hydrogen atoms are substituted with methyl groups, are also described as hydrazines. 1,1-Dimethylhydrazine is used to make hypergolic (self-igniting) bipropellant rocket fuels.

Health effects

Breathing hydrazines may cause coughing and irritation of the throat and lungs, tremors, or seizures. Breathing hydrazines for long periods may cause liver and kidney damage, as well as serious effects on reproductive organs. Eating or drinking small amounts of hydrazines may cause nausea, vomiting, uncontrolled shaking, inflammation of the nerves, drowsiness, or coma. Hydrazine is found in chewing tobacco and cigarettes. Tumors have been seen in many organs of animals that were exposed to hydrazines by ingestion or breathing, but most tumors have been found in the lungs, blood vessels, or colon. 1,2-Dimethylhydrazine has caused colon cancer in laboratory animals following a single exposure. The Department of Health and Human Services (DHHS) has determined that hydrazine and 1,1-dimethylhydrazine are known carcinogens. The International Agency for Research on Cancer (IARC) has determined that hydrazine, 1,1-dimethylhydrazine, and 1,2-dimethylhydrazine are possible human carcinogens. The Environmental Protection Agency (EPA) has determined that hydrazine, 1,1-dimethylhydrazine, and 1,2-dimethylhydrazine are probable human carcinogens. The American Conference of Governmental Industrial Hygienists (ACGIH) currently lists hydrazine and 1,1-dimethylhydrazine as suspected carcinogens, but has recently recommended that the listing of hydrazine be changed to that of animal carcinogen, not likely to cause cancer to people under normal exposure conditions. The False Morel contains the chemical gyromitrin, which is metabolized into monomethyl hydrazine inside the body. Consequently, the toxic effects of this mushroom are the same as with hydrazine poisoning.

Use

Hydrazine is used primarily as a chemical intermediate to produce agricultural chemicals, spandex fibers, and antioxidants. Hydrazine is also used as rocket fuel, an oxygen scavenger (corrosion inhibitor) in water boilers and heating systems, a polymerization catalyst, a blowing agent, and as a scavenger for gases. Additionally, it is used for plating metals on glass and plastics and in fuel cells, solder fluxes, and photographic developers. Hydrazine is used as a reactant in fuel cells in the military, as a reducing agent in electrodeless nickel plating, as a chain extender in urethane polymerizations, as a reducing agent in plutonium extraction from reactor waste, and as a water treatment chemical. Hydrazine is also used as a chemical intermediate for blowing agents, photography chemicals, pharmaceuticals, antituberculants, textile dyes, heat stabilizers, explosives, and to make hydrazine sulfate. In addition, it has recently been determined that hydrazine increases the speed of the thin-film transistors used in liquid crystal displays, a discovery that promises to revolutionize the manufacture of LCD computer monitors. Hydrazine in a 70% solution is used to power the EPU (Emergency Power Unit) on the F-16 fighter plane. Hydrazine is also used as low-power propellant for Space Shuttle maneuvers in orbit, as hydrazine can be decomposed from a liquid into gaseous components despite the absence of oxygen, allowing the high pressure gaseous products to be expanded out of a nozzle. Category:Bases Category:Rocket fuels Category:Nitrogen compounds ja:ヒドラジン

Nitrogen

Nitrogen is the chemical element in the periodic table that has the symbol N and atomic number 7. Commonly a colorless, odorless, tasteless and mostly inert diatomic non-metal gas, nitrogen constitutes 78 percent of Earth's atmosphere and is a constituent of all living tissues. Nitrogen forms many important compounds such as amino acids, ammonia, nitric acid, and cyanides.

Notable characteristics

Nitrogen is a non-metal, with an electronegativity of 3.0. It has five electrons in its outer shell, so is trivalent in most compounds. Pure nitrogen is an unreactive colorless diatomic gas at room temperature, and comprises about 78.08% of the Earth's atmosphere. It condenses at 77 K at atmospheric pressure and freezes at 63 K. Liquid nitrogen is a common cryogen.

Applications

Nitrogen Compounds

Molecular nitrogen in the atmosphere is relatively non-reactive, but in nature it is slowly converted into biologically (and industrially) useful compounds by some living organisms, notably certain bacteria (see Biological role below). The ability to combine or fix nitrogen is a key feature of modern industrial chemistry, where nitrogen (along with natural gas) is converted into ammonia (via the Haber process). Ammonia, in turn, can be used directly (primarily as a fertilizer), or as a precursor of many other important materials including explosives, largely via the production of nitric acid by the Ostwald process. The salts of nitric acid include important compounds like potassium nitrate (or saltpeter, important historically for its use in gunpowder) and ammonium nitrate, an important fertilizer. Various other nitrated organic compounds, such as nitroglycerin and trinitrotoluene, are used as explosives. Nitric acid is used as an oxidizer in liquid fueled rockets. Hydrazine and hydrazine derivatives find use as rocket fuels.

Molecular nitrogen (gas and liquid)

Nitrogen gas is readily produced by allowing liquid nitrogen (see below) to warm up and evaporate. It has a wide variety of applications, including serving as a more inert replacement for air where oxidation is undesirable;
- to preserve the freshness of packaged or bulk foods (by delaying rancidity and other forms of oxidative damage)
- on top of liquid explosives for safety It is also used in:
- the production of electronic parts such as transistors, diodes, and integrated circuits
- the manufacture of stainless steel
- filling automotive tires due to its inertness and lack of moisture or oxidative qualities, as opposed to air. A further example of its versitility is its use (as a preferred alternative to carbon dioxide) to pressurize kegs of some beers, particularly thicker stouts and Scottish and English ales, due to the smaller bubbles it produces, which make the dispensed beer smoother and headier. A modern application of a pressure sensitive nitrogen capsule known commonly as a "widget" now allows nitrogen charged beers to be packaged in cans and bottles. A very popular example of this is Guinness Draught. Liquid nitrogen is produced industrially in large quantities by distillation from liquid air and is often referred to by the quasi-formula LN₂. It is a cryogenic (extremely cold) fluid which can cause instant frostbite on direct contact with living tissue. When appropriately insulated from ambient heat it serves as a compact and readily transported source of nitrogen gas without pressurization. Further, its ability to maintain an unearthly temperature as it evaporates (77 K, -196 °C or -320 °F) makes it extremely useful in a wide range of applications as an open-cycle refrigerant, including;
- the immersion freezing and transportation of food products
- the preservation of bodies, reproductive cells (sperm and egg), and biological samples and materials
- in the study of cryogenics
- for demonstrations in science education
- in dermatology for removing unsightly or potentially malignant skin lesions,e.g., warts, actinic keratosis, etc.

