• When substances are heated, the particles absorb thermal energy which is converted into kinetic energy. This is the basis of the kinetic theory of matter

• Heating a solid causes its particles to vibrate more and as the temperature increases, they vibrate so much that the solid expands until the structure breaks and the solid melts

• On further heating, the now liquid substance expands more and some particles at the surface gain sufficient energy to overcome the intermolecular forces and evaporate

• When the b.p. temperature is reached, all the particles gain enough energy to escape and the liquids boils

• These changes in state can be shown on a graph called a heating curve

• Cooling down a gas has the reverse effect and this would be called a cooling curve

• These curves are used to show how changes in temperature affect changes of state

• The horizontal sections occur when there is a change of state but there is no change in temperature

A heating curve showing the states, state changes and temperature changes as time progresses

A cooling curve is like a heating curve, but is the mirror image