A fuel is a substance that reacts chemically with another substance to produce heat, or that produces heat by nuclear processes. The term fuel is normally limited to substances that burn readily in air or oxygen, emitting large quantities of heat. Fuels are used for heating, for the production of steam for heating and power purposes, for powering internal-combustion engines, and for a direct source of power in jet and rocket propulsion. In cases where a fuel must supply its own oxygen, as in many rockets and jet propulsion, an oxidizing agent such as hydrogen peroxide or nitric acid is added to the fuel mixture.Chemical reactions in the combustion of all ordinary fuels involve the combination of oxygen with any carbon, hydrogen, or sulphur present in the fuels. The end products are carbon dioxide, water, and sulphur dioxide. Other substances present in fuels do not contribute to the combustion but either are driven off in the form of vapour or remain after combustion in the form of ash. Fuel efficiency or heating value of a fuel is usually measured in terms of the thermal energy, or heat, evolved when a given amount of the fuel is burned under standard conditions.