For this heating curve, energy was added at a constant rate. This is obvious in regions where the temperature steadily increases (AB ; CD ; EF). In these regions, the energy added is used to increase the temperature. The question is, what is happening to the energy being added in regions BC and DE. From the Law of Conservation of Energy we know that energy can never be created or destroyed, i.e. it is not just disappearing!

From our earlier work with heating and cooling curves, we also know that in interval BC the ice is melting, and in interval DE the water is boiling. It appears that even though energy is continually being added, the temperature remains constant during phase changes. The reason the temperature remains constant is because the energy that is being added is used to change state, rather than increase temperature. We say that the H2O molecules are gaining potential energy (i.e. they are increasing their potential to do work) as they change from solid to liquid, and then again as they change from liquid to gas.

In fact, to change 1 g of ice into 1 g of water at 0 C, it takes 335 J of energy. The amount of energy required to melt one gram of solid at its melting point is called its heat of fusion. Therefore, the heat of fusion for ice is 335 J/g. The reverse of this process is solidification (freezing). The heat of solidification is the amount of heat given up as one gram of liquid changes to solid at its melting point. Because melting and freezing are reverse processes, the magnitudes (sizes) of the heat of fusion and heat of solidification are identical.

Similar energy changes take place when a liquid changes to a gas. The number of joules required to change one gram of liquid into gas at the normal boiling point is known as the liquids heat of vaporization. When a vapor condenses to a liquid the opposite heat transfer occurs. The heat of vaporization is passed to anything on which the vapor condenses.

The heat released when one gram of a gas condenses to a liquid at the normal boiling point is known as the heat of condensation. Since vaporization and condensation are reverse processes, the heat of vaporization of a substance equals its heat of condensation. The heat of vaporization for water is 2260 J/g.

Sample Problem: How much energy is necessary to convert 10. g of ice at o C to steam at o C? Given: c ice = 2.06 J/g o CH fusion = 334 J/g c water = 4.18 J/g o CH vaporization = 2260 J/g c steam = 2.02 J/g o C