Question

How are gases liquefied?

Answer 1

#Gases can be liquefied by pressurise it.
Gases can pressurise easily as you know. So, when we pressurise the gases, its molecules get tighted, which is a property of a liquid. Liquid's molecules are closer to each other.

#Gases can be liquefied by condensing it.
If we condense(cold) gases, its molecules slower downs and can not move freely everywhere, this is also a property of liquid that this cannot freely movable. By condensing gases, its molecules get closer due to cold.

Answer 2

L.P.G. which is used for cooking. In the cylinder petrol is in liquid form. When it is lighted it becomes gas.