Diffusion

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

Diffusion is the spreading out of particles from a region of higher concentration to a region of lower concentration until they are evenly mixed. It happens because the particles in liquids and gases move randomly all the time. Nothing needs to be stirred or shaken.

Diffusion is fast in gases, because the particles move quickly and there are large spaces between them. It is much slower in liquids, where the particles are close together and collide very often.

At the same temperature, gas molecules with a smaller relative molecular mass (Mr) move faster, so lighter gases diffuse faster. Raising the temperature makes every gas diffuse faster.

Rules to remember

Common mistake

When two gases meet in a tube, students often put the meeting point nearer the lighter gas. The lighter gas travels further in the same time, so the gases meet nearer the end where the heavier gas started.

Worked example

Ammonia, NH3, hydrogen sulfide, H2S, and sulfur dioxide, SO2, are released at the same moment at one end of a room. Which gas reaches the other end first? (H 1, N 14, O 16, S 32)

  1. Mr of NH3 = 14 + (3 × 1) = 17.
  2. Mr of H2S = (2 × 1) + 32 = 34.
  3. Mr of SO2 = 32 + (2 × 16) = 64.
  4. The gas with the smallest Mr has the fastest molecules at the same temperature, so it diffuses fastest.

Answer: Ammonia arrives first (Mr 17), then hydrogen sulfide (34), then sulfur dioxide (64).

Practice questions

Try each one, then open the answer.

1. A purple crystal of potassium manganate(VII) is placed at the bottom of a beaker of water. Without stirring, the colour slowly spreads through all the water. Which explanation is correct?

  1. A
    random movement of particles gives a net movement from high to low concentration
  2. B
    the particles only move upwards because they are less dense than water
  3. C
    the crystal particles expand to fill the beaker as they take in water
  4. D
    the water particles are attracted to the crystal and drag it along
Show answer

Answer: A. Diffusion is the net movement of particles from a region of higher concentration to lower concentration, caused by their random motion.

2. Which pair of gases would diffuse at the same rate at the same temperature? (C 12, N 14, O 16)

  1. A
    nitrogen, N2, and oxygen, O2
  2. B
    carbon monoxide, CO, and carbon dioxide, CO2
  3. C
    nitrogen, N2, and carbon monoxide, CO
  4. D
    oxygen, O2, and carbon dioxide, CO2
Show answer

Answer: C. Rate of diffusion depends on relative molecular mass. N2 = 28 and CO = 12 + 16 = 28, so they diffuse at the same rate.

3. Why does diffusion happen faster in a gas at a higher temperature?

  1. A
    the particles become larger and so collide more often
  2. B
    the particles are attracted to each other more strongly
  3. C
    the gas becomes denser, so the particles have less distance to travel
  4. D
    the particles have more kinetic energy and move faster
Show answer

Answer: D. At higher temperature particles move faster, so they spread through the container more quickly.

More questions on this topic

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