Brownian Motion – IGCSE Chemistry Definition

Brownian motion is the irregular, random, jerky movement of small visible particles suspended in a gas or liquid. The movement is caused by uneven bombardment from the much smaller, invisible molecules of the surrounding gas or liquid. It provides direct evidence that the molecules in gases and liquids are in constant random motion, which is a central idea of kinetic particle theory.

How it works

When a visible particle such as a smoke particle or a pollen grain is surrounded by air molecules, those air molecules collide with it from all sides. Because the collisions are random and unequal, the visible particle receives a net push in one direction for a brief moment, then another push in a different direction. The result is a jerky, zigzag path that can be observed under a microscope.

The smoke cell experiment is the classic IGCSE demonstration. A glass cell containing smoke is illuminated and viewed through a microscope. Bright specks of light (smoke particles reflecting the light) are seen moving randomly. The smoke particles themselves are much larger than the air molecules hitting them, but the uneven collisions produce visible movement.

Exam context

Examiners expect candidates to connect Brownian motion to the kinetic particle theory. A common Paper 4 question provides a description of the smoke cell experiment and asks candidates to explain the observation. The key marking points are: (1) the smoke particles are hit by air molecules, (2) the air molecules are in constant random motion, and (3) unequal numbers of collisions on different sides cause the visible particle to change direction erratically.

Worked exam question

A student observes tiny particles of pollen on the surface of still water through a microscope. The pollen grains move in random, jerky paths. Explain this observation. (3 marks)

The water molecules are in constant random motion [1]. They collide with the pollen grains [1]. The collisions are uneven on different sides, causing the pollen to move randomly and change direction [1].

Common mistakes

Candidates often say the smoke particles are colliding with each other --- this is incorrect. The visible particles are too far apart to collide frequently. The random motion is caused by invisible molecules hitting the visible particles. Another error is failing to explain why the movement is jerky: it is the unequal bombardment (more hits on one side than another at any instant) that pushes the particle in a new direction each moment.