Re: If hydrogen and oxygen are gases at room temp, why isn't water?

     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.

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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.