Why does water have such a high specific heat capacity?

The important word in that question is "specific". That means per unit of mass; the specific heat capacity would be the amount of energy that must be added to a unit mass to raise the temperature by one degree. But what really matters in determining thermodynamic properties is molecules, not mass. The heat capacity per molecule is related (to a first approximation) to the number of ways it can store energy (translational, vibrational, rotational). On a per molecule (or per mole) basis, water's heat capacity would be about the same as for other bent triatomic molecules (at a similar vapor or liquid condition) such as H2S. But because of the low molar mass of water (18), the *specific* heat capacity is larger. NH3 (molecular weight 17) has even a slightly higher *specific* heat capacity than water at similar conditions, again mainly because of the low molar mass.

So there is nothing really peculiar about the specific heat capacity of water. It is higher than most other liquids, but that is mainly because a given mass of water contains more molecules (and therefore more degrees of freedom in which to store energy) than the same mass of other liquids.

This page updated November 21, 2000