A phase change is a change in the states of matter. For example, a solid may become a liquid. This phase change is called melting. When a solid changes into a gas, it is called sublimation. When a gas changes into a liquid, it is called condensation. When a liquid changes into a solid, it is called solidification. When a gas changes into a solid, it is called deposition . When a liquid changes into gas, it is called evaporation or boiling. Phase changes are usually caused by changes in temperature or pressure.