Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Fuel, substance that reacts chemically with another to produce heat, or that produces heat by nuclear processes. The term fuel is generally limited to those 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 torpedoes, 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 sulfur present in the fuels. The end products are carbon dioxide, water, and sulfur dioxide. Other substances present in fuels do not contribute to the combustion but either are driven off in the form of vapor or remain after combustion in the form of ash.

Fuel efficiency or heating value of a fuel is usually measured in terms of the number of Btu (see British Thermal Unit) that are produced when a given amount of the fuel is burned under standard conditions. Heating values for solid and liquid fuels are stated in terms of Btu per lb, and values for gases in Btu per cu ft. A distinction is sometimes made between higher heating value, the entire heat evolved during combustion, and lower heating value, the net heat evolved, with allowance for the heat lost in the vaporization of the water produced by combustion. Approximate higher heating values of common fuels are: Solid fuels (Btu per lb): coal 12,000 to 15,000; lignite 6000 to 7400; coke 12,400; dry wood 8500. Liquid fuels: alcohol 11,000; fuel oil 19,000; gasoline 20,750; kerosene 19,800. Gaseous fuels (Btu per cu ft): acetylene 1480; blast-furnace gas 93; carbon monoxide 317; coke-oven gas or coal gas about 600; hydrogen 319; natural gas 1050 to 2220; oil gas 516; producer gas 136. See separate articles on most of these fuels.

See also Fuel Gases; Gasohol; Nuclear Energy; Petroleum; Solar Energy.