Solids, liquids and gases

O Level / IGCSE Chemistry · States of matter. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

All matter is made of tiny particles. The kinetic particle theory explains the three states by how close the particles are, how they are arranged and how they move. A solid has a fixed shape and volume. A liquid has a fixed volume but takes the shape of its container. A gas fills any container and can be compressed.

Heating gives particles more kinetic energy. At the melting point and the boiling point the energy is used to overcome the forces of attraction between particles, so the temperature stays constant until the change of state is complete.

Evaporation happens only at the surface, at any temperature below the boiling point. The fastest particles escape, so the liquid left behind cools.

Rules to remember

Common mistake

Many students write that particles get bigger or expand when heated. The particles stay the same size: they move faster and, in a gas that is free to expand, move further apart.

Worked example

A pure substance is cooled from 120 °C to −130 °C. Its cooling curve is flat at 78 °C and flat again at −114 °C. What happens at 78 °C, and what is the state of the substance at 20 °C?

  1. The substance starts as a gas, so the first flat section (78 °C) is the gas turning into a liquid: this is condensing, and 78 °C is the boiling point.
  2. The second flat section (−114 °C) is the liquid turning into a solid: this is freezing, and −114 °C is the melting point.
  3. 20 °C is above −114 °C and below 78 °C, so it lies between the melting point and the boiling point.

Answer: At 78 °C the gas condenses to a liquid. At 20 °C the substance is a liquid.

Practice questions

Try each one, then open the answer.

1. A gas is trapped in a syringe at constant temperature. The plunger is pushed in, increasing the pressure. What happens to the volume of the gas?

  1. A
    it stays the same, because the number of particles is fixed
  2. B
    it decreases, because the particles become smaller
  3. C
    it increases, because the particles move faster
  4. D
    it decreases, because the particles are pushed closer together
Show answer

Answer: D. Gas particles are far apart, so increasing the pressure squeezes them closer together and the volume falls. The particles themselves do not change size.

2. A beaker of water is left in a warm room and slowly evaporates. Why does the water that remains become slightly cooler?

  1. A
    evaporation only happens at the boiling point, so the cooling must come from the room
  2. B
    the remaining particles become heavier and slower as they clump together
  3. C
    particles from the air enter the water and take energy from it as they dissolve
  4. D
    the fastest particles escape, so the average kinetic energy of the particles left falls
Show answer

Answer: D. Only the fastest particles at the surface have enough energy to escape. Losing them lowers the average kinetic energy, and so the temperature, of the liquid left behind.

3. Which statement describes the particles in a liquid?

  1. A
    in a regular lattice, held rigidly so that the particles do not move
  2. B
    close together, vibrating about fixed positions in a fixed pattern
  3. C
    far apart, moving randomly at high speed in all directions
  4. D
    close together in an irregular arrangement, moving randomly past one another
Show answer

Answer: D. Liquid particles touch but are not in a fixed pattern; they slide past each other, which is why a liquid flows but is hard to compress.

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