Question

State one condition necessary liquefy gases in liquid

Answer 1

Liquiefing gas means changing of the substances in gaseous states are changed into liquid state. 
To liquefy gas we must-
<>Decrease temperature <>Increase pressure
By doing this the molecules of gas will come closer to each other and temperature is reduced so it will remove the enough energy to convert gas into liquid state. Thomas Andrews did work on liquefaction of gases like carbon dioxide.
another condition necessary to liquefy gasses are applying pressure and decreasing the temperature.
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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.