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van der Waals' equation of state
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| Graph of the van der Waal's equation for a real gas (red). If a quantity of gas is compressed at constant temperature under normal conditions, the red curve is not followed all the way: at a certain critical pressure a change of state occurs – there is a sudden reduction in volume and the gas liquefies (broken blue line). A whole family of related van der Waals curves exist for different temperatures in three dimensions these give rise to a pressure-volume-temperature surface. For a certain critical temperature, there is no well in the curve, only a point of inflection. At and above this temperature, there is no definable phase transition between the gaseous and liquid states.
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for a gram-molecule of a substance in the gaseous and liquid phases, where P = pressure, V = volume, T = temperature, and R = the gas constant. The term a/V 2 is a correction for the mutual attraction of the molecules, and b is a correction for the actual volume of the molecules themselves. (The values of the constants a and b depend on the particular gas in question.) The attractive forces between molecules are known as van der Waal's forces.
Related categories
• PHYSICAL CHEMISTRY
• HEAT AND THERMODYNAMICS
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