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Practice questions · Brownian motion

Random jiggles, real evidence.

Six original Cambridge-style questions on constant particle motion, the smoke-cell observation, why the path is jerky, and what we actually see versus what we work out.

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.
The chain of reasoning to reproduce

Small fast particles, uneven hits, random path.

01
[2 marks]

State what the kinetic particle model says about the particles in a gas.

  • A gas is made of tiny particles that are far apart. ✓
  • They are in constant, rapid, random motion in all directions. ✓
02
[2 marks]

Smoke is placed in a small glass cell and viewed through a microscope. Describe what is seen.

  • Small bright specks of light (the smoke particles) are seen. ✓
  • They move with a random, jerky, zig-zag motion. ✓
03
Analysis
[3 marks]

Explain, in terms of particles, why the smoke particles move in this random, jerky way.

  • The smoke particles are constantly bombarded by much smaller air particles. ✓
  • The air particles move fast and hit from random directions, so the hits are uneven. ✓
  • The changing net force pushes the smoke particle a different way each moment, giving a random jerky path. ✓
04
Analysis
[2 marks]

In this experiment, state what is actually seen through the microscope, and explain why the air particles themselves are not seen.

  • What is seen is the larger smoke particles (as points of light). ✓
  • The air particles are far too small to be seen, even under the microscope; their presence is only inferred from the motion they cause. ✓
05
[2 marks]

State how the motion of the particles in a gas changes when its temperature is increased, and what this means for their kinetic energy.

  • The particles move faster on average. ✓
  • Their average kinetic energy increases. ✓
06
Analysis
[2 marks]

Explain how the observation of Brownian motion provides evidence for the existence of fast-moving particles in the air.

  • The smoke particles could only be pushed about randomly if something kept striking them unevenly. ✓
  • This requires many small, fast-moving particles in the air, so the motion is evidence that such particles exist. ✓

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.