For chemical reactions and phase transformations, the energy absorbed or liberated is measured as heat. The principal unit for reporting heat is the calorie, which is defined as the energy needed to raise the temperature of 1 gram of water at 14.5 degrees C by a single degree. The term kilocalorie refers to 1,000 calories. Another unit of energy is the joule (rhymes with school), which is equal to 0.239 calories. Conversely, a calorie is 4.184 joules. The translation of calories to joules, or kilocalories to kilojoules, is so common in chemical calculations that you should memorize the conversion factors.

If a substance is heated without a change of state, the amount of heat required to change the temperature of 1 gram by 1 degree C is called the specific heat capacity of the substance. Similarly, the molar heat capacity is the amount of heat needed to raise the temperature of 1 mole of a substance by 1 degree C.