Solids, liquids and gases are made of tiny particles. This particle model is called the particle theory.

Solids are made of particles that remain fixed in position and simply vibrate. Such solids keep their shape. As a solid is heated the particles start to vibrate strongly.

In the liquid form particles move about but remain attracted to each other. When a liquid is heated strongly it forms a gas, this process is called evaporation. A gas is formed from particles that travel at great speeds and move apart from each other.

The process by which a solid turns into a liquid when heated is called melting.

Evaporation is the process by which a liquid is turned into a gas due to heating.