Question

Gases can be liquefied by:
A. Increasing pressure
B. Decreasing temperature
C. Both A and B
D. Decreasing pressure

Answer 1

Hint: A substance is said to be gaseous if it possesses neither definite volume nor definite shape. The gases occupy all the available space and also acquire the shape of the vessel.

Complete Step by Step Solution:
The process of liquefaction of gases generally occurs as a result of an increase in intermolecular forces. When a gas is cooled to a low temperature (decreasing temperature) or subjected to high pressure (increasing pressure), molecules come closer leading to an increase in Van der Waals forces and the gas gets liquefied.

At low temperatures, the kinetic energy of atoms (inert gases) or molecules decreases. Thus, the magnitude of Van der Waals forces increases. Again, at high pressure, the atoms (inert gases) or molecules come closer. The interaction between them decreases and hence, the magnitude of Van der Waals forces increases.
 Therefore, option C is correct.

Additional information: The attractive interactions between the different molecules of a substance are called intermolecular forces. The magnitude of these forces is maximum in the solids and decreases on passing from solid to liquid and from liquid to gaseous state. Van der Waal successfully explained the liquefaction of gases on the basis of intermolecular forces. Van der Waals forces are considered to be purely electrostatic and hence they are physical in nature.

Note: Van der Waals forces exist between nonpolar atoms like inert gases, \[{\rm{He, Ne, Ar}}\]etc. These also exist between nonpolar molecules, i.e., homonuclear molecules \[{{\rm{H}}_{\rm{2}}}{\rm{,}}\,{{\rm{N}}_{\rm{2}}}\] etc. These are known as Van der Waals forces after the name of the discoverer. Van der Waals forces are also known as London forces or dispersion forces.