Ideal Gases
These are the gases, which obey gas laws at all temperatures and pressures. No such gas is known.
Non-Ideal gases (Real gases)
These are the gases, which obey gas laws but only at high temperatures and low pressures.
These gases do not obey gas laws at low temperature and high pressure. Under these conditions they deviate from ideal behavior.
The extent of deviation from ideal behavior depends on the followings:
(1) Intermolecular forces:
Stronger the intermolecular forces of a gas, greater would be its deviation or vice versa.
(2) Temperature:
Lower the temperature, greater would be the deviation or vice versa.
(3) Pressure:
Higher the pressure, greater would be the deviation or vice versa.
Examples of non-ideal or real gases:
All known gases such as H2, O2, N2, He, CH4, CO, CO2 etc. are non-ideal gases.
At the same low temperature and high pressure these gases deviate from ideal behavior to different extent, which depends on the strength of intermolecular forces of gases.
Conditions under which gases deviate from ideal behavior:
(i) Low temperature:
At low temperature, the kinetic energy of the gas molecules decreases. This makes the intermolecular forces significant to such an extent that these cannot be neglected. As a result gases deviate from ideal behavior.
(ii) High pressure:
At high pressure, intermolecular distance decreases. This makes the actual volume occupied by the gas molecules significant to such an extent that it cannot be neglected. As a result gases deviate from ideal behavior.
Causes of deviation from ideal behavior
Van der Waals noted that gases deviate from ideal behavior because of the following two incorrect postulates of kinetic molecular theory of gases.
- Molecules of the gases do not attract each other.
- Actual volume of the molecules is negligible as compared to their occupied volume in the gaseous state.
High boiling points of some gases indicate the presence of strong attractive forces between their molecules. This is against the first postulate mentioned above. Existence of attractive forces between the molecules becomes the cause of deviation from ideal behavior.
