Molal heat capacity at constant pressure is
                                                                                         (9)

                                                                   (10)   
 

Also change in heat content can be determined by mean heat capacity C_m

                                                                                     (11)   

                                                                                                    (12)

We may write

                                                                                    (13)

Equation 13 may be used to calculate heat content for rapid calculations. More accurate would be to use equation 8 or equations 9 and 10.

1. Changes in state of aggregation

When solid is heated to melting point additional heat must be supplied to melt it. At the melting print, this additional heat does not increase the temperature. The heat content for melting at constant pressure is called latent heat of fusion  and this heat must be supplied at the melting point to transform solid into liquid. An equal quantity of heat is liberated during solidification so that

                                                                        (14)

Negative sign indicates liberation of heat. Similarly, heat effects accompanying evaporation and allotropic changes in solids are measured by latent heat of vaporization and heats of transformation.

Value of  for changes in state of aggregation vary with temperature and pressure under which change is carried out. For example heat of vaporization of water at 1000C is 542cal/g, whereas it is 583 cal/g at 25⁰C.

Example
Calculate latent heat of fusion of Cu
The melting point of Cu is 1357K.

For solid

                                       at melting point

For liquid

                                       at melting point

Therefore