Phase change refers to the transformation of a substance from one state of matter to another, such as from solid to liquid or liquid to gas. This process occurs when energy is added or removed, causing changes in temperature and pressure. Common examples include melting ice into water and boiling water into steam. During a phase change, the temperature of the substance remains constant until the entire material has transformed. This is because the energy supplied is used to break or form bonds between particles rather than increasing temperature. Understanding phase changes is essential in fields like chemistry and physics.