Heat of fusion

Heat of fusion is the heat energy which must be withdrawn to solidify a liquid or added to melt a solid.

When you withdraw heat energy from a liquid or solid, the temperature falls. When you add heat energy the temperature rises. However, at the transition point between solid and liquid (the melting point), extra energy is required (the heat of fusion). To go from liquid to solid, the molecules of a substance must become more ordered. For them to maintain the order of a solid, extra heat must be withdrawn. In the other direction, to create the disorder from the solid crystal to liquid, extra heat must be added.

The heat of fusion can be observed if you measure the temperature of water as it freezes. If you plunge a closed container of room temperature water into a very cold environment (say −20 °C), you will see the temperature fall steadily until it drops just below the freezing point (0 °C). The temperature then rebounds and holds steady while the water crystalizes. Once completely frozen, the temperature will fall steadily again.

The temperature stops falling at (or just below) the freezing point due to the heat of fusion. The energy of the heat of fusion must be withdrawn (the liquid must turn to solid) before the temperature can continue to fall.

The units of heat of fusion are usually expressed as

joules per mole (the SI units)

or

calories per gram (old metric units now little used outside of nutritional contexts)

or

British thermal units per pound or Btu per pound-mole

• Note: These are not food calories. Food calories are kilocalories (1000 calories). 1000 calories = 1 Kilocalorie = 1 food calorie.

Reference Values

The heat of fusion of water is

79.72 calories per gram

or 333 548.48 joules per kilogram

The heat of fusion of a few more common substances: Given in calories per gram.

These values are from the CRC Handbook of Chemistry and Physics, 62nd edition.

Application

To heat one kilogram (about 1 liter) of cool water 20 °C from 10 °C to 30 °C requires 20 kcal.
However, to melt ice and raise the resulting water temperature 20 °C requires extra energy. To heat ice from 0 °C to water at 20 °C requires:

(1) 80 cal/g (heat of fusion of ice) = 80 kcal for 1 kg

PLUS

(2) 1 cal/(g·°C) = 20 kcal for 1 kg to go up 20 °C

= 100 kcal

See also

Heat of vaporization