IGCSE Diffusion: Complete Guide for Cambridge IGCSE Chemistry

IGCSE diffusion is a fundamental topic in Cambridge IGCSE Chemistry that explains how particles move and spread out. Mastering diffusion processes, factors affecting diffusion rate, and concentration gradients is essential for understanding how substances move and mix, both in chemistry and biological systems.

This comprehensive IGCSE diffusion guide covers everything you need to know, including what diffusion is, how it works in gases and liquids, factors affecting diffusion rate, real-world examples, step-by-step worked examples, common exam questions, and expert tips from Tutopiya’s IGCSE chemistry tutors. We’ll also show you how to avoid the most common mistakes that cost students valuable marks.

🎯 What you’ll learn: By the end of this guide, you’ll know how to define diffusion, explain how it occurs, identify factors that affect diffusion rate, and apply these concepts to solve problems in IGCSE Chemistry exams.

Already studying with Tutopiya? Practice these skills with our dedicated IGCSE Chemistry practice deck featuring exam-style questions and instant feedback.

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Why IGCSE Diffusion Matters

IGCSE diffusion is an important topic that appears throughout the IGCSE Chemistry curriculum. Here’s why it’s so important:

• Foundation concept: Essential for understanding how particles move and substances mix
• High frequency topic: Diffusion questions appear regularly in IGCSE chemistry papers
• Real-world applications: Used in everyday life, biology, and industrial processes
• Exam weight: Typically worth 4-8 marks per paper
• Link to other topics: Connects to particle theory, states of matter, and kinetic theory

Key insight from examiners: Students often confuse diffusion with other processes or struggle to explain factors affecting diffusion rate. This guide will help you master these concepts systematically.

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Understanding Diffusion

Diffusion is the movement of particles from an area of high concentration to an area of low concentration until they are evenly distributed.

Key Characteristics of Diffusion

• Spontaneous process: Happens naturally without external energy
• Down concentration gradient: Always from high to low concentration
• Particles move randomly: Due to their kinetic energy
• Results in uniform distribution: Particles spread out evenly
• Occurs in all states: But at different rates

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How Diffusion Works

The Process

1. Particles have kinetic energy and are in constant random motion
2. In a region of high concentration, there are more particles
3. Particles collide and move randomly
4. More particles move from high concentration areas to low concentration areas
5. Eventually, particles become evenly distributed throughout the available space

Concentration Gradient

Concentration gradient is the difference in concentration between two areas.

• Steep gradient: Large difference in concentration → faster diffusion
• Shallow gradient: Small difference in concentration → slower diffusion
• No gradient: Uniform concentration → no net diffusion (equilibrium reached)

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Diffusion in Gases

Diffusion occurs very quickly in gases because:

• Gas particles have high kinetic energy
• Particles move rapidly in all directions
• There are large spaces between particles
• Particles collide frequently and change direction

Examples of Gas Diffusion

1. Perfume in a Room

• Perfume particles evaporate and spread throughout the room
• You can smell it even far from the source
• Diffusion happens quickly in air

2. Ammonia and Hydrogen Chloride

• When cotton wool soaked in ammonia and another in concentrated hydrochloric acid are placed at opposite ends of a tube
• White ammonium chloride forms in the middle
• Shows that lighter ammonia particles (NH₃) diffuse faster than heavier hydrogen chloride particles (HCl)

3. Bromine Gas

• Brown bromine vapor spreads through air
• Shows visible diffusion of a colored gas

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Diffusion in Liquids

Diffusion occurs more slowly in liquids because:

• Liquid particles have less kinetic energy than gas particles
• Particles move slower than in gases
• Particles are closer together than in gases
• More collisions slow down movement

Examples of Liquid Diffusion

1. Food Coloring in Water

• Drop food coloring into still water
• Color spreads slowly through the water
• Eventually, the water becomes uniformly colored

2. Copper(II) Sulfate in Water

• Blue copper(II) sulfate crystals dissolve
• Blue color diffuses through water
• Shows diffusion in a visible way

3. Potassium Permanganate

• Purple potassium permanganate solution diffuses in water
• Can observe the spreading of color

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Diffusion in Solids

Diffusion occurs very slowly in solids because:

• Solid particles are closely packed in a regular pattern
• Particles can only vibrate in fixed positions
• Particles cannot move from their positions easily
• Requires high temperatures for significant diffusion

