Explain why gases can be compressed much more easily than liquids.

Cambridge International IGCSE Chemistry (0620) — 1.1 Solids, liquids and gases · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Syringes are used in medicine to transfer fluids. A syringe filled with air can be compressed by pushing the plunger, but a syringe filled with water cannot be compressed in the same way.

Model answer (4 marks)

In a gas the particles are widely spaced, leaving large gaps between them. When pressure is applied these gaps can be reduced, so the particles are pushed closer together and the gas volume decreases.
In a liquid the particles are already almost touching, with only very small spaces between them. Because there is little or no space left to reduce, applying pressure does not significantly change the volume, so liquids are essentially incompressible.

Examiner tips

  • Use the words "far apart" and "large spaces" for gases, "close together" or "touching" for liquids. Show the link between pressure and reduction of gaps. Keep the answer short and to the point to hit all four marks.

Common mistakes

  • Confusing the behaviour of solids with liquids. Using vague terms like "dense" instead of "close together". Forgetting to mention that pressure reduces the gaps in a gas.

Mark scheme (4 marks)

  1. In a gas, particles are far apart / large spaces between particles
  2. These spaces can be reduced when pressure is applied / particles can be pushed closer together
  3. In a liquid, particles are already close together / touching / very little space between particles
  4. There is no space to reduce in a liquid, so it cannot be compressed

Key terms in this question

compressed · gas · liquid

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