Change of State

When we consistently (and persistently) heat a substance, we observe two things:

• an increase in temperature
• a change in state (melting or evaporating)

When we investigate this in more detail, we notice something peculiar: at the melting/boiling point there is no real change in temperature (until after all of the substance has melted/evaporated).

This is because at melting/boiling point, the heat energy is not being transformed into kinetic energy (observed as a temperature increase). Instead, it is being used to break the bonds between the particles. The amount of energy required to do this is called Latent Heat. It is an enthalpy change.

LATENT HEAT OF FUSION

LATENT HEAT OF VAPORISATION