What is Diffusion?

Diffusion is defined as:

The net movement of particles from a region of higher concentration to a region of lower concentration due to random motion of particles.

This movement continues until the particles are evenly distributed.

Example of Diffusion in Everyday Life

Diffusion can be observed easily in daily life.

Examples include:

• The smell of perfume spreading across a room
• A drop of ink spreading in water
• Gas exchange in the lungs
• The smell of food spreading in the kitchen

In each of these cases, particles move from an area where they are more concentrated to where they are less concentrated.

Diffusion in Gases

Diffusion happens fastest in gases because gas particles:

• Are very far apart
• Move very quickly
• Experience very weak forces of attraction

For example, if someone sprays perfume on one side of a room, the smell spreads throughout the room quickly.

This happens because gas particles move rapidly in all directions.

Diffusion in Liquids

Diffusion also occurs in liquids but it happens more slowly than in gases.

In liquids:

• Particles are closer together
• Movement is more restricted

For example, when a drop of food coloring is added to water, it slowly spreads throughout the water.

Diffusion and Molecular Mass

The rate of diffusion depends on the relative molecular mass of particles.

• Lighter particles diffuse faster
• Heavier particles diffuse slower

For example:

• Ammonia gas (NH₃) diffuses faster than

• Hydrogen chloride gas (HCl)

This happens because lighter particles move faster.

Factors Affecting Diffusion

Several factors affect the rate of diffusion.

1. Temperature

Higher temperature means particles move faster.

Therefore:

Higher temperature → faster diffusion

2. Particle Mass

Lighter particles move faster.

Therefore:

Lower molecular mass → faster diffusion

3. Concentration Difference

A larger difference in concentration causes faster diffusion.

Therefore:

Greater concentration gradient → faster diffusion

Why Diffusion is Important in Chemistry

Diffusion explains many chemical and biological processes such as:

• Movement of gases in the lungs
• Spread of pollutants in the air
• Mixing of gases in the atmosphere
• Chemical reactions in solutions

It also helps students understand gas behaviour and reaction rates.

Common Exam Questions

In Cambridge O Level exams, students may be asked to:

• Define diffusion
• Explain diffusion using kinetic particle theory
• Compare diffusion in gases and liquids
• Explain why lighter gases diffuse faster

Students should always mention random particle motion when explaining diffusion.

Quick Revision Summary

• Diffusion is the movement of particles from high concentration to low concentration
• Diffusion occurs because particles move randomly
• Diffusion is fastest in gases
• Lighter particles diffuse faster than heavier particles
• Higher temperature increases diffusion rate

Understanding diffusion helps explain many natural and chemical processes.

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