Substances can change state, usually when they are heated or cooled. For example, liquid water turns into steam when it is heated enough, and it turns into ice when it is cooled enough. State changes are reversible – ice can be melted and then frozen again. No new elements or compounds are formed.
The closeness, arrangement and motion of the particles in a substance change when it changes state.
Materials are a store of internal energy, due to the motion of particles and the chemical bonds between them.
When a substance is heated, its internal energy increases:
When a substance is cooled, its internal energy decreases:
The particles in a substance stay the same when it changes state - only their closeness, arrangement or motion change. This means that:
For example, 10 g of water boils to form 10 g of steam, or freezes to form 10 g of ice. This is called conservation of mass.