The addition of some solutes to a solvent will raise the temperature of the solution, while others may lower the temperature and still others will have no noticeable effect. This behavior depends on the heat of solution of the solute in the given solvent. The heat of solution, i.e., the amount of heat given off or absorbed during the process of solution, is equal to the difference between the energy that must be supplied to break up the crystals of the solute and the energy that is released when the solute particles are taken into solution by the solvent (see enthalpy). If the heat of solution is negative (i.e., more energy is required to break up the crystal than is released in forming the solution), then the temperature will decrease; if the heat of solution is positive, the temperature will increase.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.