Explaining changes of state (S)

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Freezing and melting / boiling

When a solid is heated, its particles gain more energy.
This makes the particles vibrate more, which weakens the forces (bonds) that hold the solid together. This makes the solid expand
At a certain temperature, the particles have enough energy to break free from their positions. This is called melting and the solid turns into a liquid
When a liquid is heated, the particles get even more kinetic energy
This energy makes the particles move faster, which weakens and breaks the bonds holding the liquid together
At a certain temperature, the particles have enough kinetic energy to break their bonds. This is called boiling and the liquid turns into a gas.
The amount of energy needed to cause a substance to change state depends on the strength of the forces between the particles