The amount of energy needed to change state from solid to liquid and from…

The idea

Changing state requires energy to overcome the forces between particles. Melting needs energy to break down the regular solid arrangement; boiling needs energy to separate liquid particles into a gas. If the forces between particles are strong, more energy is needed, so the melting point and boiling point are higher. If the forces are weak, less energy is needed, so the melting point and boiling point are lower.

For example: Water has stronger forces between its particles than methane, so water boils at 100 °C while methane boils at about −162 °C. The energy supplied during melting or boiling increases the potential energy of the particles rather than their average kinetic energy, so the temperature stays constant while the state changes.

What earns the marks

• States that energy is needed to overcome the forces between particles when a solid melts or a liquid boils.
• Links stronger forces between particles to a greater amount of energy needed to change state.
• Links stronger forces between particles to higher melting and boiling points, and weaker forces to lower melting and boiling points.
• Explains that during melting and boiling the energy supplied increases the potential energy of the particles rather than their average kinetic energy, so temperature remains constant.
• Compares two substances, such as water and methane, using the strength of forces between their particles to account for different boiling points.
• Recognises that the same idea applies in reverse: freezing and condensing release energy as forces between particles form.

Common mistakes and how to put them right

• Saying that strong forces between particles make a substance melt or boil more easily.

  Put it right by stating that stronger forces require more energy to overcome, so melting and boiling points are higher.

• Saying that temperature rises throughout melting.

  Put it right by stating that temperature stays constant during a change of state because energy is used to overcome forces between particles.

• Confusing forces between particles with chemical bonds within particles.

  Put it right by stating that changes of state involve overcoming forces between particles, not breaking chemical bonds.

Exam tips

• Compare substances by naming the strength of forces between their particles and linking this to the energy needed and the melting or boiling point.
• When explaining a flat section on a heating curve, state that energy is used to overcome forces between particles rather than to raise temperature.
• Use the phrase forces between particles rather than bonds to keep the explanation at the correct level.