K323 / 2027
Kinetic particle model of matter overview

Lesson 3 of 4

Brownian motion

  • I can infer random molecular movement from observations in a Brownian motion experiment.Syllabus

    Syllabus K323, 7(c). Infer random molecular movement from observations in a Brownian motion experiment.

Make a guess. You are not marked.

Smoke specks in air are watched under a microscope. The air is then warmed. What happens to the motion of the specks?

  1. It becomes smooth and slow, because the warm air is thinner.
  2. It stops, because warm air rises and carries the specks out of view.
  3. It becomes faster and more jerky, because air molecules hit them harder.
Show the answer

It becomes faster and more jerky, because air molecules hit them harder.

Warmer air molecules move faster on average. They hit the specks harder and more often, so the specks' random jiggling becomes more vigorous.

The irregular movement of a small suspended particle provides evidence for the random motion of the surrounding fluid's molecules.

A fluid is a liquid or gas. In the particle model, its molecules move randomly. A Brownian-motion experiment links that model to an observable effect.

What the smoke-cell experiment shows

A transparent smoke cell holds small smoke particles suspended in air. Light shines into it from the side, and a microscope is focused on the illuminated region. Light scattered by the smoke particles makes them appear as small bright specks.

Observe a speck over time. It moves irregularly, with changing direction and speed, rather than following one smooth straight path. This irregular motion of a small particle suspended in a fluid is Brownian motion.

Keep the observation separate from its explanation

1. Apparatus: illuminate the smoke from the side

A smoke cell observed through a microscopeA microscope objective is above a glass-topped cell containing smoke suspended in air. Light enters the cell from the side. Smoke specks scatter light and can appear as bright points in the microscope. The surrounding air molecules are too small to be resolved. The drawing is a schematic arrangement, not a molecular view.MicroscopeSide lightGlasswindowSmoke cell: smoke in air

2. Observation: bright specks move irregularly

Schematic microscope view of visible smoke specksBright points in a dark circular field represent smoke specks, not air molecules. A line joins seven irregular successive positions of one smoke speck. This trace illustrates the kind of motion to look for; it is not an experimental recording. Other specks are shown separately. The surrounding air molecules cannot be resolved in this observation.One speck's successive positions

This is a schematic trace. In the experiment, look for irregular jiggling, and distinguish it from a shared drift of many specks.

3. Model: unseen air molecules collide with the speck

A model of the molecular explanation for Brownian motionA large smoke speck is surrounded by much smaller air molecules. The blue arrows show varied molecular motion, including molecules approaching the speck from different directions and others moving away. Changing, unequal collisions from the unseen molecules account for the observed irregular motion of the speck. This molecular view is an explanatory model; air molecules are not visible in the microscope view. Relative sizes and distances are schematic.SmokespeckSmall circles: air molecules

The explanation uses molecules too small to see in the experiment. The large speck and small molecules are not drawn to a true size ratio.

Keep the observation separate from the explanation. The microscope view shows suspended smoke particles. The model explains one particle's motion through changing collisions from much smaller, unseen air molecules. Any drawn trace illustrates successive positions; it is not a recorded air-molecule trajectory.

Visible specks are not air molecules

The bright objects are smoke particles, each much larger than an air molecule. Individual air molecules are not seen in this microscope view. Their motion is inferred from what happens to the suspended particles.

  1. Air molecules move randomly and continually collide with a suspended smoke particle.
  2. At a particular instant, the collisions are not exactly balanced on all sides, so there is a resultant force on the smoke particle.
  3. The imbalance changes from one instant to the next, changing the smoke particle's motion and producing the irregular observed path.

Worked explanation

A speck turns without meeting another visible speck

Observation: the smoke speck changes direction, although no other visible smoke particle touches it.

Explanation: unseen air molecules still bombard it. A changing imbalance in these collisions changes the force and the speck's motion. A visible collision between two smoke specks is not required.

Inference: the air molecules are moving randomly. The experiment gives evidence through their effects; it does not show their individual paths directly.

Small particles suspended in water can also show Brownian motion. In that case, changing unequal collisions from water molecules explain the suspended particles' irregular movement. The inference concerns random molecular motion in a liquid rather than in a gas.

Distinguish jiggling from a common drift

Focus sharply enough to follow individual illuminated specks, and observe several over time. A speck moving out of focus has not necessarily stopped moving; it may have left the focused region.

If many specks travel together steadily in one direction, the fluid itself may be flowing. That common drift alone does not establish the random-collision explanation. Reduce bulk movement and look for irregular changes superimposed on any drift.

Say what is seen and what is inferred. The suspended specks are observed. Random molecular motion is inferred. A drawing or animation can illustrate the model but is not itself experimental evidence.

Check your understanding

Through a microscope, a bright smoke speck changes direction without meeting another visible speck. Which conclusion distinguishes observation from inference?

  1. The bright speck is an individual air molecule whose motion is directly observed.
  2. No collision can have occurred because no other visible speck touched it.
  3. The speck is observed; collisions from unseen air molecules are inferred to move it.
Show the answer

The speck is observed; collisions from unseen air molecules are inferred to move it.

The visible speck is a suspended smoke particle, much larger than an air molecule. Its irregular motion is evidence for the random movement of the molecules that bombard it.

Practise an exam question

Write your answer first, then tick the mark points it makes.

Exam-style question

Smoke particles in air are seen through a microscope as bright specks that move about in a jerky, random way. Explain this observation. [3]

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