HEAT OF FUSION

It takes a certain amount of energy to change a sample of solid matter to its liquid state, assuming that the matter is of the sort that can exist in either of these two states. (Water, glass, most rocks, and most metals fill this bill, but wood does not.) In the case of ice formed from pure water, it takes 80 cal to convert 1 g of ice at 0°C to 1 g of pure liquid water at 0°C. This quantity varies for different substances and is called the heat of fusion for the substance. In the reverse scenario, if 1 g of pure liquid water at 0°C freezes completely solid and becomes ice at 0°C, it gives up 80 cal of heat. The heat of fusion is thus expressed in calories per gram (cal/g). It also can be expressed in kilocalories per kilogram (kcal/kg) and will yield exactly the same numbers as the cal/g figures for all substances. When the substance is something other than water, then the freezing/melting point of that substance must be substituted for 0°C in the discussion.
Heat of fusion is sometimes expressed in calories per mole (cal/mol) rather than in calories per gram. However, unless it is specifically stated that the units are intended to be expressed in calories per mole, you should assume that they are expressed in calories per gram. If the heat of fusion (in calories per gram) is symbolized hf, the heat added or given up by a sample of matter (in calories) is h, and the mass of the sample (in grams) is m, then the following formula holds:
h_f = h/m