The measurement of the quantity of heat energy involved in processes such as chemical reactions, changes of state, and mixing of substances, or in the determination of heat capacities of substances. The unit of energy in the International System of Units is the joule, symbolized J. Another unit still being used, but is strongly discouraged, is the calorie, symbolized cal and defined as 4.184 J. Most calorimetric measurements are made at constant pressure, and the measured change is called the enthalpy change.