Changes of state and internal energy

A substance can change between solid, liquid and gas:

• Melting: solid → liquid
• Freezing: liquid → solid
• Boiling or evaporating: liquid → gas
• Condensing: gas → liquid
• Sublimating: solid → gas, without becoming a liquid first

Changes of state are physical changes, not chemical ones. No new substance is made, and if you reverse the change, the material gets back its original properties. Ice melts into water, and the water can be frozen back into ice.

Mass is conserved: the number of particles stays the same. Melting 100 g of ice gives exactly 100 g of water.

The particles in a substance have two kinds of energy:

• Kinetic energy, because they move or vibrate.
• Potential energy, because of the forces between them and how far apart they are.

The internal energy of a system is the total kinetic and potential energy of all its particles.

The temperature of a substance is related to the average kinetic energy of its particles. The faster they move, the hotter it is.

Heating a substance increases its internal energy. This does one of two things:

1. Raises the temperature. The particles' kinetic energy increases, so they move faster. How much the temperature rises depends on the mass, the material (its specific heat capacity) and the energy put in.
2. Changes the state. The energy goes into the particles' potential energy as they break free of each other and move further apart. The temperature stays the same while the state changes.