Water has a remarkably high
boiling point
for a substance with such small molecules. In order for a substance to boil, the molecules of the liquid must have enough energy to overcome the forces of attraction between them. Generally, boiling points of related compounds increase with molar mass. When the boiling points of the hydrogen compounds (hydrides) of Group VI elements, H2O, H2S, H2Se, and H2Te are plotted versus molar mass, water is far out of line with the heavier compounds. If water followed the trend for the two heaviest hydrides, its boiling point would be about −90 C. The boiling point of water is 100 C, which is 190 C above the extrapolated value. It requires a high temperature to give water molecules enough kinetic energy to overcome the extensive hydrogenbonded network among them. For similar reasons, water also has a higher melting point than would be expected for its low molar mass. The H2S molecule is only slightly polar, but enough to make its boiling point a bit higher than predicted by the extrapolation from the non-polar heaviest hydrides.