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IGCSE · Practice questions · Diffusion

Follow the particles.

Six original Cambridge-style questions on diffusion: the definition, experiments in gases and liquids, temperature, and the effect of relative molecular mass.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Random motion does the work.

01
Recall
[2 marks]

Define diffusion.

  • The net movement of particles from a region of higher concentration to a region of lower concentration, ✓
  • as a result of their random motion. ✓
02
Explain
[3 marks]

A crystal of potassium manganate(VII) is placed at the bottom of a beaker of water and left without stirring. Describe what is seen over several hours and explain it using the kinetic particle theory.

  • The purple colour spreads slowly upwards and outwards until all the water is evenly purple. ✓
  • The crystal dissolves and its particles, and the water particles, move randomly and collide. ✓
  • There is a net movement of manganate(VII) particles from high to low concentration until the concentration is the same everywhere. ✓
03
Analysis
[4 marks]

(Extended) In the ammonia and hydrogen chloride tube experiment, write the equation, with state symbols, for the reaction that forms the white ring, state where the ring forms and explain why.

  • NH3(g) + HCl(g) → NH4Cl(s). ✓
  • The ring forms nearer the hydrochloric acid end of the tube. ✓
  • Ammonia has a lower relative molecular mass (17) than hydrogen chloride (36.5). ✓
  • So ammonia particles move faster and diffuse further in the same time. ✓
04
Analysis
[2 marks]

(Extended) Put these gases in order of rate of diffusion, fastest first: carbon dioxide (Mr 44), helium (Mr 4), oxygen (Mr 32), methane (Mr 16). Explain your order.

  • helium, methane, oxygen, carbon dioxide. ✓
  • At the same temperature, particles with a lower relative molecular mass move faster, so they diffuse faster. ✓
05
Explain
[2 marks]

Explain why a smell spreads across a room faster on a hot day than on a cold day.

  • At a higher temperature the particles have more kinetic energy and move faster. ✓
  • So they spread out, and diffuse through the air, more quickly. ✓
06
Explain
[2 marks]

Explain why diffusion does not noticeably happen in solids.

  • Particles in a solid are held in fixed positions by strong forces of attraction. ✓
  • They only vibrate about those positions, so they cannot move past one another to mix. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.