When a liquid is heated to its boiling point, it changes into a gas. Explain, in terms of particles, what happens to the arrangement and movement of particles during this change of state.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In a liquid the particles are close together and in contact, so they can slide past one another but are still held by intermolecular forces. When the liquid is heated to its boiling point the particles absorb energy, gaining kinetic energy. This energy overcomes the attractive forces between the particles, allowing them to separate. As a result the particles move rapidly in all directions and are now far apart, with no fixed positions, forming a gas.
Examiner tips
- Use the terms "intermolecular forces" and "kinetic energy" to show understanding of the energy change.
- Show the change in particle distance and freedom of movement to hit the 4 marks.
Common mistakes
- Confusing the liquid and gas particle arrangements; writing that particles in a gas are still close together.
- Failing to mention that the energy gained is kinetic energy that overcomes intermolecular forces.
Mark scheme (4 marks)
- In a liquid, particles are close together / touching / in contact with each other, whereas in a gas the particles are far apart / spread out / much greater distances between them
- In a liquid, particles move randomly but are able to flow / move past each other; in a gas, particles move randomly in all directions with no fixed positions
- The particles gain energy (from heating), so the intermolecular forces between particles are overcome / broken
- The particles in the gas have more energy / move faster than in the liquid, so there are no significant forces of attraction between them
Key terms in this question
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More States of matter questions
- Solid carbon dioxide (dry ice) turns directly into a gas at −78 °C without first…
- Describe the arrangement and movement of particles in a liquid, and explain why …
- Explain why a gas can be compressed into a much smaller volume, but a solid cann…
- Iron has a melting point of 1538 °C, whereas oxygen has a boiling point of −183 …
- Explain why a liquid has a fixed volume but can change its shape to fill the bot…
- Explain why a substance in the gaseous state diffuses much more quickly than the…