physical laws describing the behavior of a gas under various conditions of pressure, volume, and temperature. Experimental results indicate that all real gases behave in approximately the same manner, having their volume reduced by about the same proportion of the original volume for each drop of 1° on the Celsius temperature scale. Graphs drawn to describe this behavior can be extrapolated, and all converge to a point corresponding to about −273 degrees Celsius (−459 degrees Fahrenheit)—this point is called absolute zero. A temperature scale defined so that zero degrees corresponds to this zero-volume temperature coordinate is known as an absolute scale. The Kelvin temperature scale begins at this absolute zero and has degrees the same size as those of the Celsius scale.

Gas Laws Relating Two Variables

The simplest gas laws relate pressure, volume, and temperature in pairs. Boyle's law (advanced by Robert Boyle in 1662) states that the pressure and volume of a gas are inversely proportional to one another, or PV = k, where P is pressure, V is volume, and k is a constant of proportionality. Charles's law (published by Jacques A. C. Charles in 1787), sometimes known as Gay-Lussac's law (independently demonstrated by Joseph Gay-Lussac in 1802), states that the volume of an enclosed gas is directly proportional to its temperature, or V = kT. This expression is strictly true only if the temperature is measured on an absolute scale. A third law states that the pressure is directly proportional to the absolute temperature, or P = kT.

Gas Laws Relating Three Variables

The three gas laws relating two variables can be combined into a single law relating pressure, temperature, and volume, which states that the product of pressure and volume is directly proportional to the absolute temperature, or PV = kT. This law describes the behavior of real gases only with a certain range of values for the variables. At temperatures or pressures near those at which the gas condenses to a liquid, the behavior departs from this equation. Nevertheless, it is useful to consider an ideal gas, or perfect gas, an imaginary substance that conforms to this equation for all values of the variables.

The behavior of an ideal gas can be described in terms of the kinetic-molecular theory of gases and leads directly to the relationship PV = kT, which is therefore called the ideal gas law, or general gas law. The constant of proportionality k is usually expressed as the product of the number of moles, n, of the gas and a constant R, known as the universal gas constant. In MKS units, R has the value 8.3149 × 10³ joules/kilogram-mole-degree. The ideal gas law can be further simplified by replacing the ordinary volume V by the specific volume v, which is equal to V/n. The law then has the form Pv = RT. This form has the advantage that all of the variables are intensive; that is, none of the variables depends on the mass of the gas.

The van der Waals equation (for the Dutch physicist Johannes van der Waals) is another gas law involving pressure, temperature, and volume. It takes into account the variations in behavior of different real gases from that of an ideal gas. The van der Waals equation is usually given as (P + a/v²) (v − b) = RT, where a and b are constants that have different particular values for different real gases. Other, more complicated equations exist that describe the behavior of real gases over an even wider range of values for pressure, temperature, and volume.

See also thermodynamics.

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