Heat of fusion

Heat of fusion is the amount of heat energy which must be absorbed or lost for 1 gram of a substance to change states from a solid to a liquid or vice versa. It is also called the latent heat of fusion or the enthalpy of fusion, and the temperature at which it occurs is called the melting point.

Related Topics:
Temperature - Melting point

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When you withdraw thermal 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 For them to maintain the order of a solid, extra heat must be withdrawn.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.

Related Topics:
Thermal energy - Temperature - Melting point - Molecule

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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.

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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 The energy of the heat of fusion must be withdrawn (the liquid must turn to solid) before the temperature can continue to fall.

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The units of heat of fusion are usually expressed as

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: joules per mole (the SI units)

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or

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: calories per gram (old metric units now little used, except for a different, larger calorie used in nutritional contexts)

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or

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: British thermal units per pound or Btu per pound-mole

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• Note: These are not the calories found in food. The calories found in food are kilogram calories, equal to 1000 small calories. 1000 calories = 1 kilocalorie = 1 food calorie. Food calories are sometimes abbreviated as kcal as if small calories were being used, while calories are abbreviated as cal. Another distinguishing method, though often confusing, uses capitalization. A Calorie is a food calorie, or 1000 calories. So 1 Cal = 1000 cal.
 
Heat of fusion: Reference Values ►