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Sodium Nitrate

Sodium nitrate

Properties
General
Name	Sodium nitrate
Chemical formula	Na NO₃
Appearance	White powder or colorless crystals
Physical
Formula weight	85.0 amu
Melting point	580 K (307 °C)
Boiling point	decomposes at 653 K (380 °C)
Density	2.3 ×10³ kg/m³
Crystal structure	?
Solubility	92 g in 100mL water
Thermochemistry
Δ_fH⁰_liquid	-452 kJ/mol
Δ_fH⁰_solid	-468 kJ/mol
S⁰_solid	117 J/mol·K
Safety
Ingestion	May cause gastroenteritis and abdominal pains.
Inhalation	respiratory irritation
Skin	May cause irritation.
Eyes	May cause irritation.
More info	[http://www.skylighter.com/msds/SODIUM%20NITRATE%20MSDS.htm MSDS]
SI units were used where possible. Unless otherwise stated, standard conditions were used. Disclaimer and references

Sodium nitrate is a type of salt (NaNO₃) which has long been used as an ingredient in explosives and in solid rocket propellants, as well as in glass and pottery enamel, and as a food preservative (such as in hot dogs), and has been mined extensively for those purposes. It is also variously known as caliche, Chile saltpeter, saltpeter, and soda niter. The world's largest natural deposits of caliche ore were in the Atacama desert of Chile, and many deposits were mined for over a century, until the 1940s. The former Chilean saltpeter mining communities of Humberstone and Santa Laura were declared Unesco World Heritage sites in 2005. Chile still has the largest reserves of caliche, with active mines in such locations as Pedro de Valdivia, Maria Elena and Pampa Blanca. Sodium nitrate, potassium nitrate, sodium sulfate and iodine are all obtained by the processing of calichehttp://www.sqm.com/ingles/NC_history.htm. Sodium nitrate is also manufactured synthetically by reacting nitric acid with soda ash. The compound has antimicrobial properties when used as a food preservative. It is found naturally in leafy green vegetables. It has possible health benefits for increasing oxygen to blood, as well as known health side effects in particular at high doses.

External links

- [http://www.inchem.org/documents/jecfa/jecmono/v38aje06.htm FAO/WHO report] Category:Nitrates Category:Preservatives Category:Sodium compounds

Chemical formula

A chemical formula (also called molecular formula) is a concise way of expressing information about the atoms that constitute a particular chemical compound. It identifies each type of chemical element by its element symbol and identifies the number of atoms of such element to be found in each discrete molecule of that compound. The number of atoms (if greater than one) is indicated as a subscript. For non-molecular substances the subscripts indicate the ratio of elements in the empirical formula. Chemical formula used for a series of compounds that differ from each other by a constant unit is called general formula. Such a series is called the homologous series, while its members are called homologs.

Elements

In organic chemistry most compounds consist of the following five chemical elements:
- C carbon
- H hydrogen
- N nitrogen
- O oxygen
- S sulfur For other element symbols see list of elements by symbol.

Molecular and structural formulas

For example methane, a simple molecule consisting of one carbon atom bonded to four hydrogen atoms has the chemical formula: : CH₄ and glucose with six carbon atoms, twelve hydrogen atoms and six oxygen atoms has the chemical formula: : C₆H₁₂O₆. A chemical formula may also supply information about the types and spatial arrangement of bonds in the chemical, though it does not necessarily specify the exact isomer. For example ethane consists of two carbon atoms single-bonded to each other, each having three hydrogen atoms bonded to it. Its chemical formula can be rendered as CH₃CH₃. If there were a double bond between the carbon atoms (and thus each carbon only had two hydrogens), the chemical formula may be written: CH₂CH₂, and the fact that there is a double bond between the carbons is assumed. However, a more explicit and correct method is to write H₂C:CH₂ or H₂C=CH₂. The two dots or lines indicate that a double bond connects the atoms on either side of them. A triple bond may be expressed with three dots or lines, and if there may be ambiguity, a single dot or line may be used to indicate a single bond. Molecules with multiple functional groups that are the same may be expressed in the following way: (CH₃)₃CH. However, this implies a different structure from other molecules that can be formed using the same atoms (isomers). The formula (CH₃)₃CH implies a chain of three carbon atoms, with the middle carbon atom bonded to another carbon: Carbon chain and the remaining bonds on the carbons all leading to hydrogen atoms. However, the same number of atoms (10 hydrogens and 4 carbons, or C₄H₁₀) may be used to make a straight chain: CH₃CH₂CH₂CH₃. The alkene 2-butene has two isomers which the chemical formula CH₃CH=CHCH₃ does not identify. The relative position of the two methyl groups must be indicated by additional notation denoting whether the methyl groups are on the same side of the double bond (cis or Z) or on the opposite sides from each other.(trans or E)

