Heating and changes of state

When a material is heated or cooled, two changes may happen to the particles within the material:

  • Chemical bonds between the particles may form, break or stretch. There is a change in the chemical potential store of energy in the material.
  • The material will heat up or cool down as the particles within it gain or lose speed. There is a change in the thermal store of energy within the material.
The internal energy is the total amount of energy in the thermal and chemical potential stores of all the particles in the system.
Flow chart showing the particle model changes between solid, liquid and gas, as internal energy is increased.

When energy is given to raise the temperature, particles speed up and gain energy.

The conservation of energy means that, assuming no energy is lost to the environment, any energy transferred to a material will be distributed between the chemical store and the thermal store of the internal energy.

When the substance melts or boils, energy is put in to breaking the bonds that are holding particles together, which increases the energy in the system. Throughout all of these changes the total number of particles, or mass, does not change.

The bonds between particles in one substance may have a different strength to the bonds between particles in another substance. If these bonds are stronger, more internal energy will be needed to break them and so the temperature for melting or boiling will be higher.

A change of state is a physical change because it can be reversed. Heating ice will turn it into water; freezing water turns it back to ice.