History

Nitrogen (Latin nitrum, Greek Nitron meaning "native soda", "genes", "forming") is formally considered to have been discovered by Daniel Rutherford in 1772, who called it noxious air or fixed air. That there was a fraction of air that did not support combustion was well known to the late 18th century chemist. Nitrogen was also studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley, who referred to it as burnt air or phlogisticated air. Nitrogen gas was inert enough that Antoine Lavoisier referred to it as azote, which stands for without life; this term has become the French word for "nitrogen" and later spread out to many other languages. Compounds of nitrogen were known in the Middle Ages. The alchemists knew nitric acid as aqua fortis. The mixture of nitric and hydrochloric acids was known as aqua regia, celebrated for its ability to dissolve gold. The earliest industrial and agricultural applications of nitrogen compounds used it in the form of saltpeter (sodium- or potassium nitrate), notably in gunpowder, and much later, as fertilizer, and later still, as a chemical feedstock.

Occurrence

Nitrogen is the largest single component of the Earth's atmosphere (78.084% by volume, 75.5% by weight) and is acquired for industrial purposes by the fractional distillation of liquid air or by mechanical means of gaseous air (i.e. pressurised reverse osmosis membrane or PSA (Pressure Swing Adsorption). Compounds that contain this element have been observed in outer space. Nitrogen-14 is created as part of the fusion processes in stars. Nitrogen is a large component of animal waste (for example, guano), usually in the form of urea, uric acid, and compounds of these nitrogenous products. Molecular nitrogen has been known to occur in Titan's atmosphere for some time, and has now been detected in interstellar space by David Knauth and coworkers using the Far Ultraviolet Spectroscopic Explorer.

Compounds

The main hydride of nitrogen is ammonia (NH₃) although hydrazine (N₂H₄) is also well known. Ammonia is somewhat more basic than water, and in solution forms ammonium ions (NH₄⁺). Liquid ammonia is in fact slightly amphiprotic and forms ammonium and amide ions (NH₂^-); both amides and nitride (N^3-) salts are known, but decompose in water. Singly and doubly substituted compounds of ammonia are called amines. Larger chains, rings and structures of nitrogen hydrides are also known but virtually unstable. Other classes of nitrogen anions are azides (N₃^-), which are linear and isoelectronic to carbon dioxide. Another molecule of the same structure is dinitrogen monoxide (N₂O), or laughing gas. This is one of a variety of oxides, the most prominent of which are nitrogen monoxide (NO) and nitrogen dioxide (NO₂), which both contain an unpaired electron. The latter shows some tendency to dimerize and is an important component of smog. The more standard oxides, dinitrogen trioxide (N₂O₃) and dinitrogen pentoxide (N₂O₅), are actually fairly unstable and explosive. The corresponding acids are nitrous (HNO₂) and nitric acid (HNO₃), with the corresponding salts called nitrites and nitrates. Nitric acid is one of the few acids stronger than hydronium.

Biological role

Nitrogen is an essential part of amino and nucleic acids which makes nitrogen vital to all life. Legumes like the soybean plant, can recover nitrogen directly from the atmosphere because their roots have nodules harboring microbes that do the actual conversion to ammonia in a process known as nitrogen fixation. The legume subsequently converts ammonia to nitrogen oxides and amino acids to form proteins.

Isotopes

There are two stable isotopes: N-14 and N-15. By far the most common is N-14 (99.634%), which is produced in the CNO cycle in stars. The rest is N-15. Of the ten isotopes produced synthetically, one has a half life of nine minutes and the remaining isotopes have half lives on the order of seconds or less. Biologically-mediated reactions (e.g., assimilation, nitrification, and denitrification) strongly control nitrogen dynamics in the soil. These reactions almost always result in N-15 enrichment of the substrate and depletion of the product. Although precipitation often contains subequal quantities of ammonium and nitrate, because ammonium is preferentially retained by the canopy relative to atmospheric nitrate, most of the atmospheric nitrogen that reaches the soil surface is in the form of nitrate. Soil nitrate is preferentially assimilated by tree roots relative to soil ammonium.

Precautions

Nitrate fertilizer washoff is a major source of ground water and river pollution. Cyano (-CN) containing compounds form extremely poisonous salts and are deadly to many animals and all mammals.

References

- [http://periodic.lanl.gov/elements/7.html Los Alamos National Laboratory – Nitrogen]

External links

- [http://www.webelements.com/webelements/elements/text/N/index.html WebElements.com – Nitrogen]
- [http://education.jlab.org/itselemental/ele007.html It's Elemental – Nitrogen]
- [http://www.sunysccc.edu/academic/mst/ptable/n.html Schenectady County Community College – Nitrogen]
- [http://www.uigi.com/nitrogen.html Nitrogen N2 Properties, Uses, Applications]
- [http://box27.bluehost.com/~edsanvil/wiki/index.php?title=Nitrogen_gas Computational Chemistry Wiki] Category:Nonmetals Category:Pnictogens Category:Nitrogen metabolism ko:질소 ja:窒素 simple:Nitrogen th:ไนโตรเจน

Color

Color or colour is the perception of the frequency (or wavelength) of light, and can be compared to how pitch (or a musical note) is the perception of the frequency or wavelength of sound. It is a perception which in humans derives from the ability of the fine structures of the eye to distinguish (usually three) differently filtered analyses of a view. The perception of color is influenced by biology (some people are born seeing colors differently or not at all; see color blindness), long-term history of the observer, and also by short-term effects such as the colors nearby. (This is the basis of many optical illusions.) The science of color is sometimes called chromatics. It includes the perception of color by the human eye, the origin of color in materials, color theory in art, and the physics of color in the electromagnetic spectrum.

Physics of color

The colors of the visible light spectrum.

color	wavelength interval	frequency interval
red	~ 625-740 nm	~ 480-405 THz
orange	~ 590-625 nm	~ 510-480 THz
yellow	~ 565-590 nm	~ 530-510 THz
green	~ 500-565 nm	~ 600-530 THz
cyan	~ 485-500 nm	~ 620-600 THz
blue	~ 440-485 nm	~ 680-620 THz
violet	~ 380-440 nm	~ 790-680 THz
Continuous optical spectrum Image:Spectrum441pxWithnm.png Designed for monitors with gamma 1.5.
Computer "spectrum" Image:Computerspectrum.png The bars below show the relative intensities of the three colors mixed to make the color immediately above.

Color, frequency, and energy of light.

