perfect gas


perfect gas
[1840-50]

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Gas whose physical behaviour conforms to the general gas law, which states that for a given quantity of gas, the product of the volume V and pressure P is proportional to the absolute temperature T, or PV = kT, where k is a constant.

A perfect gas is assumed to consist of a large number of molecules in random motion, which obey Newton's laws of motion. Their volume is assumed to be negligibly small, and no forces are presumed to act on the molecules except during momentary collisions. Though no gas has these properties, real gases at sufficiently high temperatures and low pressures can be described this way.

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▪ chemistry and physics
also called  ideal gas 

      a gas that conforms, in physical behaviour, to a particular, idealized relation between pressure, volume, and temperature called the general gas law. This law is a generalization containing both Boyle's law and Charles's law as special cases and states that for a specified quantity of gas, the product of the volume v and pressure p is proportional to the absolute temperature t; i.e., in equation form, pv = kt, in which k is a constant. Such a relation for a substance is called its equation of state (state, equation of) and is sufficient to describe its gross behaviour.

      The general gas law can be derived from the kinetic theory of gases and relies on the assumptions that (1) the gas consists of a large number of molecules (molecule), which are in random motion and obey Newton's laws of motion; (2) the volume of the molecules is negligibly small compared to the volume occupied by the gas; and (3) no forces act on the molecules except during elastic collisions of negligible duration.

      Although no gas has these properties, the behaviour of real gases is described quite closely by the general gas law at sufficiently high temperatures and low pressures, when relatively large distances between molecules and their high speeds overcome any interaction. A gas does not obey the equation when conditions are such that the gas, or any of the component gases in a mixture, is near its condensation point, the temperature at which it liquefies.

      The general gas law may be written in a form applicable to any gas, according to Avogadro's law, if the constant specifying the quantity of gas is expressed in terms of the number of molecules of gas. This is done by using as the mass unit the gram- mole; i.e., the molecular weight expressed in grams. The equation of state of n gram-moles of a perfect gas can then be written as pv/t = nR, in which R is called the universal gas constant (molar gas constant). This constant has been measured for various gases under nearly ideal conditions of high temperatures and low pressures, and it is found to have the same value for all gases: R = 8.314472 joules per mole-kelvin.

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Universalium. 2010.

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