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AS · Practice questions · The liquid state

Escape from the surface.

Six original Cambridge-style questions on the liquid state: particle behaviour, changes of state, vapour pressure and boiling.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Boiling is where vapour pressure meets outside pressure.

01
Recall
[2 marks]

Describe the arrangement and motion of the particles in a liquid.

  • The particles are close together and randomly arranged. ✓
  • They move randomly and slide past one another. ✓
02
Explain
[2 marks]

For water, ΔHfus = +6.0 kJ mol−1 and ΔHvap = +41 kJ mol−1. Explain the difference.

  • On melting the molecules stay close together, so only some hydrogen bonds are overcome. ✓
  • On vaporisation the molecules are separated completely, so all the intermolecular forces must be overcome, which needs much more energy. ✓
03
Explain
[3 marks]

Some water is placed in a sealed flask at 25 °C. Explain how a constant vapour pressure is established.

  • Molecules with enough energy escape from the surface of the liquid (evaporation). ✓
  • As vapour builds up, more vapour molecules return to the liquid (condensation). ✓
  • Eventually the rates of evaporation and condensation are equal: a dynamic equilibrium, and the vapour pressure stays constant. ✓
04
Explain
[2 marks]

Use Fig. 1 to explain why ethanol boils at a lower temperature than water.

  • Ethanol has weaker intermolecular forces overall, so it has a higher vapour pressure at any temperature. ✓
  • Its vapour pressure reaches 101 kPa at 78 °C, whereas water's reaches it at 100 °C. ✓
05
Explain
[2 marks]

Explain why food cooks faster in a pressure cooker.

  • The pressure inside is above atmospheric, so the vapour pressure of the water must be higher to equal it. ✓
  • Water therefore boils at a higher temperature (above 100 °C), and the food is heated more. ✓
06
Explain
[2 marks]

State two differences between evaporation and boiling.

  • Evaporation happens only at the surface; boiling happens throughout the liquid. ✓
  • Evaporation happens at any temperature; boiling happens only when the vapour pressure equals the external pressure. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.