Physical properties of materials and systems can often be categorized as being either intensive or extensive, according to how the property changes when the size (or extent) of the system changes. According to IUPAC, an intensive quantity is one whose magnitude is independent of the size of the system^{[1]} whereas an extensive quantity is one whose magnitude is additive for subsystems.^{[2]} This reflects the corresponding mathematical ideas of mean and measure, respectively.
An intensive property is a bulk property, meaning that it is a local physical property of a system that does not depend on the system size or the amount of material in the system. Examples of intensive properties include temperature, T; refractive index, n; density, ?; and hardness of an object, ?.
By contrast, extensive properties such as the mass and entropy of systems are additive for subsystems because they increase and decrease as they grow larger and smaller, respectively.^{[3]}
These two categories are not exhaustive, since some physical properties are neither intensive nor extensive.^{[4]} For example, the electrical impedance of two subsystems is additive when -- and only when -- they are combined in series; whilst if they are combined in parallel, the resulting impedance is less than that of either subsystem.
The terms intensive and extensive quantities were introduced by Richard C. Tolman in 1917.^{[5]}
An intensive property is a physical quantity whose value does not depend on the amount of the substance for which it is measured. For example, the temperature of a system in thermal equilibrium is the same as the temperature of any part of it. If the system is divided, the temperature of each subsystem is identical. The same applies to the density of a homogeneous system; if the system is divided in half, the mass and the volume are both divided in half and the density remains unchanged. Additionally, the boiling point of a substance is another example of an intensive property. For example, the boiling point of water is 100 °C at a pressure of one atmosphere, which remains true regardless of quantity.
The distinction between intensive and extensive properties has some theoretical uses. For example, in thermodynamics, according to the state postulate: "The state of a simple compressible system is completely specified by two independent, intensive properties". Other intensive properties are derived from those two variables.
Examples of intensive properties include:^{[3]}^{[5]}^{[4]}
See List of materials properties for a more exhaustive list specifically pertaining to materials.
An extensive property is a physical quantity whose value is proportional to the size of the system it describes, or to the quantity of matter in the system. For example, the mass of a sample is an extensive quantity; it depends on the amount of substance. The related intensive quantity is density which is independent of the amount. The density of water is approximately 1g/mL whether you consider a drop of water or a swimming pool, but the mass is different in the two cases.
Dividing one extensive property by another extensive property generally gives an intensive value--for example: mass (extensive) divided by volume (extensive) gives density (intensive).
Examples of extensive properties include:^{[3]}^{[5]}^{[4]}
The ratio of two extensive properties of the same object or system is an intensive property. For example, the ratio of an object's mass and volume, which are two extensive properties, is density, which is an intensive property.^{[8]}
More generally properties can be combined to give new properties, which may be called derived or composite properties. For example, the base quantities^{[9]} mass and volume can be combined to give the derived quantity^{[10]} density. These composite properties can also be classified as intensive or extensive.^{[dubious – discuss]} Suppose a composite property is a function of a set of intensive properties and a set of extensive properties , which can be shown as . If the size of the system is changed by some scaling factor, , only the extensive properties will change, since intensive properties are independent of the size of the system. The scaled system, then, can be represented as .
Intensive properties are independent of the size of the system, so the property F is an intensive property if for all values of the scaling factor, ,
(This is equivalent to saying that intensive composite properties are homogeneous functions of degree 0 with respect to .)
It follows, for example, that the ratio of two extensive properties is an intensive property. To illustrate, consider a system having a certain mass, , and volume, . The density, is equal to mass (extensive) divided by volume (extensive): . If the system is scaled by the factor , then the mass and volume become and , and the density becomes ; the two s cancel, so this could be written mathematically as , which is analogous to the equation for above.
The property is an extensive property if for all ,
(This is equivalent to saying that extensive composite properties are homogeneous functions of degree 1 with respect to .) It follows from Euler's homogeneous function theorem that
where the partial derivative is taken with all parameters constant except .^{[11]} This last equation can be used to derive thermodynamic relations.
A specific property is the intensive property obtained by dividing an extensive property of a system by its mass. For example, heat capacity is an extensive property of a system. Dividing heat capacity, C_{p}, by the mass of the system gives the specific heat capacity, c_{p}, which is an intensive property. When the extensive property is represented by an upper-case letter, the symbol for the corresponding intensive property is usually represented by a lower-case letter. Common examples are given in the table below.^{[3]}
Extensive property |
Symbol | SI units | Intensive (specific) property |
Symbol | SI units | Intensive (molar) property |
Symbol | SI units |
---|---|---|---|---|---|---|---|---|
Volume | Specific volume* | Molar volume | ||||||
Internal energy | Specific internal energy | Molar internal energy | ||||||
Enthalpy | Specific enthalpy | Molar enthalpy | ||||||
Gibbs free energy | Specific Gibbs free energy | Chemical potential | ||||||
Entropy | Specific entropy | Molar entropy | ||||||
Heat capacity at constant volume |
Specific heat capacity at constant volume |
Molar heat capacity at constant volume |
||||||
Heat capacity at constant pressure |
Specific heat capacity at constant pressure |
Molar heat capacity at constant pressure |
If the amount of substance in moles can be determined, then each of these thermodynamic properties may be expressed on a molar basis, and their name may be qualified with the adjective molar, yielding terms such as molar volume, molar internal energy, molar enthalpy, and molar entropy. The symbol for molar quantities may be indicated by adding a subscript "m" to the corresponding extensive property. For example, molar enthalpy is H_{m}.^{[3]} Molar Gibbs free energy is commonly referred to as chemical potential, symbolized by ?, particularly when discussing a partial molar Gibbs free energy ?_{i} for a component i in a mixture.
For the characterization of substances or reactions, tables usually report the molar properties referred to a standard state. In that case an additional superscript ° is added to the symbol. Examples:
The use of the term intensive is potentially confusing. The meaning here is "something within the area, length, or size of something", and often constrained by it, as opposed to "extensive", "something without the area, more than that".
The general validity of the division of physical properties into extensive and intensive kinds has been addressed in the course of science.^{[12]}Redlich noted that, although physical properties and especially thermodynamic properties are most conveniently defined as either intensive or extensive, these two categories are not all-inclusive and some well-defined physical properties conform to neither definition.^{[4]} Redlich also provides examples of mathematical functions that alter the strict additivity relationship for extensive systems, such as the square or square root of volume, which may occur in some contexts, albeit rarely used.^{[4]}
Other systems, for which standard definitions do not provide a simple answer, are systems in which the subsystems interact when combined. Redlich pointed out that the assignment of some properties as intensive or extensive may depend on the way subsystems are arranged. For example, if two identical galvanic cells are connected in parallel, the voltage of the system is equal to the voltage of each cell, while the electric charge transferred (or the electric current) is extensive. However, if the same cells are connected in series, the charge becomes intensive and the voltage extensive.^{[4]} The IUPAC definitions do not consider such cases.^{[3]}
Some intensive properties do not apply at very small sizes. For example, viscosity is a macroscopic quantity and is not relevant for extremely small systems. Likewise, at a very small scale color is not independent of size, as shown by quantum dots, whose color depends on the size of the "dot".