This Demonstration shows the heating or cooling effects achieved by salt packs. The chemical processes involved in producing efficient heat and cold packs depend on the enthalpy of dissolution of the salts and the specific heat of water. We investigate the theoretical effect of adding various amounts of salt to one kilogram of water at an initial temperature of 25 °C. The relevant formulas are and mol, where is the heat transferred (in J), is the mass of salt (in g), is the specific heat (in J/g K), is the enthalpy change in the solute (J/mol) and is the resultant temperature change (K). You can select among several salts and specify the number of moles added to determine the final temperature of the water in the heat or cold pack.