heat of fusion

[′hēt əv ′fyü·zhən]

(thermodynamics)

The increase in enthalpy accompanying the conversion of 1 mole, or a unit mass, of a solid to a liquid at its melting point at constant pressure and temperature. Also known as latent heat of fusion.

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Heat of Fusion

(or latent heat of fusion), the amount of heat that must be supplied to a substance in an equilibrium constant-pressure and constant-temperature process to convert the substance from the solid (crystalline) state to the liquid state. The same amount of heat is liberated when the substance crystallizes.

The heat of fusion is a special case of the heat of a first-order transition. For a given substance, the heat of fusion may be determined per unit mass or per mole. In the former case, the heat of fusion is measured in, for example, joules per kg (J/kg) or kilocalories

Table 1. Heat of fusion of several substances
Substance	t_m(°C)	L_f(kcal/kg)	L_f(J/kg)
Hydrogen ...............	–259.1	13.89	58,200
Nitrogen ...............	–209.86	6.09	25,500
Mercury ...............	–38.89	2.82	11,800
Ice ...............	0	79.4	333,000
Tin ...............	231.9	14.4	60,300
Lead ...............	327.4	5.89	24,700
Copper ...............	1083	48.9	205,000
Iron ...............	1539	65	272,000

per kg (kcal/kg). In the latter case, the heat of fusion may be expressed in joules per mole. The term “molar heat of fusion” is sometimes applied to the heat of fusion per mole. Table 1 gives the values of the heat of fusion per kg L_f for several substances at atmospheric pressure (760 mm Hg, or 101,325 newtons per m²) and at the melting point t_m.

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