How do the molecules behave in the three states?

In a solid there are strong bonds linking the molecules into a lattice structure. The molecules can vibrate but cannot move around, so the solid has a fixed shape. However in a liquid some of these bonds have been broken, allowing the molecules to move freely and take up the shape of the container. In water vapor even more of these bonds have been broken so that not only does the gas take up the shape of the container but it is able to expand and fill the entire volume of the container, no matter how big it is.

A crucial point is that in liquids molecules are moving at different speeds all the time. How fast a molecule moves depends on how much energy it has. This is called kinetic energy. It's a liquid's average energy that we measure when we find the temperature. More energy — and so faster-moving molecules — means the liquid will be hotter, less energy means the liquid will be colder.

Heat transfer 

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