Polymers

For polymers, parentheses are placed around the repeating unit. For example, a hydrocarbon molecule that is described as: CH₃(CH₂)₅₀CH₃, is a molecule with 50 repeating units. If the number of repeating units is unknown or variable, the letter n may be used to indicate this: CH₃(CH₂)_nCH₃.

Ions

For ions, the charge on a particular atom may be denoted with a right-hand superscript. For example Na⁺, or Cu²⁺. The total charge on a charged molecule or a polyatomic ion may also be shown in this way. For example: hydronium, H₃O⁺ or sulfate, SO₄^2-.

Isotopes

Although isotopes are more relevant to nuclear chemistry or stable isotope chemistry than to conventional chemistry, different isotopes may be indicated with a left-hand superscript in a chemical formula. For example, the phosphate ion containing radioactive phosphorus-32 is ³²PO₄^3-. Also a study involving stable isotope ratios might include ¹⁸O:¹⁶O. A left-hand subscript is sometimes used to indicate redundantly, for convenience, the atomic number.

Empirical formula

In chemistry, the empirical formula of a chemical is a simple expression of the relative number of each type of atom or ratio of the elements in it. Empirical formulas are the standard for ionic compounds, such as CaCl₂, and for macromolecules, such as SiO₂. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. The term empirical refers to the process of elemental analysis, a technique of analytical chemistry used to determine the relative percent composition of a pure chemical substance by element. For example, hexane could have a chemical formula of CH₃CH₂CH₂CH₂CH₂CH₃, implying that it has a straight chain structure, 6 carbon atoms, and 14 hydrogen atoms. However the empirical formula for the same molecule would be C₃H₇.

Nitrate

In inorganic chemistry, nitrates are the salts of nitric acid. The nitrate ion is the polyatomic ion with empirical formula N O₃⁻; it is the conjugate base of nitric acid. The nitrate ion is trigonal planar and can be represented as a hybrid of the following resonance structures: nitric acid A nitrate salt forms when a positively charged ion attaches to one of the negatively charged oxygen atoms of the nitrate ion.

Uses

Nitrates such as potassium nitrate (saltpeter) and ammonium nitrate are an important source of nitrogen in fertilizers. These nitrates must be used quickly by plants because they are easily lost through leaching or denitrification by bacteria. Nitrate pollution has become an environmental issue in rivers and oceans. According to the Black Hawk County Green Party, the Cedar River (Iowa) has the highest nitrate levels of any river in the world. Nitrates are also oxidizing agents. When mixed with hydrocarbons or carbohydrates, nitrates can form a flammable or even explosive mixture. For example, potassium nitrate is the oxidizing ingredient in black gunpowder. In medicine, organic nitrates such as nitroglycerin, isosorbide mononitrate (ISMN) and isosorbide dinitrate (ISDN) are particularly useful for prevention and treatment of angina pectoris. However they can cause a dangerous reaction if taken within 24 hours of taking sildenafil citrate (Viagra) or similar drugs.

