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Chemistry notebook Cambridge IGCSE 0620
Contains the answers

Answers · States and changes of state

States and changes of state — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. SR1 · Return

Model answer. A solid has a fixed shape and volume; its particles are close together in a regular pattern and vibrate about fixed positions. A liquid has a fixed volume but takes the shape of its container; its particles are close together, random and move around each other. A gas fills its container and can be compressed; its particles are far apart, random and move quickly in all directions.

A full answer includes

  • Solid: one property, such as a fixed shape; particles close together in a regular pattern, vibrating about fixed positions.
  • Liquid: one property, such as taking the shape of its container; particles close together, random, moving around each other.
  • Gas: one property, such as filling its container or being easy to compress; particles far apart, random, moving quickly in all directions.

Also accept

  • Solid: fixed shape, regular, vibrating. Liquid: takes container's shape, close and random, moving around each other. Gas: fills container, far apart, moving quickly in all directions.

Watch for

  • “Gas particles are bigger than liquid particles.” — The particles are the same size. The spaces between them are much larger in a gas.

2. SR2 · Return

Model answer. While ice melts, the energy supplied overcomes the forces of attraction between the molecules instead of making them move faster, so the temperature stays at 0 °C until all the ice has melted. Evaporation happens only at the surface and below the boiling point; boiling happens throughout the liquid, with bubbles, at the boiling point.

A full answer includes

  • I said the energy overcomes forces of attraction between molecules, so they don't move faster and the temperature stays constant.
  • Evaporation happens at the surface; boiling happens throughout the liquid.
  • Evaporation happens below the boiling point; boiling happens at the boiling point.

Also accept

  • Energy breaks the forces between molecules, not raising their speed. Evaporation: surface, below boiling point. Boiling: throughout, at boiling point.

Watch for

  • “No energy goes in while the ice is melting.” — Energy keeps going in; it changes the state instead of raising the temperature.

3. SR3 · Return

Model answer. (a) Condensing. (b) Freezing. (c) As the liquid freezes, forces of attraction pull the particles into fixed positions and energy is released. This replaces the energy lost to the surroundings, so the temperature stays constant until all of it has frozen. (d) A substance is a gas at 25 °C if its boiling point is below 25 °C.

A full answer includes

  • (a) Condensing.
  • (b) Freezing.
  • (c) Energy is released as forces of attraction pull the particles into fixed positions; it replaces the energy lost, so the temperature stays constant.
  • (d) Its boiling point is below 25 °C.

Also accept

  • Condensing; freezing; energy released as particles are pulled together balances the energy lost; boiling point below 25 °C.

Watch for

  • “Water vapour turning into water is evaporation.” — Evaporation goes from liquid to gas. The reverse, gas to liquid, is condensing.