According to the dictionary a phase is "a homogeneous, physically distinct, and mechanically separable portion of matter present in a nonhomogeneous physical-chemical system". For our purposes we can consider three phases of matter, solid, liquid, and gaseous.

Phase Changes

• Vaporization 

Vaporization is the conversion of a liquid to a gas.  Vaporization is an endothermic process, since energy must be supplied to overcome the intermolecular forces that hold molecules together as a liquid.  The difference in enthalpy per mole of molecules between the gaseous and liquid states of a substance is called the enthalpy of vaporization, H_vap.

• Fusion (Melting) 

Fusion is the conversion of a solid to a liquid.  Fusion, like vaporization, is always an endothermic process, since energy must be supplied  to overcome the intermolecular forces that hold molecules together as a solid.  The difference in enthalpy per mole of molecules between the liquid and solid states of a substance is called the enthalpy of fusion, H_fus.  Since the change from a solid to a liquid, is not as drastic as the change from a liquid to a gas, enthalpies of fusion are smaller than enthalpies of vaporization for the same substance.

• Sublimation 

Sublimation is the conversion of a solid directly to a gas without going through the liquid state.  Sublimation, like fusion and vaporization, is always an endothermic process, since energy must be supplied  to overcome the intermolecular forces that hold molecules together as a solid.  The difference in enthalpy per mole of molecules between the gaseous and solid states of a substance is called the enthalpy of sublimation, H_sub.  Since the change from a solid to a gas is a larger change than vaporization or fusion, enthalpies of sublimation are larger than enthalpies of vaporization or fusion for the same substance. In fact the enthalpy of sublimation is the sum of the enthalpies of vaporization and fusion.

The three phase changes described above are reversible.  The enthalpy of the reverse phase change will have the same value but opposite sign.  For example the enthalpy of fusion of water is +6.0 kJ · mol^-1 and its enthalpy of freezing, the reverse of fusion, is -6.0 kJ · mol^-1.  As is the case with chemical reactions, the enthalpies of phase changes can be determined experimentally with a calorimeter.  Also as with chemical reactions if we know the enthalpies of formation of initial and final substances we can calculate the enthalpy change for the process.  To illustrate this consider the following example.

Use values of H^o_f  to calculate the enthalpy of vaporization of water at 25 ^oC. 

Solution

The process is          H₂O(l) ==> H₂O(g)

H^o_vap = n H^o_f (products) - m H^o_f (reactants) =  H^o_f [H₂O(g)] -  H^o_f [H₂O(l)] = [(-241.8) - (-285.8)]kJ = 44.0 kJ