Changes of State (Edexcel IGCSE Physics): Revision Note

Changes of state

• When a solid is heated, it melts to form a liquid

  • When it reaches the melting point, further energy supplied is transferred to the potential store of the particles

  • This breaks the rigid bonds between the particles so they can flow over each other

• When a liquid is heated, it boils to form a gas

  • When it reaches the boiling point, further energy supplied is transferred to the potential store of the particles

  • This overcomes the intermolecular bonds completely, so the particles spread far apart and move randomly

• Evaporation can also turn a liquid into a gas, but it is different from boiling:

  • Evaporation can happen at any temperature, not just the boiling point

  • Only the most energetic particles at the surface of the liquid have enough kinetic energy to escape the intermolecular bonds

  • Bubbles of gas form in the liquid during boiling, but not during evaporation

Changing between states of matter

Changing the temperature of a solid, liquid or gas changes its state

Heat & temperature

• Heating a system increases the internal energy of its particles

• The internal energy is made up of two parts:

  • The kinetic energy of the molecules, due to their motion

  • The potential energy of the molecules, due to their position relative to each other (the intermolecular bonds)

• The temperature of the material is related to the average kinetic energy of the molecules

• The increase in internal energy from heating can:

  • cause the temperature to increase — the energy goes into the kinetic store of the molecules, so they move around faster

  • produce a change of state (e.g., solid to liquid or liquid to gas) — the energy goes into the potential store of the molecules, breaking the intermolecular bonds, while the kinetic energy stays constant, so the temperature does not rise

• The higher the temperature, the higher the average kinetic energy of the molecules, and vice versa

The relationship between temperature and internal energy

As the container is heated up, the gas molecules move faster with higher kinetic energy. The energy stored within the system - the internal energy - therefore increases