Color	$\lambda \,\!$ /nm	$\nu \,\!$ /10¹⁴ Hz	$\nu_b \,\!$ /10⁴ cm^-1	$E \,\!$ /eV	$E \,\!$ /kJ mol^-1
Infrared	>1000	<3.00	<1.00	<1.24	<120
Red	700	4.28	1.43	1.77	171
Orange	620	4.84	1.61	2.00	193
Yellow	580	5.17	1.72	2.14	206
Green	530	5.66	1.89	2.34	226
Blue	470	6.38	2.13	2.64	254
Violet	420	7.14	2.38	2.95	285
Near ultraviolet	300	10.0	3.33	4.15	400
Far ultraviolet	<200	>15.0	>5.00	>6.20	>598

Electromagnetic radiation is a mixture of radiation of different wavelengths and intensities. When this radiation has a wavelength inside the human visibility range (approximately from 380 nm to 740 nm), it is known as light within the (human) visible spectrum. The light's spectrum records each wavelength's intensity. The full spectrum of the incoming radiation from an object determines the visual appearance of that object, including its perceived color. As we will see, there are many more spectra than color sensations; in fact one may formally define a color to be the whole class of spectra which give rise to the same color sensation, although any such definition would vary widely among different species and also somewhat among individuals intraspecifically. A surface that diffusely reflects all wavelengths equally is perceived as white, while a dull black surface absorbs all wavelengths and does not reflect (for mirror reflection this is different: a proper mirror also reflects all wavelengths equally, but is not perceived as white, while shiny black objects do reflect). The familiar colors of the rainbow in the spectrum—named from the Latin word for appearance or apparition by Isaac Newton in 1671—contains all those colors that consist of visible light of a single wavelength only, the pure spectral or monochromatic colors. The frequencies are approximations and given in terahertz (THz). The wavelengths, valid in vacuum, are given in nanometers (nm). A list of other objects of similar size is available.

Important note

The color table should not be interpreted as a definite list – the pure spectral colors form a continuous spectrum, and how it is divided into distinct colors is a matter of taste and culture. Similarly, the intensity of a spectral color may alter its perception considerably; for example, a low-intensity orange-yellow is brown, and a low-intensity yellow-green is olive-green.

Spectral versus non-spectral colors

Most light sources are not pure spectral sources; rather they are created from mixtures of various wavelengths and intensities of light. To the human eye, however, there is a wide class of mixed-spectrum light that is perceived the same as a pure spectral color. In the table above, for instance, when your computer screen is displaying the "orange" patch, it is not emitting pure light at a fixed wavelength of around 600 nm (which is something most computer screens are unable to do). Rather, it is emitting a mixture of about two parts red to one part green light. Were you to print this page on a color printer, the orange patch on the paper, when lit with white light, would reflect yet another, more continuous spectrum. We cannot see those differences (although many animals can), and the reason has to do with the pigments that make up our color vision cells (see below). A useful quantification of this property is the dominant wavelength, which matches a wavelength of spectral light to a non-spectral source that evokes the same color perception. Dominant wavelength is the formal background for the popular concept of hue. In addition to the many light sources that can appear to be pure spectral colors but are actually mixtures, there are many color perceptions that by definition cannot be pure spectral colors due to desaturation or because they are purples (which are a mixture of red and violet light, from either end of the spectrum). Some examples of necessarily non-spectral colors are the achromatic colors (black, gray and white) and other colors such as pink, tan and magenta. See metamerism (color) for a basic introduction as to why color matching challenges exist.

Physical basis of color

A light wave can be analyzed as a superposition of sine waves, each of which has a specific frequency and wavelength. The eye gives limited information about the relative intensities of these sine waves (but not their phases — the eye is even more blind to phase than the ear, which can detect phase relationships of sounds only in certain very specific contexts). To understand which particular color perception will arise from a particular physical spectrum requires knowledge of the physiology of the retina. The human eye is also insensitive to polarization in most cases (though see Haidinger's brush), whereas some fish and mollusks can perceive it.

Color vision

Though the exact status of color is a matter of current philosophical dispute, color is arguably a psychophysical phenomenon that exists only in our minds. (See Qualia, for some of that dispute.) A "red" apple does not give off "red light", and it is misleading to think of things that we see, or of light itself, as objectively colored at all. Rather, the apple simply absorbs light of various wavelengths shining on it to different degrees, in such a way that the unabsorbed light which it reflects is perceived as red. An apple is perceived to be red only because normal human color vision perceives light with different mixes of wavelengths differently—and we have language to describe that difference. language In 1931, an international group of experts called the Commission Internationale d'Eclairage (CIE) developed a mathematical color model. The premise used by the CIE is that color is the combination of three things: a light source, an object, and an observer. The CIE tightly controlled each of these variables in an experiment that produced the measurements for the system. Although Aristotle and other ancient scientists speculated on the nature of light and color vision, it was not until Newton that light was correctly identified as the source of the color sensation. Goethe studied the theory of colors, and in 1801 Thomas Young proposed his trichromatic theory which was later refined by Hermann von Helmholtz. That theory was confirmed in the 1960s and will be described below. Hermann von Helmholtz The retina of the human eye contains three different types of color receptor cells, or cones. One type, relatively distinct from the other two, is most responsive to light that we perceive as violet, with wavelengths around 420 nm (cones of this type are sometimes called short-wavelength cones, S cones, or, most commonly but quite misleadingly, blue cones). The other two types are closely related genetically, chemically and in response. Each type is most responsive to light that we perceive as green or greenish. One of these types (sometimes called long-wavelength cones, L cones, or, misleadingly, red cones) is most sensitive to light we perceive as yellowish-green, with wavelengths around 564 nm; the other type (sometimes called middle-wavelength cones, M cones, or misleadingly green cones) is most sensitive to light perceived as green, with wavelengths around 534 nm. The term "red cones" for the long-wavelength cones is deprecated as this type is actually maximally responsive to light we perceive as greenish, albeit longer wavelength light than that which maximally excites the mid-wavelength/"green" cones. The sensitivity curves of the cones are roughly bell-shaped, and overlap considerably. The incoming signal spectrum is thus reduced by the eye to three values, sometimes called tristimulus values, representing the intensity of the response of each of the cone types. Because of the overlap between the sensitivity ranges, some combinations of responses in the three types of cone are impossible no matter what light stimulation is used. For example, it is not possible to stimulate only the mid-wavelength/"green" cones: the other cones must be stimulated to some degree at the same time, even if light of some single wavelength is used (including that to which the target cones are maximally sensitive). The set of all possible tristimulus values determines the human color space. It has been estimated that humans can distinguish roughly 10 million different colors, although the identification of a specific color is highly subjective, since even the two eyes of a single individual perceive colors slightly differently. This is discussed in more detail below. The rod system (which vision in very low light relies on exclusively) does not by itself sense differences in wavelength; therefore it is not normally implicated in color vision. But experiments have conclusively shown that in certain marginal conditions a combination of rod stimulation and cone stimulation can result in color discriminations not based on the mechanisms described above. While the mechanisms of color vision at the level of the cones in the retina are well described in terms of tristimulus values (see above), color processing and perception above that base level are organized differently. A dominant theory of the higher neural mechanisms of color vision proposes three opponent processes, or opponent channels, constructed out of the raw input from the cones: a red-green channel, a blue-yellow channel, and a black-white ("luminance") channel. This theory tries to account for the structure of our subjective color experience (see discussion below). Blue and yellow are considered complementary colors, or opposites: you could not experience a bluish yellow (or a greenish red), any more than you could experience a dark brightness or a hot coldness. The four "polar" colors proposed as extremes in the two opponent processes other than black-white have some natural claim to being called primary colors. This is in competition with various sets of three primary colors proposed as "generators" of all normal human color experience (see below).

