IGCSE Chemistry: States of Matter Practice Questions
The kinetic particle model explains the three states of matter by the arrangement, movement and energy of their particles. Solids have particles packed closely in a fixed lattice, liquids have particles close but able to move past each other, and gases have particles far apart moving randomly at high speed.
Topic 1 of Cambridge IGCSE Chemistry 0620 is where students first meet the requirement to explain in terms of particles. Answers that describe what happens rather than explaining it in terms of particle arrangement, movement and energy score badly. The questions below train the explanation.
What you need to know for States of Matter
- SolidParticles are packed closely together in a regular fixed arrangement. They vibrate about fixed positions but cannot move past each other, so a solid has a fixed shape and volume and cannot be compressed.
- LiquidParticles are still close together but arranged randomly and able to move past each other. A liquid has a fixed volume but takes the shape of its container.
- GasParticles are far apart in a random arrangement, moving quickly in all directions. A gas has no fixed shape or volume and can be compressed because of the large spaces between particles.
- Changes of stateMelting is solid to liquid, boiling and evaporation are liquid to gas, condensing is gas to liquid, freezing is liquid to solid, and sublimation is solid directly to gas.
- DiffusionThe movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. It is fastest in gases and does not occur in solids.
- Rate of diffusion and massAt a given temperature, particles with a lower relative molecular mass move faster on average, so lighter gases diffuse more quickly.
IGCSE Chemistry States of Matter questions and answers
4 exam-style questions written to the 0620 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Explain, in terms of particles, why a gas can be compressed easily but a solid cannot.
Show the worked answer
- In a gas the particles are very far apart, with large spaces between them.
- Applying pressure pushes the particles closer together into those spaces, so the volume decreases and the gas is compressed.
- In a solid the particles are already packed closely together in a fixed arrangement, touching their neighbours.
- There are almost no spaces to push the particles into, so the volume barely changes and the solid cannot be compressed. Note that the particles themselves do not change size in either case.
A student heats a pure solid and records the temperature every minute. The graph shows a horizontal section partway up. Explain why the temperature stays constant during this section even though heating continues.
Show the worked answer
- The horizontal section occurs at the melting point of the substance.
- During melting the energy being supplied is used to overcome the forces of attraction holding the particles in their fixed positions in the lattice.
- The energy is not increasing the average kinetic energy of the particles, so the temperature does not rise.
- Once all the solid has melted, further heating increases the kinetic energy of the particles again and the temperature starts to rise.
Cotton wool soaked in concentrated ammonia solution is placed at one end of a long glass tube and cotton wool soaked in concentrated hydrochloric acid at the other. A white ring of ammonium chloride forms nearer the hydrochloric acid end. Explain why.
Show the worked answer
- Both substances give off gases that diffuse along the tube because their particles are in constant random motion.
- Ammonia has a relative molecular mass of 17, while hydrogen chloride has a relative molecular mass of 36.5.
- At the same temperature, lighter molecules move faster on average, so ammonia molecules diffuse more quickly than hydrogen chloride molecules.
- In the time taken for the two gases to meet, the ammonia has travelled further along the tube, so the white ring of ammonium chloride forms nearer the hydrochloric acid end.
Explain what happens to the volume of a fixed mass of gas when the temperature is increased at constant pressure.
Show the worked answer
- Raising the temperature increases the average kinetic energy of the gas particles, so they move faster.
- The particles collide with the container walls more frequently and with greater force.
- At constant pressure the gas must expand so that the collisions become less frequent per unit area, so the volume increases.
- The number of particles does not change. Only their speed and the space they occupy change.
Common mistakes in this topic
- Writing that particles change size during compression, heating or a change of state.
- Confusing evaporation with boiling. Evaporation happens at any temperature and only at the surface. Boiling happens at a fixed temperature throughout the liquid.
- Saying diffusion happens in solids. Particles in a solid vibrate but cannot move past each other.
- Describing melting as particles breaking. Bonds within molecules are not broken, only the forces between particles are overcome.
- Giving only the arrangement of particles when the question asks for arrangement, movement and energy.
Exam tips
- Answer every state of matter question with three elements: arrangement, movement and separation of the particles.
- For heating curve questions, name the change of state occurring at each plateau.
- In diffusion comparisons, always quote the two relative molecular masses. The comparison is where the mark is.
- Use the word average when discussing kinetic energy. Particles have a range of speeds.
- Remember that the forces overcome during melting and boiling are the forces between particles, not covalent bonds.
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States of Matter FAQs
Why can gases be compressed but liquids and solids cannot?
Gas particles are very far apart with large spaces between them, so applying pressure pushes them closer together and the volume decreases. In liquids and solids the particles are already touching, so there are almost no spaces to push them into and the volume barely changes.
Why does temperature stay constant during melting?
The energy supplied during melting is used to overcome the forces of attraction holding particles in the solid lattice, rather than to increase their average kinetic energy. Since temperature depends on average kinetic energy, it stays constant until all the solid has melted.
What is the difference between evaporation and boiling?
Evaporation occurs only at the surface of a liquid and can happen at any temperature below the boiling point, as the faster particles escape. Boiling occurs throughout the liquid at a fixed temperature, the boiling point, with bubbles of vapour forming inside the liquid.
Why do lighter gases diffuse faster?
At a given temperature all gas particles have approximately the same average kinetic energy. Since kinetic energy depends on both mass and speed, particles with a smaller relative molecular mass must move faster on average, so they diffuse more quickly and travel further in the same time.
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Written to the published Cambridge IGCSE Chemistry (0620) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.