The Engineer posted on November 10, 2006 
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An ideal gas is defined as one in which all collisions between atoms or molecules are perfectly elastic and in which there are no intermolecular attractive forces. 
Ideal gas law is a generalization containing both Boyle's law and Charles's law as special cases and states that: 
In such a gas, all the internal energy is in the form of kinetic energy and any change in internal energy is accompanied by a change in temperature. An ideal gas can be characterized by three state variables: 
 absolute pressure (P),
 volume (V),
 and absolute temperature (T).

The relationship between them may be deduced from kinetic theory and is called the Ideal gas law. 
PV = kT = nRT 
where 
 n is the total number of moles,
 N_{A} = Avogadro's Number = 6.02217 · 10^{23} molecules/mole,
 R = Universal gas constant = 8.314 J/K · mol ,
 k = Boltzmann Constant = R/N_{A} = 1.380622 · 10^{23} J/K.

The ideal gas law can be viewed as arising from the kinetic pressure of gas molecules colliding with the walls of a container in accordance with Newton's laws. But there is also a statistical element in the determination of the average kinetic energy of those molecules. The temperature is taken to be proportional to this average kinetic energy; this invokes the idea of kinetic temperature. 