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pure substance: A substance— either an element or compound—that has an unvarying composition. This means that by changing the proportions of atoms, the result would be an entirely different substance. Compare with mixture.
qualitative: Involving a comparison between qualities that are not precisely defined, such as “fast” and “slow” or “warm” and “cold.”
quantitative: Involving a comparison between precise quantities—for instance, 10 lb vs. 100 lb, or 50 MPH vs. 120 MPH.
saturated: A qualitative term describing a solution that contains as much solute as it can dissolve at a given temperature.
sdlubility: In a broad, qualitative sense, solubility refers to the property of being soluble. Chemists, however, usually
apply the word in a quantitative sense, to indicate the maximum amount of a substance that dissolves in a given amount of solvent at a specific temperature. Solubility is usually expressed in grams of solute per 100 g of solvent.
soluble: Capable of dissolving in a solvent.
sdlute: The substance or substances that are dissolved in a solvent to form a solution.
sdlutidn: A homogeneous mixture in which one or more substances (the solute) is dissolved in a solvent—for example, sugar dissolved in water.
solvent: The substance or substances that dissolve a solute to form a solution.
surfactant: A substance made up of molecules that are both water- and oil-soluble, which acts as an agent for joining other substances in an emulsion.
unsaturated: A term describing a solution that is capable of dissolving additional solute, if that solute is introduced to it.
um chromate (K2Cr04) is added to an aqueous solution of barium nitrate (Ba[N03]2 to form solid barium chromate (BaCr04) and a solution of potassium nitrate (KN03). This reaction is described in the essay on Chemical Reactions.
We have primarily discussed liquid solutions, and in particular aqueous solutions. It should be stressed, however, that solutions can also exist in the gaseous or solid phases. The air we breathe is a solution, not a compound: in other words, there is no such thing as an “air molecule.” Instead, it is made up of diatomic ele-
ments (those in which two atoms join to form a molecule of a single element); monatomic elements (those elements that exist as single atoms); one element in a triatomic molecule; and two compounds.
The “solvent” in air is nitrogen, a diatomic element that accounts for 78% of Earth’s atmosphere. Oxygen, also diatomic, constitutes an additional 21%. Argon, which like all noble gases is monatomic, ranks a distant third, with 0.93%. The remaining 0.07% is made up of traces of other noble gases; the two compounds men-
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tioned, carbon dioxide and water (in vapor form); and, high in the atmosphere, the triatom-ic form of oxygen known as ozone (03).
The most significant solid solutions are alloys of metals, discussed in the essay on Mixtures, as well as in essays on various metal families, particularly the Transition Metals. Some well-known alloys include bronze (three-quarters copper, one-quarter tin); brass (two-thirds copper, one-third zinc); pewter (a mixture of tin and copper with traces of antimony); and numerous alloys of iron—particularly steel—as well as alloys involving other metals.
WHERE TO LEARN MORE
“Aqueous Solutions” (Web site). <http://www.tannerm. com/aqueous.htm> (June 6, 2001).
Gibson, Gary. Making Things Change. Brookfield, CT: Copper Beech Books, 1995.
Hauser, Jill Frankel. Super Science Concoctions: 50 Mysterious Mixtures for Fabulous Fun. Charlotte, VT: Williamson Publishing, 1996.
“Introduction to Solutions” (Web site). <http://edie.
cprost.sfu.ca/~rhlogan/solintro.html> (June 6, 2001).
Knapp, Brian J. Air and Water Chemistry. Illustrated by David Woodroffe. Danbury, CT: Grolier Educational, 1998.
Seller, Mick. Elements, Mixtures, and Reactions. New York: Shooting Star Press, 1995.
“Solubility ” Spark Notes (Web site). <http://www.
sparknotes.com/chemistry/solutions/solubility/terms. html> (June 6, 2001).
“Solutions and Solubility” (Web site). <http://educ. queensu.ca/~science/main/concept/chem/cl0/cl01a mol.htm> (June 6, 2001).
Watson, Philip. Liquid Magic. Illustrated by Elizabeth Wood and Ronald Fenton. New York: Lothrop, Lee, and Shepard Books, 1982.
Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.
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The term osmosis describes the movement of a solvent through a semipermeable membrane from a less concentrated solution to a more concentrated one. Water is sometimes called “the perfect solvent,” and living tissue (for example, a human being’s cell walls) is the best example of a semipermeable membrane. Osmosis has a number of life-preserving functions: it assists plants in receiving water, it helps in the preservation of fruit and meat, and is even used in kidney dialysis. In addition, osmosis can be reversed to remove salt and other impurities from water.