Question

Ideal gas cannot be liquefied because:
(A) Ideal gas has highest intermolecular force of attraction
(B) Intermolecular forces in ideal gas are negligible
(C) Ideal gas has high density
(D) None of the above

Answer 1

Hint: A gas that follows all the gas law and gas equation at every possible temperature and pressure is known as ideal gas or perfect gas. An ideal gas follows the equation\[\text{PV}\,\text{=}\,\,\text{nRT}\].
Density of a gas depends on the pressure and temperature. \[\text{P}\,\,\text{ }\!\!\propto\!\!\text{ }\,\,\text{d}\]

Complete Solution :
An ideal gas cannot be liquefied because there is no intermolecular force of attraction between the ideal gas molecules.
(A) Ideal gas molecules are separated from each other by large space and the actual volume of the gas molecule is negligible with respect to the volume of gas. Due to large intermolecular spaces, there is no force of attraction between the ideal gas molecules and they are completely independent from each other.
Non-ideal gases show high intermolecular interaction, so liquification of these gases are controlled by two factors – lowering in temperature and increase in pressure. Liquification of gases is determined by van der Waals equation.

(B) Ideal gases show negligible intermolecular force of attraction and do not follow the van der Waals equation, so liquification of these gases is not possible.

(C) The density of an ideal gas is very low because gas molecules are present in a large space
So $\text{d}\,\,\text{=}\dfrac{\text{m}}{\text{V}}$ due to high volume density is low.
However, non-ideal gas molecules show intermolecular attraction, so these gas molecules have less volume and high density in respect to the ideal gas.
So, the correct answer is “Option B”.

Note: The non-ideal gases are highly compressible. Liquification of a gas also depends on its compressibility. Non-ideal gases have low volume and high density therefore the attractive force becomes large and molecules are crowded together. So, high pressure is necessary for the liquefaction of gases.