A property is any characteristic of a system that can be given a numerical value without regard to the history of the system. Properties are classified as either intensive or extensive.
An intensive property has a value at a point, and its value is independent of the extent or size of the system. The value of an intensive property may vary with time and its position within the system. Examples of intensive properties include temperature, velocity, mass density, specific volume, and specific energy.
An extensive property does not have a value at a point, and its value depends on the extent or size of the system. The amount of an extensive property for a system can be determined by summing the amount of the extensive property for each subsystem within the system. Examples of extensive properties include mass, charge, linear momentum, volume, and energy.
Empirical evidence as codified by science has identified a class of extensive properties that can neither be created nor destroyed. An extensive property that satisfies this requirement is called a conserved property. When the accounting principle is developed for a conserved property, both the generation rate and the consumption rate are identically zero. We typically consider five conserved properties in engineering science: mass, energy, charge, linear momentum, and angular momentum. Another important property, entropy, is not conserved. Nonetheless, the accounting framework also provides a useful vehicle for analysis, since entropy can only be produced as a consequence of the Second Law of Thermodynamics.