Phase transition 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 or boiling water into steam. During a phase transition, the properties of the material change significantly. For instance, in the case of water, the molecules in ice are tightly packed, while in steam, they are much more spread out. Understanding phase transitions is crucial in various fields, including chemistry and material science.