Atmospheric gases can be liquefied by applying pressure and lowering temperature. When enough pressure is applied, the gases are highly compressed into a small volume. The particles of gases get so close together that they start attracting one another sufficiently to form a liquid. When a gas is compressed too much by applying high pressure, a lot of heat is produced. So, while applying pressure to liquefy gases, it is necessary to lower their temperature (or cool them) to take away the heat produced during compression.
