Ideal gas – it is an abstract, mathematical gas model that fulfills the following conditions:
-no intermolecular interactions except for repulsion at the moment of collision of particles
-the volume of particles is negligible in relation to the gas volume
-particle collisions are perfectly elastic
-the molecules are in continuous chaotic motion

These assumptions explained the basic properties of gases. After the discovery of the properties of particles in quantum mechanics, these assumptions were also applied to quantum particles..

Such a gas in classical mechanics describes the Clapeyron equation (ideal gas state equation), showing the relationship between gas pressure p, its volume V, temperature T and the number of n expressed in moles:
$$ pV = nRT $$
where:
\( p \) - gas pressure,
\( V \) - gas volume
\( n \) - amount of substance of the gas (in moles)
\( R \) - gas constant (8.314 J·K⁻¹·mol⁻¹)
\( T \) - absolute temperature

After transformations

Gas pressure:
$$ p = {nRT\over{V}} $$

Gas volume:
$$ V = {nRT\over{p}}$$

Amount of substance:
$$ \rho = \frac{pV}{RT} $$

Temperature:
$$ T = \frac{pV}{Rn} $$