Clinical issues

If one or more types of a person's color-sensing cones are missing or less responsive than normal to incoming light, that person has a smaller or skewed color space and is said to be color deficient. Another term frequently used is color blind, although this can be misleading; only a small fraction of color deficient individuals actually see completely in black and white, and most simply have anomalous color perception. Some kinds of color deficiency are caused by anomalies in the number or nature of cones of the various types, as just described. Others (like central or cortical achromatopsia) are caused by neural anomalies in those parts of the brain where visual processing takes place. Some animals may have more than three different types of color receptor (most marsupials, birds, reptiles, and fish; see tetrachromat, below) or fewer (most mammals; these are called dichromats and monochromats). Humans and other old-world primates are actually rather unusual in possessing three kinds of receptors. An unusual and elusive neurological condition sometimes affecting color perception is synaesthesia.

Tetrachromat

A normal human is a trichromat (from Greek: tri=three, chroma=color). In theory it may be possible for a person to have four, rather than three, distinct types of cone cell. If these four types are sufficiently distinct in spectral sensitivity and the neural processing of the input from the four types is developed, a person may be a tetrachromat (tetra=four). Such a person might have an extra and slightly different copy of either the medium- or long-wave cones. It is not clear whether such people exist or that the human brain could actually process the information from such an extra cone type separately from the standard three. However, strong evidence suggests that such people do exist, they are all female by genetic imperative, and their brains gladly adapt to use the additional information. For many species, tetrachromacy is the normal case, although the cone cells of animal tetrachromats have a very different (more evenly-spaced) spectral sensitivity distribution than those of possible human tetrachromats.

Color perception

There is an interesting phenomenon which occurs when an artist uses a limited color palette: the eye tends to compensate by seeing any grey or neutral color as the color which is missing from the color wheel. E.g.: in a limited palette consisting of red, yellow, black, and white, a mixture of yellow and black will appear as a variety of green, a mixture of red and black will appear as a variety of purple, and pure grey will appear bluish. When the eye shifts attention after viewing a color for some time, then an afterimage of the complement of that color (the color opposite to it in the color wheel) is perceived by the eye for some time wherever it moves. This effect of color perception was utilised by Vincent van Gogh, a Post-Impressionist painter.

Effect of luminosity

Note that the color experience of a given light mixture may vary with absolute luminosity, because both rods and cones are active at once in the eye, with each having different color curves, and rods taking over gradually from cones as the brightness of the scene is reduced. This effect leads to a change in color rendition with absolute illumination levels that can be summarised in the "Kruithof curve".

Cultural influences

Different cultures have different terms for colors, and may also assign some color names to slightly different parts of the spectrum, or have a different color ontology: for instance, the Han character 青 (pronounced qīng in Mandarin and aoi in Japanese) has a meaning that covers both blue and green; blue and green are traditionally considered shades of 青; In more contemporary terms, they are 藍 (lán) and 綠 (lǜ) respectively. Similarly, languages are selective when deciding which hues are split into different colors on the basis of how light or dark they are. Apart from the black-grey-white continuum, English splits some hues into several distinct colors according to lightness: such as red and pink or orange and brown. To English speakers, these pairs of colors, which are objectively no more different that light green and dark green, are conceived as totally different. An Italian will make the same red-pink and orange-brown distinctions, but will also make a further distinction between blu and azzurro, which English speakers would simply call dark and light blue. To Italian speakers, blu and azzurro are as separate as red and pink or orange and brown. Color terms evolve. It is argued that there are a limited number of universal "basic color terms" which begin to be used by individual cultures in a relatively fixed order. For example, a culture would start with only two terms, meaning roughly 'dark' (covering black, dark colors and cold colors such as blue ) and 'bright' (covering white, light colors and warm colors such as red), before adding more specific color names, in the order of red; green and/or yellow; blue; brown; and orange, pink, purple, and/or gray. Older arguments for this theory also stipulated that the acquisition and use of basic color terms further along the evolutionary order indicated a more complex culture with more highly developed technology. A somewhat dated example of a universal color categories theory is Basic Color Terms: Their Universality and Evolution (1969) by Brent Berlin and Paul Kay. A more recent example of a linguistic determinism theory might be Is color categorisation universal? New evidence from a stone-age culture (1999) by Jules Davidoff et al. The idea of linguistically determined color categories is often used as evidence for the Sapir-Whorf hypothesis (Language, Thought, and Reality (1956) by Benjamin Lee Whorf). Additionally, different colors are often associated with different emotional states, values, or groups, but these associations can vary between cultures. In one system, red is considered to motivate action; orange and purple are related to spirituality; yellow cheers; green creates cosiness and warmth; blue relaxes; and white is associated with either purity or death. These associations are described more fully in the individual color pages, and under color psychology. See also: National colors

Color constancy

The trichromatric theory discussed above is strictly true only if the whole scene seen by the eye is of one and the same color, which of course is unrealistic. In reality, the brain compares the various colors in a scene, in order to eliminate the effects of the illumination. If a scene is illuminated with one light, and then with another, as long as the difference between the light sources stays within a reasonable range, the colors of the scene will nevertheless appear constant to us. This was discovered by Edwin Land in the 1970s and led to his retinex theory of color constancy.

Contrast

Note: the following comparison requires an all-digital display setup (commonly, a laptop or DVI-connected LCD) to avoid errors caused by an unfortunate interaction between frequency response and gamma curves. Compare the visibility of the RGB primary and secondary colors against a white background:

red

green

blue

red+green

green+blue

red+blue

red+green+blue

zero light

Again, compare variations on gray backgrounds—#7f7f7f, #5f5f5f & #9f9f9f—the eight RGB primaries are equidistant from #7f7f7f in a 3-d geometrical representation of RGB color space—a reminder of the importance of background color for color perception. Background = #7f7f7f

red

green

blue

red+green

green+blue

red+blue

red+green+blue

zero light

And let's look at black again, for completeness. (Note that your monitor background probably is not perfectly black, as you can see by switching off the monitor.) Background = #000000

