Only below the critical temperature, by the application of pressure, a gas can be liquefied. CO₂ has critical temperature as 303.98 K. Room temperature means (30 + 273 K) 303 K. At room temperature, (critical temperature) even by applying large amount of pressure CO₂ cannot be liquefied. Only below the critical temperature, it can be liquefied. At room temperature, CO₂ remains as gas.

The deviation of real gases from ideal behaviour is measured in terms of a ratio of PV to nRT. This is termed as compression factor.
Compression factor = Z = \(PV\over nRT\)
For ideal gases Z = 1 at all temperature and pressures.

(i) Graham's law of diffusion is useful to determine the molecular mass of the gas if the rate of diffusion is known.
(ii) Graham's law forms the basis of the process of enriching the isotopes of U²³⁵ from other isotopes and also useful in isotopic separation of deuterium and protium.

Take two balloons, one is filled with air and another with helium. After one day, the helium balloon was shrunk, because helium being lighter diffuses out faster than the air.

Graham's law of diffusion: The rate of diffusion or effusion is inversely proportional to the square root of molecular mass of a gas through an orifice.
\(\frac{r_A}{r_B}=\sqrt{\frac{M_B}{M_A}}\)
r_A, r_B = rate of diffusion of gases A, B
M_A, M_B = Molecular mass of gases A, B.