Final answer:
A phase change is a physical process in which a substance goes from one phase to another. The melting point is the temperature at which a substance goes from a solid to a liquid, while the boiling point is the temperature at which a substance goes from a liquid to a gas.
Step-by-step explanation:
A phase change is a physical process in which a substance goes from one phase to another. The melting point is the temperature at which a substance goes from a solid to a liquid, while the boiling point is the temperature at which a substance goes from a liquid to a gas. The nature of the phase change depends on the direction of the heat transfer. Adding heat changes a substance from a solid to a liquid or a liquid to a gas, while removing heat changes a gas to a liquid or a liquid to a solid.