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, a process known as melting. Conversely, when water is cooled, it freezes into ice, demonstrating the phase change of freezing. Another common phase change is evaporation, where liquid water turns into vapor when heat is added. This process can be reversed through condensation, where vapor cools and becomes liquid again. Understanding these phase changes is essential in fields like chemistry and physics, as they explain how substances behave under varying conditions.