Specific Heat Capacity at the Molecular Level
by Roger Tobin
The heat capacity of a material is a measure of the amount of heat energy that needs to be added to a given mass of the material in order to raise its temperature by a given amount. It depends primarily on three things:
- How many molecules does it take to make that much mass? The more molecules per gram, the more energy will be required.
- How complicated are the molecules? The more ways the molecule has of vibrating and rotating, the more energy it can hold at a given temperature, and the more energy is required to raise its temperature by a given amount.
- How free are the molecules to move? In a liquid or a gas, molecules are relatively unconstrained. In a solid they are held much more tightly, removing some of the modes of vibration or rotation, so that the molecules act like simpler ones and have lower heat capacity.
Water is a relatively light molecule – lighter, even, than the molecules of nitrogen and oxygen that make up the air – so a gram of water contains more molecules than a gram of most materials. It is also a relatively complicated molecule, made up of three atoms in a bent geometry, which means that, for its size, it has a lot of different ways of vibrating and rotating. These two factors contribute to the high heat capacity of water relative to most other common materials. Ice has about half the heat capacity of liquid water, even though the molecules are the same, because in the solid state the molecules are not as free to move.