1-2 Phase Change, Slide 3

To change a substance from solid to liquid or gas, or the other way around, it needs energy. This energy helps the tiny particles in the substance break apart and move away from each other.

When a substance is changing its phase - like melting or freezing - energy can be put into it or taken out, but its temperature will stay the same. The temperature will only start to change once the substance has completely changed its phase.