Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. Gases diffuse faster than liquids. Lighter particles (lower relative molecular mass) move faster at the same temperature, so they diffuse more quickly; higher temperature also speeds up diffusion.

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Section 1 of 14

Learning objectives

Describe diffusion in terms of random particle motion.
Explain why gases diffuse faster than liquids.
Explain the effect of relative molecular mass and temperature on the rate of diffusion.

Pause and think

Someone opens a bottle of perfume across the room. Why do you smell it a little while later, even with no breeze?

Section 2 of 14

Core idea

Start hereThe drop of ink

Drop some ink into a still glass of water and leave it. Nobody stirs it, yet an hour later the whole glass is coloured. Nothing pushed the ink: the water particles did it by bumping randomly.

The big idea

Diffusion is the net movement of particles from high to low concentration caused by their random motion. Lighter particles and higher temperatures make it faster.

1. Why diffusion happens

Every particle moves randomly. Where there are many particles, more of them happen to move outwards than inwards, so overall they spread out. When they are evenly spread, particles still move, but there is no further net movement.

2. Speed of diffusion

Gases diffuse much faster than liquids; diffusion in solids is negligible.
At the same temperature, particles with lower $M_r$ move faster.
Higher temperature → faster particles → faster diffusion.

Worked exampleThe ammonia and hydrogen chloride tube

Cotton wool soaked in concentrated ammonia and concentrated hydrochloric acid is placed at opposite ends of a $1$ m glass tube. A white ring of ammonium chloride forms.

Goal: Predict where the ring forms and explain.
Strategy: Compare relative molecular masses.

$M_r(\mathrm{NH_3})=14+3=17$
Ammonia.
$M_r(\mathrm{HCl})=1+35.5=36.5$
Hydrogen chloride.
$\mathrm{NH_3}+\mathrm{HCl}\rightarrow\mathrm{NH_4Cl}$
The gases meet and react to form the white solid.

AnswerNearer the hydrochloric acid end, because ammonia is lighter and diffuses faster.

✓ CheckThe faster gas covers more distance in the same time ✓.

Factors affecting diffusion

Drawing diagram…

Both factors work by changing particle speed.

Exam tipTwo key words Examiners look for "net movement" and "random motion" in a definition of diffusion.

Pause and think

Why does a helium balloon go down faster than one filled with air?

Summary

Net movement from high to low concentration.
Random motion is the cause.
Lighter or hotter → faster.

Diffusion: Net movement of particles from high to low concentration by random motion.
Relative molecular mass: Sum of the relative atomic masses in a molecule.

HistoryBrownian motion. In 1827 botanist Robert Brown saw pollen grains jiggling in water under a microscope. In 1905 Einstein explained it: the grains are being hit by invisible, moving water molecules.

Fun factSmelly balloons. Helium balloons go flat overnight faster than air-filled ones: tiny, light helium atoms escape through the rubber much more quickly.

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