The principle that mass, or matter, can be neither created nor destroyed. According to the conservation of mass principle, reactions and interactions which change the properties of substances within a closed system leave the total mass within the system unchanged over time. For instance, when charcoal burns, the mass of all of the products of combustion, such as ashes, soot, and gases, equals the original mass of the charcoal and the oxygen with which it reacted (see Illustration). In this way, mass can change its location in space and its form, but the sum total mass at the beginning of the reaction or interaction must equal the mass at the end of the reaction or interaction. See also: Chemical reactor; Mass; Matter

Charcoals burning. According to the principle of the conservation of mass, assuming the burning charcoals and the oxygen enabling their burning are all within a closed system, the total amount of mass in the system prior to the charcoals burning will be the same as after the charcoals have completely burnt into ash, soot, and gases. (Credit: Lucian Coman/Hemera/Getty Images)

In order for the conservation of mass to truly hold, a system must be closed, meaning no transfers of mass or energy into the system can occur from outside the system. In most practical situations, however, including the charcoal example just given, systems are not closed in such a manner. Nevertheless, the principle of conservation of mass remains useful in many physical science fields, including chemistry and fluid mechanics. See also: Chemistry

The principle of the conservation of mass is inherent in classical mechanics, originally developed by English physicist and mathematician Isaac Newton in the late 1600s, and which accurately approximates the behavior of objects at macroscopic scales. The special theory of relativity, proposed by German-born U.S. physicist Albert Einstein in 1905, forces modifications to the conservation of mass, namely by extending the conservation to include energy because of mass-energy equivalency. The special theory shows that the mass of a body changes as the energy possessed by the body changes. Such changes in mass are routinely detected in high energy collisions of subatomic particles. See also: Classical mechanics; Collision (physics); Conservation of energy; Energy; Quantum mechanics; Relativity

The equivalence of matter and energy explains how mass (matter) may in fact be created or destroyed, in apparent defiance of the original conception of the conservation of mass. For instance, matter is created via the materialization of a virtual photon (a massless quantum of electromagnetic energy) into an electron-positron pair, which is a pair of matter and antimatter elementary particles possessing mass. Matter may also be destroyed when this pair of particles annihilates, producing a pair of photons. See also: Electron-positron pair production and annihilation; Photon; Relativistic mechanics

Vernon D. Barger