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What Is the Ideal Gas Law?

Daniel Liden
Daniel Liden

The ideal gas law is an equation used in chemistry to describe the behavior of an "ideal gas," a hypothetical gaseous substance that moves randomly and does not interact with other gases. The equation is formulated as PV=nRT, meaning that pressure times volume equals number of moles times the ideal gas constant times temperature. The ideal gas law is generally used with the SI system of units, so P is in Pascals, V is in cubic meters, n is dimensionless and represents the number of moles, R is in joules divided by kelvins times moles, and T is in kelvins. The law may also be given as PV=NkT, with number of particles (N) replacing number of moles, and the Boltzmann constant replacing the ideal gas constant.

One important element that those working with the ideal gas law must understand is that it works only in idealized, theoretical situations. Real gases do interact with each other and with themselves to varying degrees, and those interactions detract from the randomness of the movement of gas particles. At low temperatures and high pressures, for instance, the attractive forces between gases are likely to significantly alter the way that the gases behave. At sufficiently low temperatures and high pressures, many gases even become liquids, but the ideal gas law does not account for this behavior.

Scientist with beakers
Scientist with beakers

There are a variety of uses for the ideal gas law, but they almost always involve theoretical situations. One can use the ideal gas law to determine any of the unknown properties of an ideal gas, assuming that one knows the rest of the properties. If, for instance, the pressure, number of moles, and temperature are known, it is possible to calculate the volume though simple algebra. In some cases, the ideal gas law can be used in real-life situations, but only with gases whose behaviors closely follow the law at certain temperature and pressure conditions, and even then it can only be used as an approximation.

The ideal gas law is usually taught in significant detail in high school and college general chemistry classes. Students use the law to learn the basics of calculation in chemistry and are often required to make several unit conversions before actually applying the equation. The law also illustrates several important concepts about the behaviors of gases. It shows, for instance, that an increase in the pressure of a gaseous system tends to correspond to a decrease in volume, and vice versa. It is important to understand the relationships demonstrated, even if the equation cannot be used for precise calculations about actual gaseous systems.

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