The Heat of Fusion is the energy needed to turn 1 g of solid at the melting point into 1 g of liquid at the melting point (no temperature change). The heat of fusion is a form of internal potential energy. It is the energy that was required to break the bonds that held the atoms/molecules of the substance together as a solid. Since this energy does not contribute to the speed of atomic motion, it increases the internal energy of the substance (more atomic potential energy), but does not change the temperature of the substance (same atomic kinetic energy).

Every substance has its own heat of fusion. Water’s heat of fusion is 80 cal/gram, which is much higher than most other substances.

Q: How much energy is needed to turn 35 g of frozen water at 0 ^o C into 35 g of liquid

water at 0 ^o C?

A: Since water’s heat of fusion is 80 cal/gram, we get the following:

35 g ´ 80 cal = 2800 cal

g

If melting absorbs heat energy (80 cal/gram for water), then freezing releases the same amount of energy. Because water molecules release potential energy when returning to a solid state, the temperature of a cloud goes up when snow particles begin to form. Every gram of snow that forms releases 80 cal of energy into the cloud.

The Heat of Vaporization is the energy needed to turn 1 g of a liquid at the into 1 g of gas. The heat of vaporization is a form of internal potential energy. It is the energy that was required to pull the atoms/molecules of the substance away from each other. Since this energy does not contribute to the speed of atomic motion, it increases the internal energy of the substance (more atomic potential energy), but does not change the temperature of the substance (same atomic kinetic energy).

Every substance has its own heat of vaporization.

Water’s heat of vaporization is 540 cal/gram, which is much higher than most other substances.

Water’s high heat of vaporization explains why a steam burn is much worse than a water burn. If 1g of water at 100^o C gets on your skin, it will cool down to the temperature of your body (37^o C), a change of 63^o C, which means it needs to release 63 calories of heat into your skin. But 1 g of steam must first release its heat of vaporization (540 cal) in order to turn from a gas at 100^o C into a liquid at 100^o C, and then release the other 63 cal it takes for the liquid to cool from 100^o C down to 37^o C, for a total of 603 calories! Ouch!

Below is a heating curve, showing the change in the temperature of one gram of water as it absorbs heat energy. The lower “plateau” is the melting point, and the upper “plateau” is the boiling point. All pure substances will produce a curve like this when heated, though the melting points, boiling points, and heats of fusion and vaporization will be different for each substance.