Related materials

Nitrates should not be confused with nitrites, the salts of nitrous acid. Organic compounds containing the nitro functional group (which has the same formula and structure as the nitrate ion save that one of the O⁻ atoms is replaced by the R group) are known as nitro compounds.

External links

- [http://www.compchemwiki.org/index.php?title=Nitrate Computational Chemistry Wiki] Category:Oxoanions Category:Nitrogen metabolism

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

Melting point

The melting point of a solid is the temperature at which it changes state from solid to liquid. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point. For most substances, melting and freezing points are equal. For example, the melting point and freezing point of the element mercury is 234.32 kelvins (−38.83 °C or −37.89 °F). However, certain substances possess differing solid-liquid transition temperatures. For example, agar melts at 85 °C (185 °F) and solidifies from 32 to 40 °C (89.6 to 104 °F); this phenomenon is known as hysteresis. Certain materials, such as glass, may harden without crystalizing; this is called an amorphous solid. Unlike the boiling point, the melting point is relatively insensitive to pressure. The material with the highest known melting point at atmospheric pressure is graphite, with a melting point of 3,948 kelvins (3,674.8 °C or 6,646.5 °F). Water's Melting/Freezing point is 0 C, or 32 F. Melting point is often used to ascertain purity of and characterise organic compounds. The melting point of a pure substance is always higher than the melting point of an impure sample of that particular substance. When two chemical substances are mixed, the melting point of the resultant mixture will be lower than the melting point of either constituent. The mixing ratio that results in the lowest possible melting point is known as the eutectic point.

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

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

Celsius

History

Naming

Application

Trivia

Kilogram

:For other uses of 'kg' see kg (disambiguation) kg (disambiguation) The kilogram or kilogramme, (symbol: kg) is the SI base unit of mass. It is defined as being equal to the mass of the international prototype of the kilogram. It is the only SI base unit that employs a prefix, and the only SI unit that is still defined in relation to an artifact rather than to a fundamental physical property.

History

The kilogram was originally defined as the mass of one litre of pure water at a temperature of 3.98 degrees Celsius and standard atmospheric pressure. This definition was hard to realize accurately, partially because the density of water depends ever-so-slightly on the pressure, and pressure units include mass as a factor, introducing a circular dependency in the definition of the kilogram. To avoid these problems, the kilogram was redefined as precisely the mass of a particular standard mass created to approximate the original definition. Since 1889, the SI system defines the unit to be equal to the mass of the international prototype of the kilogram, which is made from an alloy of platinum and iridium of 39 mm height and diameter, and is kept at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures). Official copies of the prototype kilogram are made available as national prototypes, which are compared to the Paris prototype ("Le Grand Kilo") roughly every 10 years. The international prototype kilogram was made in the 1880s. By definition, the error in the repeatability of the current definition is exactly zero; however, in the usual sense of the word, it can be regarded as of the order of 2 micrograms. This is found by comparing the official standard with its official copies, which are made of roughly the same materials and kept under the same conditions. There is no reason to believe that the official standard is any more or less stable than its official copies, thus giving a way to estimate its stability. This procedure is performed roughly once every forty years. The international prototype of the kilogram seems to have lost about 50 micrograms in the last 100 years, and the reason for the loss is still unknown (reported in Der Spiegel, 2003 #26). The observed variation in the prototype has intensified the search for a new definition of the kilogram. It is accurate to state that any object in the universe (other than the reference metal in France) that had a mass of 1 kilogram 100 years ago, and has not changed since then, now has a mass of 1.000 000 05 kg. This perspective is counterintuitive and defeats the purpose of a standard unit of mass, since the standard should not change arbitrarily over time.

The gram

The gram or gramme is the term to which SI prefixes are applied. The gram was the base unit of the older cgs system of measurement, a system which is no longer widely used.

Proposed future definitions

There is an ongoing effort to introduce a new definition for the kilogram by way of fundamental or atomic constants. The proposals being worked on are:

Atom-counting approaches

- The Avogadro approach attempts to define the kilogram as a fixed number of silicon atoms. As a practical realization, a sphere would be used and its size would be measured by interferometry.
- The ion accumulation approach involves accumulation of gold atoms and measuring the electrical current required to neutralise them.

