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In physics and chemistry, an extensive property of a substance is a property that depends on the amount of that substance in a physical system. Conversely, intensive properties do not have any dependence on the amount of the material in the system — the intensive properties of a substance will not change regardless of how much there is. Popular examples of the extensive properties of a substance include its mass and volume.
One of the most basic illustrations of the differences between extensive properties and intensive properties can be found in the property of density. The density of an object can be found by dividing an object's mass by its volume. Its mass and volume are said to be extensive properties because they depend on the amount of material in the object — the more material, the greater the mass and volume. Assuming that the object's composition is uniform, however, the density will remain the same regardless of the size.
A simple way of remembering the differences between an intensive property and an extensive property is that when two identical systems are combined, the intensive properties will remain the same whereas an extensive property would be doubled. For example, connecting two identical gas canisters with identical contents would double the mass of the system, which is an extensive property. The pressure inside the canisters is intensive and would stay the same.
In thermodynamics, internal energy, entropy, Gibbs free energy and enthalpy are said to be extensive properties. Their amounts are dependent on the size of the thermodynamic system. These values are not to be confused with specific energy, specific entropy, specific Gibbs free energy or specific enthalpy, which are intensive.
In general, an extensive property divided by another extensive property will result in an intensive property. Density, expressed as the ratio of mass to volume, is one example. Specific properties in thermodynamics are also intensive. For example, the specific heat capacity of a substance can be expressed as the ratio of the substance's heat capacity to the substance's mass.
Confusion most commonly exists in regard to the differences between extensive properties and intensive properties, but confusion also exists between extensive properties and system perceptions. A perception about a particular system describes an observer's thoughts on an abstract concept, such as color or smell. Unlike an abstract concept, an extensive property can be quantified and measured.
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