Most substances exist in one of three states of matter. Whenever a change occurs, the substance has new properties. This happens because the molecles are arranged differently. Each pure substance has a melting point, or temperature at which a solid changes into a liquid. Each also has a boiling point at which the liquid changes into a gas. The process by which a liquid slowly turns into a gas is known as evaporation. The temperature at which a liquid changes into a solid is known as the freezing point. Most materials contract, or become smaller, as they get cooler. They expand, or become larger, as they get hotter.