red

green

blue

red+green

green+blue

red+blue

red+green+blue

zero light

Measurement and reproduction of color

monitor Two different light spectra which have the same effect on the three color receptors in the human eye will be perceived as the same color. This is exemplified by the white light that is emitted by fluorescent lamps, which typically has a spectrum consisting of a few narrow bands, while daylight has a continuous spectrum. The human eye cannot tell the difference between such light spectra just by looking into the light source, although reflected colors from objects can look different. (This is often exploited e.g. to make fruit or tomatoes look more brightly red in shops.) Similarly, most human color perceptions can be generated by a mixture of three colors called primaries. This is used to reproduce color scenes in photography, printing, television, and other media. There are a number of methods or color spaces for specifying a color in terms of three particular primary colors. Each method has its advantages and disadvantages depending on the particular application. No mixture of colors, though, can produce a fully pure color perceived as completely identical to a spectral color, although one can get very close for the longer wavelengths, where the chromaticity diagram above has a nearly straight edge. For example, mixing green light (530 nm) and blue light (460 nm) produces cyan light that is slightly desaturated, because response of the red color receptor would be greater to the green and blue light in the mixture than it would be to a pure cyan light at 485 nm that has the same intensity as the mixture of blue and green. Because of this, and because the primaries in color printing systems generally are not pure themselves, the colors reproduced are never perfectly saturated colors, and so spectral colors cannot be matched exactly. However, natural scenes rarely contain fully saturated colors, thus such scenes can usually be approximated well by these systems. The range of colors that can be reproduced with a given color reproduction system is called the gamut. The CIE chromaticity diagram can be used to describe the gamut. Another problem with color reproduction systems is connected with the acquisition devices, like cameras or scanners. The characteristics of the color sensors in the devices are often very far from the characteristics of the receptors in the human eye. In effect, acquisition of colors that have some special, often very "jagged", spectra caused for example by unusual lighting of the photographed scene can be relatively poor. Species that have color receptors different from humans, e. g. birds that may have four receptors, can differentiate some colors that look the same to a human. In such cases, a color reproduction system `tuned' to a human with normal color vision may give very inaccurate results for the other observers. The next problem is different color response of different devices. For color information stored and transferred in a digital form, color management technique based on color profiles attached to color data and to devices with different color response helps to avoid deformations of the reproduced colors. The technique works only for colors in gamut of the particular devices, e.g. it can still happen that your monitor is not able to show you real color of your goldfish even if your camera can receive and store the color information properly and vice versa.

Pigments and reflective media

When producing a color print or painting a surface, the applied paint changes the surface; if the surface is then illuminated with white light (which consists of equal intensities of all visible wavelengths), the reflected light will have a spectrum corresponding to the desired color. If a dab of paint looks red in white light, that is because the reflection of all non-red wavelengths is interrupted by the pigment, such that only red light is reflected into one's eye.

Structural color

Structural color is a property of some surfaces that are scored with fine parallel lines, formed of many thin parallel layers, or otherwise composed of periodic microstructures on the scale of the color's wavelength, to make a diffraction grating. The grating reflects some wavelengths more than others due to interference phenomena, causing white light to be reflected as colored light. Variations in the pattern's spacing often give rise to an iridescent effect, as seen in peacock feathers, films of oil, and mother of pearl, because the reflected color depends upon the viewing angle. Structural color is studied in the field of thin-film optics. A layman's term that describes particularly the most ordered structural colors is iridescence.

Footnotes

# The spelling color is predominant in American English, while colour is used in Commonwealth English. See our/or.

External links and sources

- [http://www.physicstoday.org/vol-55/iss-7/p43.html Comparative Article examining Goethean and Newtonian Color]
- [http://palimpsest.stanford.edu/waac/wn/wn21/wn21-3/wn21-308.html Kruithof curve citation]
- [http://www.soluxtli.com/edu13.htm Article by technical lighting manufacturer on rod/cone vision, with cites to literature]
- [http://www.angelfire.com/psy/reading/Colour.html The Psychology of Colour]
- [http://plato.stanford.edu/entries/color/ Stanford Encyclopedia of Philosophy entry]
- [http://webexhibits.org/causesofcolor/ Why are things colored?]
- [http://www.research.ibm.com/people/l/lloydt/color/color.HTM Why Should Engineers and Scientists Be Worried About Color?]
- [http://poynterextra.org/cp/colorproject/color.html Color, Contrast & Dimension in News Design] Category:Color Category:Image processing Category:Vision ko:색 ja:色 simple:Color

Atomic weight

The atomic mass of a chemical element (also known as the relative atomic mass or average atomic mass or atomic weight) is the average atomic mass of all the chemical element's isotopes as found in a particular environment, weighted by isotopic abundance. Periodic tables usually list these with reference to the local environment of Earth's crust and atmosphere. For artificial elements the nucleon count of the most stable isotope is listed in parentheses as the atomic mass. The atomic mass of an isotope is the relative mass of the isotope, scaled with carbon-12 as exactly 12. No other isotopes have whole number masses due to the different mass of neutrons and protons, as well as loss/gain of mass to binding energy. However, since mass defect due to binding energy is minimal compared to the mass of a nucleon, rounding the atomic mass of an isotope tells you the total nucleon count. Neutron count can then be derived by subtracting the atomic number. The pattern in the amounts the atomic masses deviate from their mass numbers is as follows: the deviation starts positive at hydrogen-1, becomes negative until a minimum is reached at iron-56, then increases to positive values in the heavy isotopes, with increasing atomic number. This corresponds to the following: nuclear fission in an element heavier than iron produces energy, and fission in any element lighter than iron requires energy; the opposite is true of nuclear fusion reactions - fusion in elements lighter than iron produces energy, and fusion in elements heavier than iron requires energy. A similar definition applies to molecules; it is then called molecular mass. One can compute the molecular mass of a compound by adding the atomic masses of its constituent atoms multiplied by the ratios of elements given in the chemical formula. A similar formula mass can be calculated for those compounds which do not form molecules. Direct comparison and measurement of the masses of atoms and molecules is achieved with mass spectrometry. One mole of a substance always contains almost exactly the atomic or molecular mass of that substance, expressed in grams. For example, the atomic mass of iron is 55.847, and therefore one mole of iron has a mass of 55.847 grams.

History

Before the 1960s, this was expressed so that the oxygen-16 isotope received the atomic weight 16, however, the proportions of oxygen-17 and oxygen-18 present in natural oxygen, which were also used to calculate atomic mass led to two different tables of atomic mass. Formerly chemists and physicists used two different atomic mass scales. The chemists used a scale such that the natural mixture of oxygen isotopes had an atomic mass 16, while the physicists assigned the same number 16 to the atomic mass of the most common oxygen isotope (containing eight protons and eight neutrons). The unified scale based on carbon-12, ¹²C, met the physicists' need to base the scale on a pure isotope, while being numerically close to the old chemists' scale. The term atomic weight is being phased out slowly and being replaced by relative atomic mass, in most current usage. The term standard atomic weight refers to the mean relative atomic mass of an element.

