Re: How is heat capacity of water related to hydrogen bonding?

You are right that hydrogen bonding is an important part of the properties of 
water, though I would not say it is *as* important (at least not directly) for 
the heat capacity as it is for many other properties.  For example, the heat of 
vaporization of water is quite high because you have to break the hydrogen bonds 
to get water from the liquid to the vapor phase, and that is a pretty big 
effect.

The heat capacity measures how much heat you have to put into a substance to get 
a unit rise in temperature.  For water, the hydrogen bonding weakens as the 
temperature goes up, so that some of the energy you are putting in is going into 
breaking hydrogen bonds instead of into raising the temperature.  This makes the 
heat capacity higher than it otherwise would be, but it isn't a huge effect.

The fact that you said "specific heat capacity" makes me think that I should 
explain one more thing.  Specific heat capacity is the amount of heat to raise a 
unit *mass* one temperature unit.  But what really matters at the molecular 
level is the *molar* heat capacity.  So water may look like it has a high 
*specific* heat capacity, but that is partly an artifact of its relatively low 
molecular weight compared to other things that are liquid at similar conditions. 
 If you were looking at molar heat capacities, there would not be as much 
difference.  An illustration of this would be D2O (heavy water, with the mass-2 
deuterium isotope of hydrogen), which has about the same amount of hydrogen 
bonding as H2O (actually slightly more) and about the same molar heat capacity, 
but because of the higher molecular weight D2O would have a lower specific 
heat capacity by a ratio of 18:20.

Finally, if you are interested in the role of water in life, I will mention a 
new book by Philip Ball called "Life's Matrix: A Biography of Water" which I've 
just started reading and which looks like a nice source of information at a 
level that would be good for a secondary school teacher.  You can also find some 
scientific information about water under the "FAQs about Water and Steam" 
section of the International Association for the Properties of Water and Steam 
(IAPWS) at www.iapws.org.

Allan Harvey, aharvey@boulder.nist.gov
"Don't blame the government for what I say, or vice-versa."