Under what condition can a gas be liquefied?
(1) `T=T_(c) and P lt P_(c)`
(2) `T lt T_(c) and P=P_(c)`

Explanation

It is possible to liquefy a gas by the application of pressures. Provided that the temperature of the gas is equal to or less than its critical temperature.
If the temperature (T) of a gas is equal to its critical temperature `(T_(c))`, then it is possible to liquefy the gas if the pressure (P) of the gas is equal to or above its critical pressure `(P_(c))`. so, under the condition (1) the gas cannot be condensed into liquid. on the othre hand, if the temperature (T) of a gas is below its critical temperature `(T_(c))`, then the gas can be transformed into liquid if the applied pressure on the gas is greater than or less than or equal to its critical pressure. so, under condition (2) the gas can be liquefied.