External links

- [http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&ascii=html&isotype=some Atomic masses of all isotopes] Category:Chemical properties Category:Mass ko:원자 질량 ja:原子量 th:มวลอะตอม

Atomic mass unit

AMU redirects here, but may also refer to the Arab Maghreb Union The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic masses and molecular masses. It is defined to be 1/12 of the mass of one atom of carbon-12. :1 u = 1/N_A gram = 1/(1000 N_A) kg (where N_A is Avogadro's number) :1 u ≈ 1.66053886 x 10^-27 kg See 1 E-27 kg for a list of objects which have a mass of about 1 u. The symbol amu for atomic mass unit can sometimes still be found, particularly in older works. Atomic masses are often written without any unit and then the atomic mass unit is implied. In biochemistry and molecular biology literature (particularly in reference to proteins), the term "dalton" is used, with the symbol "Da". Because proteins are large molecules, they are typically referred to in kilodaltons, or "kDa", with one kilodalton being equal to 1000 daltons. The unified atomic mass unit is not an SI unit of mass, although it is (only by that name, and only with the symbol u) accepted for use with SI. See SI website link below. The unit is convenient because one hydrogen atom has a mass of approximately 1 u, and more generally an atom or molecule that contains n protons and neutrons will have a mass approximately equal to n u. (The reason is that a carbon-12 atom contains 6 protons, 6 neutrons and 6 electrons, with the protons and neutrons having about the same mass and the electron mass being negligible in comparison.) This is only a rough approximation however, since it does not account for the mass contained in the binding energy of an atom's nucleus; this binding energy mass is not a fixed fraction of an atom's total mass. Another reason the unit is used is that it is experimentally much easier and more precise to compare masses of atoms and molecules (determine relative masses) than to measure their absolute masses. Masses are compared with a mass spectrometer (see below). Avogadro's number (N_A) and the mole are defined so that one mole of a substance with atomic or molecular mass 1 u will have a mass of precisely 1 gram. For example, the molecular mass of water is 18.01508 u, and this means that one mole of water has a mass of 18.01508 grams, or conversely that 1 gram of water contains N_A/18.01508 ≈ 3.3428 × 10²² molecules.

Measuring relative atomic masses

The relative atomic mass is measured with a mass spectrometer. After placing a sample of the element to be measured in the mass spectrometer it is bombarded with electrons which turns the atoms into positive ions. An electric field is then used to accelerate these positive ions, after which the ions are deflected using a magnetic field. As a result the various isotopes are separated out due to the ions of lighter isotopes being deflected more than those heavier. This produces a mass spectrum. This spectrum provides two things: # Relative isotopic masses in the sample # Abundances of the isotopes

History

The chemist John Dalton was the first to suggest the mass of one atom of hydrogen as the atomic mass unit. Francis Aston, inventor of the mass spectrometer, later used 1/16 of the mass of one atom of oxygen-16 as his unit. Before 1961, the physical atomic mass unit was defined as 1/16 of the mass of one atom of oxygen-16, while the chemical atomic mass unit was defined as 1/16 of the average mass of an oxygen atom (taking the natural abundance of the different oxygen isotopes into account). Both units are slightly smaller than the unified atomic mass unit, which was adopted by the International Union of Pure and Applied Physics in 1960 and by the International Union of Pure and Applied Chemistry in 1961.

External links

- [http://www1.bipm.org/en/si/si_brochure/chapter4/table7.html SI website on acceptable non-SI units]
- [http://physics.nist.gov/cgi-bin/cuu/Value?ukg Accepted value of 1 u as of 2002] Category:Nuclear chemistry Category:Units of mass ja:原子質量単位 ko:아보가드로 수 th:หน่วยมวลอะตอม

Kelvin

The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. It is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. A temperature given in kelvins, without further qualification, is measured with respect to absolute zero, where molecular motion stops. It is also common to give a temperature relative to the reference temperature of 273.15 K, approximately the melting point of water under ordinary conditions; this convention is the Celsius temperature scale. The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin; his barony was in turn named after the River Kelvin, which runs through the grounds of the University of Glasgow.

SI multiples

Typographical conventions

The word kelvin as an SI unit is correctly written with a lowercase k (unless at the beginning of a sentence), and is never preceded by the words degree or degrees, or the symbol °, unlike degrees Fahrenheit, or degrees Celsius. This is because the latter are adjectives, whereas kelvin is a noun. It takes the normal plural form by adding an s in English: kelvins. When the kelvin was introduced in 1954 (10th General Conference on Weights and Measures (CGPM), Resolution 3, CR 79), it was the "degree Kelvin", and written °K; the "degree" was dropped in 1967 (13th CGPM, Resolution 3, CR 104). Note that the symbol for the kelvin unit is always a capital K and never italicised. There is a space between the number and the K, as with all other SI units. Unicode includes the "kelvin sign" at U+212A (in your browser it looks like K). However, the "kelvin sign" is canonically decomposed into U+004B, thereby seen as a (preexisting) encoding mistake, and it is better to use U+004B (K) directly.

Conversion factors

Kelvins and Celsius

The Celsius temperature scale is now defined in terms of the kelvin, with 0 °C corresponding to 273.15 kelvins.
- kelvins to degrees Celsius
- :

\mathrm = \mathrm - 273.15

Temperature and energy

In a thermodynamic system, the energy of the particles of a perfect gas is proportional to the absolute temperature, where the constant of proportionality is the Boltzmann constant. As a result, it is possible to determine the average kinetic energy

\overline

of the gas particles at the temperature T or to calculate the temperature of the gas from the average kinetic energy of the particles: :

\overline = \frac \cdot k_B \cdot \mathrm

External link

- [http://www1.bipm.org/en/si/si_brochure/chapter2/2-1/2-1-1/kelvin.html BIPM brochure on the kelvin] Category:SI base units Category:Units of temperature ko:켈빈 ja:ケルビン simple:Kelvin th:เคลวิน

Celsius

The degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701–1744), who first proposed a similar system in 1742. The Celsius scale sets 0.01 °C to be at the triple point of water and a degree Celsius to be 1/273.16 of the difference in temperature between the triple point of water and absolute zero. Until 1954 the scale was defined with the freezing point of water at 0 °C and the boiling point at 100 °C at standard atmospheric pressure, this definition is still a close approximation to the actual definition and is for that reason commonly (but wrongly) used to refer to the scale.

History

The Celsius temperature scale was originally designed so that the freezing point of water is 100 degrees, and its boiling point is 0 degrees at standard atmospheric pressure. This was reversed to its modern order some time after his death, in part at the instigation of Daniel Ekström, the manufacturer of most of the thermometers used by Celsius. Several other people, including Per Elvius the Elder from Sweden (1710) and Christian of Lyons (1743), independently invented the same temperature scale. The oft-quoted claim that the botanist Carolus Linnaeus (1740) is amongst those is unsubstantiated. The Delisle scale was another temperature scale that ran "downward". Since there are one hundred graduations between these two reference points, the original term for this system was centigrade (100 parts) or centesimal. In 1948 the system's name was officially changed to Celsius (a third name which had also been in use before then) by the 9th General Conference on Weights and Measures (CR 64), both in recognition of Celsius himself and to eliminate confusion caused by conflict with the use of the SI centi- prefix. While the values for freezing and boiling of water remain approximately correct, they are no longer suitable as reference points for a formal standard. The current official definition of the Celsius scale sets 0.01 °C to be at the triple point of water and a degree to be 1/273.16 of the difference in temperature between the triple point of water and absolute zero. This definition was adopted in 1954 at the 10th General Conference on Weights and Measures, the very same definition given for the kelvin. For the practical calibration of thermometers, the International Temperature Scale of 1990 defines many additional reference points.