Fundamental-constant approaches

- The Watt balance uses the current balance that was formerly used to define the ampere to relate the kilogram to a value for Planck's constant, based on the definitions of the volt and the ohm.
- The levitated superconductor approach relates the kilogram to electrical quantities by levitating a superconducting body in a magnetic field generated by a superconducting coil, and measuring the electrical current required in the coil.
- Since the values of the Josephson (CIPM (1988) Recommendation 1, PV 56; 19) and von Klitzing (CIPM (1988), Recommendation 2, PV 56; 20) constants have been given conventional values, it is possible to combine these values (K_J ≡ 4.835 979 Hz/V and R_K ≡ 2.581 280 7 Ω) with the definition of the ampere to define the kilogram as follows: :The kilogram is the mass which would be accelerated at precisely 2 m/s² if subjected to the per metre force between two straight parallel conductors of infinite length, of negligible circular cross section, placed 1 metre apart in vacuum, through which flow a constant current of exactly 6.241 509 629 152 65 elementary charges per second.

Link with weight

When the weight of an object is given in kilograms, the property intended is almost always mass. Occasionally the gravitational force on an object is given in "kilograms", but the unit used is not a true kilogram: it is the deprecated kilogram-force (kgf), also known as the kilopond (kp). An object of mass 1 kg at the surface of the Earth will be subjected to a gravitational force of approximately 9.80665 newtons (the SI unit of force). Note that the factor of 980.665 cm/s² (as the CGPM defined it, when cgs systems were the primary systems used) is only an agreed-upon conventional value (3rd CGPM (1901), CR 70) whose purpose is to define grams force. The local gravitational acceleration g varies with latitude and altitude and location on the Earth, so before this conventional value was agreed upon, the gram-force was only an ill-defined unit. (See also gee, a standard measure of gravitational acceleration.)

Examples

- Attogram: a research team at Cornell University made a detector using NEMS cantilevers with sub-attogram sensitivity.
- Yoctogram: can be used for masses of nucleons, atoms and molecules. It is a little large for light particles, but yocto- is the last official prefix in the sequence.
- The coefficient is close to the reciprocal of Avogadro's number: 1 unified atomic mass unit = 1.660 54 yg
- Although the unified atomic mass unit is often convenient as a unit, one may sometimes want to use yoctograms to relate easily to other SI values.
- Mass of a free electron: 0.000 91 yg
- Mass of a free proton : 1.672 6 yg
- Mass of a free neutron: 1.674 9 yg

SI multiples

External links

- [http://www.npl.co.uk/mass/faqs/kilogram.html National Physical Laboratory FAQ on kilogram definition, the need for a new definition, and some alternatives]
- [http://www.ex.ac.uk/trol/scol/index.htm Conversion Calculator for Units of MASS (& Weight)]
- [http://nvl.nist.gov/pub/nistpubs/jres/106/4/j64schw.pdf More on the NIST Watt Balance]
- [http://www.npl.co.uk/mass/avogadro.html More on the Avogadro project]
- [http://www.ex.ac.uk/trol/scol/ccmass.htm Conversion: Units of Weight]
- [http://www.bipm.fr Le Bureau International des Poids et Mesures]
- [http://www.hgc.cornell.edu/Nems%20Folder/Attogram%20Sensitivity%20Using%20Nanoelectromechanical.html Attogram Detection]
- [http://www.newscientist.com/article.ns?id=dn7208&feedId=online-news_rss20 World's most sensitive scales weigh a zeptogram, by New Scientist.com]
- [http://news.bbc.co.uk/1/hi/sci/tech/4394947.stm Scales tip with tiniest mass yet, by BBC News Online] Category:SI base units Category:Units of mass zh-min-nan:Kong-kin ko:킬로그램 ja:キログラム simple:Kilogram th:กิโลกรัม

Crystal structure

In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. A crystal structure is composed of a unit cell, a set of atoms arranged in a particular way; which is periodically repeated in three dimensions on a lattice. The spacing between unit cells in various directions are called its lattice parameters. The symmetry properties of the crystal are embodied in its space group. A crystal's structure and symmetry play a role in determining many of its properties, such as cleavage, electronic band structure, and optical properties.

