If we consider a sample of water in the solid phase at -25 ūC and we add heat it is interesting to plot the change in heat (enthalpy) versus temperature (see plot on left). Some interesting features are apparent in such a plot.

As heat is added to the solid at -25 ūC it begins to warm. The added heat changes the temperature of the solid. The slope of the line is the specific heat of ice. For ice the specific heat is 2.09 J·g-1C-1 . When the temperature of the sample reaches 0ūC, the melting point of water, the added heat is used to break down the attractive forces which maintain the solid phase. The amount of heat required to melt 1 gram of water (solid to liquid) is 0.334 kJ·g-1 . After the sample has melted the added heat changes the temperature. The change in temperature depends on the amount of substance and the amount of heat added. The specific heat for water in the liquid phase is 4.184 J·g-1C-1. The temperature continues to increase as heat is added until the boiling point of the liquid is reached. At that temperature the added heat is again required to change the phase of the sample. The temperature remains constant until all of the sample is vaporized. For water the amount of heat required to vaporize water is 2.26 kJ·g-1. It is interesting that it requires so much more heat to vaporize a liquid, compared to melting a solid. After converting all the liquid water at 100 C to water vapor at 100 C we need to heat the sample to 110 C. The specific heat of water in the vapor phase is 1.84 J·g-1C-1.

The total amount of heat required to change the temperature of a sample of a compound is just the sum of the heat used in each stage of the process.

Lets look at a sample problem...