Phase changes refer to the transitions between different states of matter: solid, liquid, and gas. These changes occur when energy is added or removed from a substance, causing its particles to move differently. For example, when heat is applied to ice, it melts into water, while cooling water can freeze into ice. Common phase changes include melting, freezing, condensation, and evaporation. During melting, a solid like ice turns into a liquid, while freezing is the reverse process. Evaporation occurs when a liquid, such as water, turns into a gas, and condensation happens when gas cools and becomes a liquid again.