Intensive and extensive properties

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In physics and chemistry an intensive property of a system is a physical property of the system that does not depend on the system size or the amount of material in the system. By contrast, an extensive property of a system does depend on the system size or the amount of material in the system. However, some of the intensive properties are statistical in nature (e.g. viscosity) and are relevant only in aggregate scales.

Examples of intensive properties include:

temperature
viscosity
density
electrical resistivity
melting point
boiling point
pressure
spectral absorption maxima (in solution)
flammability

Examples of extensive properties include:

Distinction from perceptions.
Certain perceptions are often described (or even "measured") as if they are an intensive or extensive physical property. However, they are fundamentally different. For example, the colour of a solution is not a physical property. A dilute solution of potassium permanganate is pink, a more concentrated solution deep purple, and a larger volume of a strong solution is black. The colour (ie, the degree of 'pinkness' or 'purpleness'), is a perception and cannot be measured, only ranked in comparison with other coloured solutions by a panel of observers. Attempts to quantitate a perception always involve an observer response, and biological variability is an intrinsic part of the process. The same volume of permanganate solution has physical properties related to the colour: the optical absorption spectrum is an extensive property, and the positions of the absorption maxima (which are relatively independent of concentration) are intensive properties. Although the colour is related to the absorption spectrum, they are not synonymous. The truth of this becomes apparent when comparing two objects that have the same colour despite very different spectra.

The confusion between perception and physical properties is increased by the existence of numeric scales for many perceived qualities. However, this is not 'measurement' in the same sense as in physics and chemistry. A numerical value for a perception is, directly or indirectly, the expected response of a group of observers when perceiving the specified physical event.

Examples of perceptions related to an intensive physical property:

loudness of sound: the related physical property is sound pressure level
hue of a solution: the related physical property is the position of the spectral absorption maximum (or maxima)

Examples of perceptions related to an extensive physical property:

color of a solution: the related physical property is the transmission or absorption spectrum

Intensive and extensive properties

See also