Ice, water, and steam are all the same stuff — just behaving differently. Today you'll learn why, then you will predict what the particles do as we heat and cool them.
Before we start — ice, water, and steam are all just water — what do you think is different inside each one?
Every material is built from particles too small to see. They are always moving — even in something that looks perfectly still, like a rock or an ice cube.
In a solid the particles are packed and only vibrate in place. In a liquid they stay close but slide past each other. In a gas they fly apart and fill all the space.
Adding heat gives particles more energy, so they move faster. Fast enough and a solid melts, then boils into gas. But during the actual change — while it is melting or boiling — the heat goes into breaking the particles free of one another rather than speeding them up, so the temperature pauses until the change is finished. Taking heat away does all of this in reverse.