Explain the following:
(i) Enthalpy of fusion
(ii) Enthalpy of vaporization
(iii) Enthalpy of sublimation.

(i) Enthalpy of fusion: It is the enthalpy change accompanying the conversion of one mole of a solid substance into the liquid state at its melting point. For example, when one mole of ice changes into the water at its melting point (273 K), 6.0 kJ of heat is absorbed (endothermic change).

(ii) Enthalpy of vaporisation: It is the enthalpy change accompanying the conversion of one mole of liquid into its vapours (gas) at its boiling point. For example, when one mole of water is converted into steam (gas) at 373 K, the enthalpy change accompanying the process is 40.6 kJ.

(iii) Enthalpy of sublimation: Sublimation is a process of direct conversion of solid into its vapours below its melting point. It is the enthalpy change accompanying the conversion of 1 mole of a solid directly into a gaseous state at the temperature below its melting point. For example; when 1 mole of iodine sublimes, the enthalpy change accompanying the process is +62.4 kJ.

The enthalpy of sublimation of a solid is equal to the sum of enthalpy of fusion and enthalpy of vaporisation.