Unit cell

A unit cell is a spatial arrangement of atoms which is tiled in three-dimensional space to describe the crystal. The positions of the atoms inside the unit cell are described by the asymmetric unit or basis, the set of atomic positions

(x_i, y_i, z_i)

measured from a lattice point. For each crystal structure there is a conventional unit cell, usually chosen to make the resulting lattice as symmetric as possible. However, the conventional unit cell is not always the smallest possible choice. A primitive unit cell of a particular crystal structure is the smallest possible unit cell one can construct such that, when tiled, it completely fills space. A Wigner-Seitz cell is a particular kind of primitive cell which has the same symmetry as the lattice.

Crystal system

The crystal system is the point group of the lattice (the set of rotation and reflection symmetries which leave a lattice point fixed), not including the positions of the atoms in the unit cell. There are seven unique crystal systems. The simplest and most symmetric, the cubic (or isometric) system, has the symmetry of a cube. The other six systems, in order of decreasing symmetry, are hexagonal, tetragonal, rhombohedral (also known as trigonal), orthorhombic, monoclinic and triclinic. Some crystallographers consider the hexagonal crystal system not to be its own crystal system, but instead a part of the trigonal crystal system.

Classification of lattices

A Bravais lattice is a set of points constructed by translating a single point in discrete steps by a set of basis vectors. In three dimensions, there are 14 unique Bravais lattices (distinct from one another in that they have different space groups) in three dimensions. All crystalline materials recognised till now (not including quasicrystals) fit in one of these arrangements. The fourteen three-dimensional lattices, classified by crystal system, are shown to the right. The crystal structure is one of the lattices with a unit cell, which contains atoms at specific coordinates, at every lattice point. Because it includes the unit cell, the symmetry of the crystal can be more complicated than the symmetry of the lattice.

Point and space groups

The crystallographic point group or crystal class is the set of non-translational symmetries that leave a point in the crystal fixed. There are 32 possible crystal classes. The space group of the crystal structure is composed of the translational symmetries in addition to the symmetries of the point group. There are 230 distinct space groups.

Defects in crystals

Real crystals feature defects or irregularities in the ideal arrangements described above and it is these defects that critically determine many of the electrical and mechanical properties of real materials. In particular dislocations in the crystal lattice allow shear at much lower stress than that needed for a perfect crystal structure.

Crystal Symmetry

Crystal structures can be divided into 32 classes, or point groups, according to the number of rotational axes and reflection planes they exhibit that leave the crystal structure unchanged. Twenty of the 32 crystal classes are piezoelectric. All 20 piezoelectric classes lack a center of symmetry. Any material develops a dielectric polarization when an electric field is applied, but a substance which has such a natural charge separation even in the absence of a field is called a polar material. Whether or not a material is polar is determined solely by its crystal structure. Only 10 of the 32 point groups are polar. All polar crystals are pyroelectric, so the 10 polar crystal classes are sometimes referred to as the pyroelectric classes. Under normal circumstances, even polar materials do not display a net dipole moment. As a consequence there are no electric dipole equivalents of bar magnets because the intrinsic dipole moment is neutralized by "free" electric charge that builds up on the surface by internal conduction or from the ambient atmosphere. Polar crystals only reveal their nature when perturbed in some fashion that momentarily upsets the balance with the compensating surface charge.

