For a gas to become liquid, the particles of the gas must:

- Move more closely together

- Move more slowly

- Have weaker forces of attraction between them

When a gas is compressed, the particles are forced to move closer together. This increases the intermolecular forces between the particles, which causes them to slow down. As the particles slow down, they have less energy to overcome the intermolecular forces, and they begin to condense into a liquid.