Atmospheric gases can be liquefied by applying pressure and reducing the temperature. The forces of attraction among the particles of gas are very weak. As a result, the particles are able to move freely. They have a lot of empty spaces between them. When pressure is applied on a gas at a reduced temperature, its molecules get closer and they start attracting one another to form a liquid. When a gas is compressed by applying high pressure, a lot of heat is produced. So, while applying high pressure, it is necessary to keep the temperature low which helps to take away the heat released during compression.