External links

- [http://www.planewave.de/icp/atoms/atoms.sgml-7.html Appendix A from the manual for Atoms, software for XAFS]
- [http://dave.ucsc.edu/myrtreia/crystal.html Intro to Minerals: Crystal Class and System]
- [http://www.rockhounds.com/rockshop/xtal/index.html Introduction to Crystallography and Mineral Crystal Systems]
- [http://www.ece.byu.edu/cleanroom/EW_orientation.phtml Crystal planes and Miller indices]
- [http://www.ibiblio.org/e-notes/Cryst/Cryst.htm Interactive 3D Crystal models] Category:Chemical properties Category:Condensed matter physics Category:Crystallography Category:Materials science Category:Mineralogy ja:結晶構造

Gram

:For other uses of the words gram or gramme, see gram (disambiguation). The gram or gramme, symbol g, is a unit of mass. It is defined as one one-thousandth of the SI base unit kilogram (i.e., 1×10⁻³ kg). Its name derives from the Greek/Latin root grámma.

History

It was the base unit of mass in the original French metric system and the later centimetre-gram-second (CGS) system of units.

Uses

The gram is today the most widely used unit of measurement for non-liquid ingredients in cooking and grocery shopping worldwide. For food products that are typically sold in quantities far less than 1 kg, the unit price is normally given per 100 g. Most standards and legal requirements for nutrition labels on food products require relative contents to be stated per 100 g of the product, such that the resulting figure can also be read as a percentage.

Conversion factors

- 1 grain = 0.06479891 gram
- 1 ounce (avoirdupois) = 28.349523125 grams
- 1 ounce (troy) = 31.1034768 grams

Joule

The joule (symbol: J) is the SI unit of energy, or work. It is named in honour of the physicist James Prescott Joule (1818–1889).

Definition

The joule is a derived unit defined as the work done, or energy required, to exert a force of one newton for a distance of one metre, so the same quantity may be referred to as a newton metre or newton-metre (also with meter spelling), with the symbol N·m or N m. It can also be written as kg·m²·s⁻². However, the newton metre is usually used as a measure of torque, not energy. One joule is also:
- The work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt, with the symbol C·V.
- The work done to produce power of one watt continuously for one second; or one watt second (compare kilowatt-hour), with the symbol W·s

Conversions

1 joule is exactly 10⁷ erg. 1 joule is approximately equal to:
- 6.241506363 eV (electron-volts)
- 0.239 cal (calorie) (small calories)
- 2.390 Calorie or kilocalorie (food)
- 9.48 BTU (British thermal unit)
- 0.738 ft·lbf (foot pound force)
- 23.7 ft·pdl (foot poundals)
- 2.7778 kilowatt-hour
- 2.7778 watt-hour
- 9.8692 litre-atmosphere
- the energy required to lift a small apple (102 g) one metre against Earth's gravity Units defined in terms of the joule include:
- 1 thermochemical calorie = 4.184 J (exact)
- 1 International Table calorie = 4.1868 J (exact)
- 1 watt-hour = 3600 J (exact)

Mole unit

The mole (symbol: mol) is the SI term identifying the number of particles in a given amount of matter. It is a dimensionless quantity (meaning a number without units) numerically equal to Avogadro's number.

Definition

The formal definition of the mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 12 grams of carbon 12, where the carbon 12 atoms are unbound, at rest and in their ground state. The number of atoms in 0.012 kilogram of carbon 12 is known as Avogadro's number. It is approximately 6.0221415 (2002 CODATA value). A mole is a dimensionless name for an integer, much like dozen or googol. Although the exact value of the mole is not known at present, it is equal to Avogadro's number, which is known to 1 part in 10 million. Because of the relationship of the atomic mass unit to Avogadro's number, a practical way of stating this for atoms or molecules is: That amount of the substance containing exactly the same number of grams as the number of the atomic weight of the substance. Since iron, for example, has an atomic weight of 55.845, there are 55.845 grams in a mole of iron.

