Real gases

Real gases are non-hypothetical gases whose molecules occupy space and have interactions; consequently, they adhere to gas laws. To understand the behaviour of real gases, the following must be taken into account:

For most applications, such a detailed analysis is unnecessary, and the ideal gas approximation can be used with reasonable accuracy. On the other hand, real-gas models have to be used near the condensation point of gases, near critical points, at very high pressures, to explain the Joule–Thomson effect and in other less usual cases. The deviation from ideality can be described by the compressibility factor Z.

Real gases are often modeled by taking into account their molar weight and molar volume

or alternatively:

Where p is the pressure, T is the temperature, R the ideal gas constant, and V_m the molar volume. a and b are parameters that are determined empirically for each gas, but are sometimes estimated from their critical temperature (T_c) and critical pressure (p_c) using these relations:

With the reduced properties ${\displaystyle \ p_{r}={\frac {p}{p_{\text{c}}}}\ ,\ V_{r}={\frac {V_{\text{m}}}{V_{\text{m,c}}}}\ ,\ T_{r}={\frac {T}{T_{\text{c}}}}\quad }$ the equation can be written in the reduced form:

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