Heat of combustion ("Δ"H"°") is the measure of the amount of energy released in the form of heat ("q") when one mole of a substance is burned (combustion). The production of heat means the reaction is an exothermic process and gives off energy. Heat of combustion is a specialized form of reaction enthalpy because it is measured at standard conditions and is limited to one mole starting material. The (°) symbol shows that the heat of combustion value is obtained at standard conditions: 25 degrees Celsius (298.15 Kelvin) and at a constant pressure. The pressure is reported both at either one bar or at one atmosphere depending on the source. 