Heat makes them vibrate faster, turning the substance liquid. Increasing the heat makes the particles move all over the place so fast that the bonds between molecules break and the substance becomes a gas. In a gas, particles are far apart, which is why gases are less dense. Liquid and gas Liquids evaporate to gas as more and more fast-moving particles break away from the surface. A gas condenses to a liquid, as particles slow down and bonds begin to hold them together.
Heat makes the particles move enough to break the pattern.