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Inthermodynamics, thereduced properties of a fluid are a set ofstate variables scaled by the fluid's state properties at itscritical point. These dimensionless thermodynamic coordinates, taken together with a substance'scompressibility factor, provide the basis for the simplest form of thetheorem of corresponding states.^[1]

Reduced properties are also used to define thePeng–Robinson equation of state, a model designed to provide reasonable accuracy near the critical point.^[2] They are also used tocritical exponents, which describe the behaviour of physical quantities near continuous phase transitions.^[3]

The reduced pressure is defined as its actual pressure${\displaystyle p}$ divided by itscritical pressure${\displaystyle p_{\mathrm{c}}}$:^[1]

${\displaystyle p_{\mathrm{r}}=\frac{p}{p_{\mathrm{c}}}}$

The reduced temperature of a fluid is its actual temperature, divided by itscritical temperature:^[1]

${\displaystyle T_{\mathrm{r}}=\frac{T}{T_{\mathrm{c}}}}$

where the actual temperature and critical temperature are expressed in absolute temperature scales (eitherKelvin orRankine). Both the reduced temperature and the reduced pressure are often used in thermodynamical formulas like the Peng–Robinson equation of state.

The reduced specific volume (or "pseudo-reduced specific volume") of a fluid is computed from theideal gas law at the substance's critical pressure and temperature:^[1]

${\displaystyle v_{\mathrm{r}}=\frac{v{p}_{\mathrm{c}}}{R{T}_{\mathrm{c}}}}$

This property is useful when the specific volume and either temperature or pressure are known, in which case the missing third property can be computed directly.

• Departure function

• Thermodynamic properties

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