Energy of Phase Changes (College Board AP® Chemistry)

Heating & Cooling

• 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

  • 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 boiling point temperature is reached, all the particles gain enough energy to escape and the liquids boil

• 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 in state

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

Heating Curve

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

Cooling Curve

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

• The energy absorbed during a phase change is equal to the energy released during a complementary phase change in the opposite direction

  • For example, the molar heat of condensation of a substance is equal to the negative of its molar heat of vaporization.