The thermal energy, or temperature, of solid water does not change as it melts into liquid water. Instead, all the thermal energy absorbed from its environment is converted into potential energy through the breaking of the rigid hydrogen bonds in water ice. The thermal energy of boiling water, as well as condensing or freezing water, also remains constant as all transferred thermal energy goes from or to potential energy.
Pure materials in general do not change into thermal energy during phase changes. However, it holds true only with pure compounds or elemental substances. Solutions such as salt water do change into thermal energy as they undergo phase changes, both because of the different thermal properties of the dissolved particles and the changes in their concentration. For instance, salt water boils at a higher temperature than fresh water, and as it boils, the concentration of salt increases, raising the boiling temperature further.
The ability of substances to absorb heat as potential energy rather than increasing in temperature is useful for situations where insulation or other temperature control is needed, according to the University of Tennessee, Knoxville. In certain advanced structural walls, for instance, tiny cells of paraffin begin to melt in hot temperatures. This reduces and slows down the transmission of heat because while it melts, the wall does not increase in temperature.