Thermochemistry is the branch of physical chemistry that deals with heat changes accompanying chemical reactions. Practical thermochemistry is mainly by calorimetry, which yields standard heats of reaction or enthalpy values (ΔH 0). If this is negative, the reaction is termed exothermic (heat-producing); if positive, endothermic (heat-absorbing). Hess' law, or the law of constant heat summation, states that the overall heat change in a chemical reaction is the same whether it takes place in one or several steps. Thus, by algebraic addition of chemical equations and their ΔH 0 values, inaccessible heats of reaction may be calculated, including the heat of formation of a compound, which is the heat change when one mole of the compound is formed from its constituent elements in their standard states.