Boiling, freezing and melting

When water boils in a kettle the addition of heat is turning the water into steam when it reaches 100°C.  We say that the water changes phase from a liquid to a gas.  Different chemicals boil at a different temperature. 

Water turns to ice and becomes a solid when its temperature goes below 0ºC.  Stored ice was used as one of the very first ways of keeping products cold - so the very first "Refrigerant". 

So most substances exist in any of three states depending on their temperature: Solid, Liquid or Gas.

SOLIDS

Solids are defined as having a fixed hard shape- like this ice cube:

LIQUIDS

Liquids have no fixed shape and will take the shape of any container they are put in - whether it's in the bottle or the glass in the picture below - but the volume is fixed and always remains the same.

GASES

Gases don't have a fixed shape or fixed volume in the way that liquids and solids do. They will spread out and their volume increases until they disperse.

What's happening?

Everything around us is made of molecules. In a solid the molecules are packed tightly together, in a liquid the molecules move around more energetically and in a gas there is a lot of space between molecules and they flow around freely.

When a substance is heated the molecules start to move around more, they take up more space and the substance expands. It is not the molecules themselves that get bigger, but the room they need to move around. The heat that is applied provides the molecules with the energy they need to increase their movement.

In the same way, if things are cooled energy is extracted as heat, and eventually, the substance will change from gas to liquid to solid.

A good example of this occurring naturally is steam condensing as water on a cold surface in a bathroom. The warm steam meets the cold mirror surface, which conducts the heat of the steam away and water droplets are formed.

A good example of this happening mechanically (by artificial means) is a vapour compression refrigeration system - your fridge at home.