Question

why are gases liquefied

Answer 1

Air is liquefied by the Linde process, in which air is alternately compressed, cooled, and expanded, each expansion results in a considerable reduction in temperature. With the lower temperature the molecules move more slowly and occupy less space, so the air changes phase to become liquid.

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.