Can a gas be returned to a liquid?

Answer 1

Yes. The three common phases of matter, solid, gas and liquid can be achieved by any material provided the right conditions, but different materials exist in different phases under the same conditions. The most common way that changes between these phases are achieved is by a change in temperature (although a change in pressure will also affect the state of matter). If a gas is cooled sufficiently, it will form a liquid. For example, if you set a glass of ice-water out, the outside of the glass will become wet. This wetness is water vapor which was in the air as a gas, but was cooled when it came into contact with the cold glass and condensed on the glass into a liquid. Even gases like Nitrogen, which makes up most of the air around us, can form a liquid, although it must be cooled to −196 °C.

Answer 2

Yes, all gases can be liquified at a sufficiently low temperature.