Naming

The degree Celsius is the only SI unit whose full unit name ("degree Celsius", not "Celsius") in English includes an upper case letter. That is a quirk of English, because it is a proper adjective rather than a noun (before the name was changed from "degree Kelvin" to "kelvin" in 1967, that was another SI unit containing a capital letter in English). While SI prefixes could be applied in principle, as in "12 m°C", they are not used in practice (ISO 1000).

Application

The Celsius scale is the world's most commonly used temperature scale. It has been adopted by virtually all the countries of the world, with the notable exceptions of the United States of America and Jamaica. In broadcast media it was still frequently referred to as centigrade until the late 1980s or early 1990s, particularly by weather forecasters on European networks such as the BBC, ITV, and RTÉ. In the United States and Jamaica, Fahrenheit remains the preferred scale for everyday temperature measurement, although Celsius or kelvin is used for aeronautical and scientific applications. In the United Kingdom, Celsius is the official scale used by the government and the media. It is also the only scale used in British cooking and temperature controllers (for example, room thermostats). Some of the British media, however, still provide Fahrenheit equivalents since many in Britain, especially older people, still use the Fahrenheit scale. Even so, many that do still switch to the use of Celsius for low temperatures.

Trivia

- The Unicode character set contains a dedicated precomposed degrees Celsius character (℃, U+2103). This character was only intended for compatibility mapping of legacy character sets that contain it as well. It should not be used in new texts. Category:SI derived units Category:Units of temperature zh-min-nan:Liap-sī ko:섭씨 ja:セルシウス度

Boiling point

:Alternate use: Boiling Point, a film by Takeshi Kitano; Boiling Points, a television series The boiling point of a substance is the temperature at which it can change its state from a liquid to a gas throughout the bulk of the liquid. A liquid may change to a gas at temperatures below the boiling point through the process of evaporation. Any change of state from a liquid to a gas at boiling point is considered vaporization. However, evaporation is a surface phenomenon, in which only molecules located near the gas/liquid surface could evaporate. Boiling on the other hand is a bulk process, so at the boiling point molecules anywhere in the liquid may be vaporized, resulting in the formation of vapor bubbles. A somewhat clearer definition of boiling point is that it is the temperature at which the vapor pressure of the liquid equals the pressure of the environment. Something that should be remembered is that boiling is evidenced by the appearance of bubbles containing vapor from the liquid. Production of this vapor requires energy and thus does not occur without some source of energy. This source can be a hot surface or even the liquid itself. Hot liquid will boil as it rises through the bulk liquid when the pressure of the environment drops to the vapor pressure of the liquid at its temperature. This production of vapor will quickly stop because the temperature of the liquid will be reduced by the vaporization thus reducing the vapor pressure. The element with the lowest boiling point is helium. Both the boiling points of rhenium and tungsten exceed 5000 K at standard pressure. Due to the experimental difficulty of precisely measuring extreme temperatures without bias, there is some discrepancy in the literature as to whether tungsten or rhenium has the higher boiling point. The boiling point corresponds to the temperature at which the vapor pressure of the substance equals the ambient pressure. Thus the boiling point is dependent on the pressure. Usually, boiling points are published with respect to standard pressure (101.325 kilopascals or 1 atm). At higher elevations, where the atmospheric pressure is much lower, the boiling point is also lower. The boiling point increases with increased ambient pressure up to the critical point, where the gas and liquid properties become identical. The boiling point cannot be increased beyond the critical point. Likewise, the boiling point decreases with decreasing ambient pressure until the triple point is reached. The boiling point cannot be reduced below the triple point. The process of changing from a liquid to a gas requires an amount of heat called the latent heat of vaporization. As heat is added to a liquid at its boiling point, all of this heat goes toward the phase change from liquid to gas, thus the temperature of the substance remains constant even though heat has been added. The word latent, which comes from Latin and means hidden, is used to describe this "disappearing" heat that is added, but doesn't result in an increase in temperature. Since heat is added with no corresponding change in temperature, the heat capacity of the liquid is essentially infinite at the boiling point.
Intermolecular interactions
In terms of intermolecular interactions, the boiling point represents the point at which the liquid molecules possess enough heat energy to overcome the various intermolecular attractions binding the molecules into the liquid (eg. dipole-dipole attraction, instantaneous-dipole induced-dipole attractions, and hydrogen bonds). Therefore the boiling point is also an indicator of the strength of these attractive forces. The boiling point of water is 100 °C (212 °F) at standard pressure. On top of Mount Everest the pressure is about 260 mbar (26 kPa) so the boiling point of water is 69 °C. For purists with a knowledge of thermodynamics, the normal boiling point of water is 99.97 degrees Celsius (at a pressure of 1 atm, i.e. 101.325 kPa). Until 1982 this was also the standard boiling point of water, but the IUPAC now recommends a standard pressure of 1 bar (100 kPa). At this slightly reduced pressure, the standard boiling point of water is 99.61 degrees Celsius. (Cf. DeVoe, Howard, Thermodynamics and Chemistry. Prentice-Hall, 2001)
See also

- Leidenfrost effect
- flash point
- boiling delay
- critical temperature
- triple point
- boiling-point elevation Category:Chemical properties Category:Thermodynamics Category:Fluid dynamics ko:끓는점 ja:沸点 th:จุดเดือด

Kelvin

SI multiples

Typographical conventions

Conversion factors

Kelvins and Celsius

The Celsius temperature scale is now defined in terms of the kelvin, with 0 °C corresponding to 273.15 kelvins.
- kelvins to degrees Celsius
- :

\mathrm = \mathrm - 273.15

Temperature and energy

\overline

of the gas particles at the temperature T or to calculate the temperature of the gas from the average kinetic energy of the particles: :

\overline = \frac \cdot k_B \cdot \mathrm

External link

Density

: For other senses of "density", see density (disambiguation). Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. The higher an object's density, the higher its mass per volume. The average density of an object equals its total mass divided by its total volume. A denser object (such as iron) will have less volume than an equal mass of some less dense substance (such as water). The SI unit of density is the kilogram per cubic metre (kg/m³) :

\rho = \frac

where :ρ is the object's density (measured in kilograms per cubic metre) :m is the object's total mass (measured in kilograms) :V is the object's total volume (measured in cubic metres) Under specified conditions of temperature and pressure, density of a fluid is defined as described above. However, the density of a solid material can be different, depending on exactly how it is defined. Take sand for example. If you gently fill a container with sand, and divide the mass of sand by the container volume you get a value termed loose bulk density. If you took this same container and tapped on it repeatedly, allowing the sand to settle and pack together, and then calculate the results, you get a value termed tapped or packed bulk density. Tapped bulk density is always greater than or equal to loose bulk density. In both types of bulk density, some of the volume is taken up by the spaces between the grains of sand. Also, in terms of candy making, density is affected by the melting and cooling processes. Loose granular sugar, like sand, contains a lot of air and is not tightly packed, but when it has melted and starts to boil, the sugar loses its granularity and entrained air and becomes a fluid. When you mold it to make a smaller, compacted shape, the syrup tightens up and loses more air. As it cools, it contracts and gains moisture, making the already heavy candy even more dense.

