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.

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
• the movement of its particles increases
• bonds between particles break when a substance melts or evaporates, or sublimes to form a gas from a solid

When a substance is cooled:

• its internal energy decreases
• the movement of its particles decreases
• bonds between particles form when a substance condenses or freezes, or sublimes to form a solid from a gas

Conservation of mass

The particles in a substance stay the same when it changes state, only their closeness, arrangement or motion change. This means that:

• the chemical nature of the substance itself stays the same
• the mass of the substance stays the same

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.