When a gas changes into liquid, it is called condensation. During condensation, the gas molecules lose energy and their kinetic energy decreases. This causes them to slow down and move closer together, resulting in the formation of a liquid.

The speed of molecules in a gas is inversely proportional to the square root of its temperature. This means that as the temperature of a gas decreases, the speed of its molecules also decreases. When the temperature of a gas reaches its boiling point, the molecules have enough energy to overcome the intermolecular forces holding them together and the gas changes into a liquid.