MadSci Network: Chemistry |
At the surface of water at room temperature in an open container, molecules of liquid water are continually escaping the liquid state and enteriing the vapor (gaseous) state. In time, all of the water, say in an open beaker, will evaporate. The molecules at the surface have enough energy of motion (kinetic energy) to overcome the forces of attraction that exist between molecules and escape, while the molecules that are not at the surface (being surrounded by other molecules) are subject to forces between molecules called dipole-dipole forces. Because of the high attraction of oxygen atoms for shared electron pairs (it is said to be highly electronegative) and the small size of hydrogen atoms (which allows close approach of molecules), these dipole-dipole forces in water are quite strong. They are given the special name 'hydrogen bonding.' This intermolecular force, hydrogen bonding, is largely responsible for water being mostly a liquid at room temperature. Reference: Zumdahl, Steven S. and Susan A., CHEMISTRY, 5th Ed., Houghton- Mifflin, NY (ISBN 0-395-98581-1), pp. 451-458. ------- Admin note: this answer has supposed that you are asking specifically about why water is not a gas, and not a more general question about why chemical compounds are usually quite different physically from the simple substances they are made from: e.g. white unreactive crystals of common salt, sodium chloride, from a soft reactive metal and a poisonous yellow-green gas. John C.
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