Elementary entities

When the mole is used to specify the amount of a substance, the kind of elementary entities (particles) in the substance must be identified. The particles can be atoms, molecules, ions, formula units, electrons, or other particles. For example, one mole of water is equivalent to about 18 grams of water and contains one mole of H₂O molecules, but three moles of atoms (two moles H and one mole O). When the substance of interest is a gas, the particles are usually molecules. However, the noble gases (He, Ar, Ne, Kr, Xe, Rn) are all monoatomic, that is each particle of gas is a single atom. All gases have the same molar volume of 22.4 litres per mole at STP (see Avogadro's Law). A mole of atoms or molecules is also called a "gram atom" or "gram molecule".

History

The name mole is attributed to Wilhelm Ostwald who introduced the concept in the year 1902. He used it to express the gram molecular weight of a substance. So, for example, 1 mole of hydrochloric acid (HCl) has a mass of 36.5 grams (atomic weights Cl: 35.5 u, H: 1.0 u). Prior to 1959 both the IUPAP and IUPAC used oxygen to define the mole, the chemists defining the mole as the number of atoms of oxygen which had mass 16 g, the physicists using a similar definition but with the oxygen-16 isotope only. The two organizations agreed in 1959/1960 to define the mole as such: The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is "mol." This was adopted by the CIPM (International Committee for Weights and Measures) in 1967, and in 1971 it was adopted by the 14th CGPM (General Conference on Weights and Measures) In 1980 the CIPM clarified the above definition, defining that the carbon-12 atoms are unbound and in their ground state.

Utility of moles

The mole is useful in chemistry because it allows different substances to be measured in a comparable way. Using the same number of moles of two substances, both amounts have the same number of molecules or atoms. The mole makes it easier to interpret chemical equations in practical terms. Thus the equation: :2H₂ + O₂ = 2H₂O can be understood as "two moles of hydrogen plus one mole of oxygen yields two moles of water." Moles are useful in chemical calculations, because they enable the calculation of yields and other values when dealing with particles of different mass. Number of particles is a more useful unit in chemistry than mass or weight, because reactions take place between atoms (for example, two hydrogen atoms and one oxygen atom make one molecule of water) that have very different weights (one oxygen atom weighs almost 16 times as much as a hydrogen atom). However, the raw numbers of atoms in a reaction are not convenient, because they are very large; for example, just one mL of water contains over 3 × 10²² (or 30,000,000,000,000,000,000,000) molecules.

Example calculation

In this example, moles are used to calculate the mass of CO₂ given off when 1 g of ethane is burnt. The equation for this chemical reaction is: :7 O₂ + 2 C₂H₆ → 4 CO₂ + 6 H₂O Here, 7 moles of oxygen react with 2 moles of ethane to give 4 moles of carbon dioxide and 6 moles of water. Notice that the number of moles does not need to balance on either side of the equation. This is because a mole does not count mass or the number of atoms involved, simply the number of individual particles. In our calculation it is first necessary to work out the number of moles of ethane that has been burnt. The mass in grams of one mole of a substance is by definition its atomic or molecular mass. The atomic mass of hydrogen is 1, and the atomic mass of carbon is 12, so the molecular mass of C₂H₆ is (2 × 12) + (6 × 1) = 30. One mole of ethane is 30 g. The amount burnt was 1 g, or 1/30th of a mole. The molecular mass of CO₂ (the atomic mass of carbon is 12 and that of oxygen is 16) is 2 × 16 + 12 = 44, so one mole of carbon dioxide is 44 g. From the formula we know that :1 mole of ethane gives off 2 moles of carbon dioxide (because 2 give off 4). We also know the masses of a mole of both ethane and carbon dioxide, so :30 g of ethane gives off 2 × 44 g of carbon dioxide. It is necessary to multiply the mass of carbon dioxide by 2 because two moles are produced. However, we also know that just 1/30th of a mole of ethane was burnt. Again: :1/30th of a mole of ethane gives off 2 × 1/30th of a mole of carbon dioxide, so finally: :30 × 1/30 g ethane gives off 44 × 2/30 g of carbon dioxide = 2.93 g.

References

# [http://www.bipm.org/en/si/base_units/ Official SI Unit definitions]