Internal energy is a measure of the total energy of all the particles in a system. This is made up of two energy stores:
1. Kinetic energy – A measure of the motion of molecules and particles. As gas particles move around more, they have more energy in the kinetic energy store.
2. Potential energy – The energy an object has due to its position relative to other objects.
Materials store internal energy. When a substance is heated, its internal energy increases and when a substance cools, its internal energy decreases.
Heating a substance will cause particles to speed up, increasing the kinetic energy of its molecules. This is why temperature is a measure of the average kinetic energy of the particles of a substance.
However, once the substance reaches a specific temperature, increasing the heat will no longer increase the energy in the kinetic store.
Due to the conservation of energy, any energy transferred to the substance must be split between the kinetic and potential energy stores.
This means that increasing the heat will now lead to an increase in the potential energy store. This breaks the bonds between the molecules in the substance, which leads to a change in state. At this point, although the substance is being heated, the temperature does not increase.