What is latent heat of fusion?

The of fusion is the amount of thermal (see note below) required to cause a a liquid to freeze (by taking that much heat out) or a solid to melt (by putting that much heat in).

For example, the latent heat of fusion for water (ice) is about 334 kJ/mol. Find the amount of heat required to melt 36.0 g of ##H_2O## that is at 0 degrees C.

First of all, the latent heat is ONLY applied when the substance is pure and is already at the temperature of the phase transition (for example, 0 deg C to melt ice, or 100 deg C to boil water).

Notice that the units are kJ/mol, but we were given grams of water, so we must convert, so we will make this a dimensional analysis problem. ##(36.0gH2O)/1 * (1molH_2O)/(18.0g) * (334kJ)/(1molH2O)=668kJ##

The 18.0g/mol came from adding up the atomic masses to find the molar mass of water.

Notice that the units in the step before cancel until we are left with kJ, which are a unit of heat.

Note: Latent heat of fusion is actually the total amount of enthalpy (a kind of energy) necessary to accomplish a phase change for a solid or liquid or gas at the freezing/melting point.

Phase changes are generally considered at constant , rather than constant volume. Because a kg of say, 0°C water, occupies less volume than a kg of 0°C ice, some has to be done to push the environment out of the way as that water expands to become ice.

Enthalpy ##H## is a quantity which includes the energy ##U## that goes into the substance at the new phase, plus the work, ##W=P\DeltaV##, to expand or contract the substance into the new phase at the same pressure:

##H = U + P\DeltaV##,

where ##P## is the ambient pressure, ##\Delta V## is the change in volume, and ##U## is the internal energy of the substance in its new phase. Latent heat of fusion can also be called the enthalpy of fusion.