Ask A Scientist Chemistry Archive

Question:  What does water expand when it gets cold, but most other 
things contract?
Skeim

Answer:  I just saw your question posted, so sorry for any delay.  Water is 
a very unique chemical.  It is its ability to extensively hydrogen bond to itself 
that causes things to get strange.  For a molecule of molecular weight of
only 18, it should technically be a gas at room temperature (consider 
hydrogen sulfide H2S: mw = 32 and it's a gas).  Anyway, when water is
cooled to its freezing point, the molecules are ordered into a 3D 
structure.  Technically, water does contract when its cooled.  It has
a maximum density at 4 degrees C.  As it continues to cool to 0 degrees C
the molecules take on a 3D structure that contains lots of empty space
between the molecules.  This accounts for the lower density of ice and
the expanding nature of the solid.  It's a good thing ice is less dense
than liquid water, too.  All the freshwater lakes and rivers would
freeze from the bottom up, killing everything in the process!
-Joe Schultz