Other units

Density in terms of the SI base units is expressed in terms of kilograms per cubic metre (kg/m³). Other units fully within the SI include grams per cubic centimetre (g/cm³) and megagrams per cubic metre (Mg/m³). Since both the litre and the tonne or metric ton are also acceptable for use with the SI, a wide variety of units such as kilograms per litre (kg/L) are also used. Imperial units or U.S. customary units, the units of density include pounds per cubic foot (lb/ft³), pounds per cubic yard (lb/yd³), pounds per cubic inch (lb/in³), ounces per cubic inch (oz/in³), pounds per gallon (for U.S. or imperial gallons) (lb/gal), pounds per U.S. bushel (lb/bu), in some engineering calculations slugs per cubic foot, and other less common units. The maximum density of pure water at a pressure of one standard atmosphere is 999.972 kg/m³; this occurs at a temperature of about 3.98 °C (277.13 K). From 1901 to 1964, a litre was defined as exactly the volume of 1 kg of water at maximum density, and the maximum density of pure water was 1.000 000 kg/L (now 0.999 972 kg/L). However, while that definition of the litre was in effect, just as it is now, the maximum density of pure water was 0.999 972 kg/dm³. During that period students had to learn the esoteric fact that a cubic centimetre and a millilitre were slightly different volumes, with 1 mL = 1.000 028 cm³. (often stated as 1.000 027 cm³ in earlier literature).

Measurement of density

A common device for measuring fluid density is a pycnometer. A device for measuring absolute density of a solid is a gas pycnometer.

Density of substances

Perhaps the highest density known is reached in neutron star matter (see neutronium). The singularity at the centre of a black hole, according to general relativity, does not have any volume, so its density is undefined. The most dense naturally occurring substance on Earth is iridium, at about 22650 kg/m³. A table of densities of various substances:

Substance	Density in kg/m³
Iridium	22650
Osmium	22610
Platinum	21450
Gold	19300
Tungsten	19250
Uranium	19050
Mercury	13580
Palladium	12023
Lead	11340
Silver	10490
Copper	8960
Iron	7870
Tin	7310
Titanium	4507
Diamond	3500
Basalt	3000
Granite	2700
Aluminium	2700
Graphite	2200
Magnesium	1740
PVC	1300
Seawater	1025
Water	1000
Ice	917
Polyethylene	910
Ethyl alcohol	790
Gasoline	730
Aerogel	.003
any gas	0.0446 times the average molecular mass, hence between 0.09 and ca. 10.0 (at standard temperature and pressure)
For example air	1.2

Note the low density of aluminium compared to most other metals. For this reason, aircraft are made of aluminium. Also note that air has a nonzero, albeit small, density. Aerogel is the world's lightest solid.

Milliliter

Litre

Solubility

A substance is soluble in a fluid if it dissolves in that fluid. The dissolved substance is called the solute and the dissolving fluid (usually present in excess) is called the solvent, which together form a solution. The process of dissolving is called solvation, or hydration if the solvent is water. A solution at equilibrium cannot hold any more solute and is said to be saturated. Solutions may, under special conditions, hold more solute than the solvent can normally dissolve. This is called supersaturation. The maximum equilibrium amount of solute which can normally dissolve per amount of solvent (or solution) is the solubility of that solute in that solvent. It is often expressed as a maximum concentration of a saturated solution. The solubility of one substance dissolving in another is determined by the intermolecular forces between the solvent and solute, temperature, the entropy change that accompanies the solvation, the presence and amount of other substances, and sometimes pressure or partial pressure of a solute gas. For salts, solubility in aqueous solutions is often dependent on a solubility constant. The solubility constant is a special case of an equilibrium constant for the reaction of dissolving the salt in question, with the concentration of undissolved compound not in the expression because it is not in the aqueous phase. The solubility constant is also "applicable" (i. e. useful) to precipitation, the reverse of the dissolving reaction. As with other equilibrium constants, temperature can affect the numerical value of solubility constant. Solvents are normally characterized as polar or nonpolar. Polar solvents will dissolve ionic compounds and covalent compounds which ionize, while nonpolar solvents will dissolve nonpolar covalent compounds. For example, ordinary table salt, an ionic compound, will dissolve in water, but not in ethanol. Common solvents used in organic chemistry include acetone, ethanol, water, and benzene. Water and nonpolar solvents are immiscible; they do not form homogeneous mixtures but separate into two distinct phases or form milky emulsions. While solutions are typically thought of as solids being mixed into liquids, any two states of matter can be mixed and be called a solution. Carbonated water is a solution of a gas in a liquid, hydrogen (a gas) can dissolve in palladium (a solid), and stainless steel is a solution of a solid in a solid (called an alloy).

Solubility of bonding type in water

Bonding type	Solubility in water	Example
ionic	most soluble	See below
metallic	insoluble	Fe
metallic	unless they react with water	K
polar covalent	soluble if it H bonds	glucose
	soluble by reaction	HCl
	insoluble otherwise	ether
non-polar covalent	most insoluble	benzene
non-polar covalent	some slightly soluble	O₂
covalent lattice	insoluble	diamond

Solubility of ionic compounds

Soluble	Insoluble
Group 1 and NH₄⁺ compounds	carbonates (except Group 1 and NH₄⁺ compounds)
nitrates	sulfites (except Group 1 and NH₄⁺ compounds)
acetates (ethanoates)	phosphates (except Group 1 and NH₄⁺ compounds)
chlorides, bromides and iodides (except Ag⁺, Pb²⁺, Cu⁺ and Hg₂²⁺)	hydroxides and oxides (except Group 1, NH₄⁺, Ba²⁺, Sr²⁺ and Ca²⁺)
sulfates (except Ag⁺, Pb²⁺, Ba²⁺, Sr²⁺ and Ca²⁺)	sulfides (except Group 1, Group 2 and NH₄⁺ compounds)

Software tools for prediction of solution

One of the most recent and prominent solution (solubility) prediction technologies is applied in [http://www.q-pharm.com/home/contents/drug_d/soft/ Quantum 3.1] that is a suite of Molecular Modeling software for Linux and Windows. The software calculates the solvation energy and solubility for a molecule or a library of molecules in a number of solvents (e.g. water and DMSO). The Quantum 3.1 [http://www.q-pharm.com/home developer] is also a service provider.

Standard enthalpy change of formation

The standard enthalpy of formation or "standard heat of formation" of a compound is the change of enthalpy that accompanies the formation of 1