Question

Gases can be liquefied by:
A. increasing pressure
B. decreasing temperature
C. both A and B
D. decreasing pressure

Answer 1

Hint: Gases have very large space between them and particles in gaseous phase have high kinetic energy which leads to them having less force of attraction between them. In case of liquids, the space between particles is less and so is the kinetic energy of the particles in liquid. So, we can liquefy a gas into liquid if we decrease space between them and also decrease their kinetic energy.

Complete answer:
Liquefaction of gases is the process by which substances in their gaseous state are converted to the liquid state. Gases are the phase of matter in which particles are usually very far apart from one another, move very quickly, and they do not have strong forces of attraction between them. There is a large empty space between particles and these particles have a lot of kinetic energy. Whereas, the particles in liquids on the other hand are bounded by comparatively very strong forces of attraction and particles here possess less kinetic energy than gaseous particles.
On increasing pressure the volume of the gas decreases which brings gaseous molecules together and decreases the space between them and on decreasing the temperature the kinetic energy of the molecules decreases and forces of attraction between them increases and thus, gas can be converted into liquid.

Hence, both increasing pressure and decreasing temperature can help in liquefaction of gases. So, the correct option is option C.

Note: Critical temperature is a temperature in which substance can be in liquid form only. After this temperature substance can be either in vapour form or gaseous form.