1-2 Phase Change, Slide 3

Energy is needed to change a substance from solid to liquid or gas. This energy helps the tiny pieces that make up the substance move apart from each other.

When a substance is changing from one form to another, like from ice to water, we might add or take away energy, but the temperature stays the same. Only after the substance has completely changed form, like all the ice has melted into water, will the temperature start to change.