Examples of Solid Diffusion

• Diffusion of atoms in metals at high temperatures
• Steel hardening processes
• Semiconductor manufacturing

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Factors Affecting Diffusion Rate

Several factors affect how fast diffusion occurs:

1. Temperature

Higher temperature → Faster diffusion

• Why: Particles have more kinetic energy and move faster
• Example: Food coloring diffuses faster in hot water than cold water
• Particles move: Faster at higher temperatures, increasing collision frequency

Lower temperature → Slower diffusion

• Why: Particles have less kinetic energy and move slower
• Example: Diffusion is slower in cold conditions

2. Particle Size (Molecular Mass)

Smaller particles → Faster diffusion

• Why: Lighter particles move faster than heavier particles at the same temperature
• Example: Hydrogen (H₂, mass 2) diffuses faster than oxygen (O₂, mass 32)
• Graham’s Law: Rate of diffusion is inversely proportional to the square root of molecular mass

Larger particles → Slower diffusion

• Why: Heavier particles move slower
• Example: Large molecules diffuse more slowly than small molecules

3. Concentration Gradient

Steeper gradient → Faster diffusion

• Why: Larger difference in concentration provides a stronger “driving force”
• Example: Strong perfume smell diffuses faster initially when concentration difference is large
• As diffusion progresses: Gradient becomes smaller, diffusion slows down

Smaller gradient → Slower diffusion

• Why: Less difference in concentration, weaker driving force
• As equilibrium approaches: Diffusion rate decreases

4. State of Matter

Gases > Liquids > Solids

• Gases: Fastest diffusion (particles move rapidly with high energy)
• Liquids: Moderate diffusion (particles move slower)
• Solids: Slowest diffusion (particles can only vibrate)

5. Distance

Shorter distance → Faster to reach equilibrium

• Why: Particles have less distance to travel
• Example: Diffusion appears faster over short distances

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Graham’s Law of Diffusion

Graham’s Law states that the rate of diffusion is inversely proportional to the square root of the molecular mass.

Formula: Rate₁ / Rate₂ = √(M₂ / M₁)

Where:

• Rate₁ and Rate₂ are diffusion rates
• M₁ and M₂ are molecular masses

Application

Example: Hydrogen (H₂, mass 2) diffuses 4 times faster than oxygen (O₂, mass 32)

Calculation:

• Rate(H₂) / Rate(O₂) = √(32/2) = √16 = 4
• Hydrogen diffuses 4 times faster

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Brownian Motion

Brownian motion is the random, erratic movement of small particles suspended in a fluid (liquid or gas).

Key Characteristics

• Random movement: Particles move in unpredictable paths
• Caused by collisions: With invisible, rapidly moving fluid particles
• Visible evidence: Of particle movement and kinetic theory
• Demonstrates: That particles are in constant motion

Example

• Smoke particles in air: Can be seen moving randomly under a microscope
• Shows that air particles (invisible) are colliding with smoke particles

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Diffusion vs Other Processes

Diffusion vs Osmosis

Diffusion:

• Movement of any particles
• Can occur in any state
• No membrane required

Osmosis:

• Movement of water molecules only
• Through a partially permeable membrane
• From dilute to concentrated solution

Diffusion vs Active Transport

Diffusion:

• Down concentration gradient (high to low)
• No energy required
• Spontaneous process

Active Transport:

• Against concentration gradient (low to high)
• Energy required (ATP)
• Uses carrier proteins

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Real-World Applications of Diffusion

In Biology

• Gas exchange in lungs: Oxygen diffuses into blood, carbon dioxide diffuses out
• Nutrient absorption: Nutrients diffuse from digestive system to blood
• Plant transpiration: Water vapor diffuses out of leaves

In Everyday Life

• Cooking: Aromas diffuse through the air
• Air fresheners: Fragrance diffuses to fill a room
• Tea brewing: Tea compounds diffuse into hot water

In Industry

• Separation techniques: Based on different diffusion rates
• Food preservation: Understanding diffusion helps preserve food
• Chemical processes: Many reactions depend on diffusion

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Step-by-Step Method for Diffusion Problems

1. Identify the particles involved (what is diffusing?)
2. Determine the state (gas, liquid, or solid?)
3. Consider the factors affecting rate (temperature, particle size, gradient)
4. Explain using particle theory (kinetic energy, collisions, movement)
5. Use diagrams if helpful to show concentration differences
6. Check your answer - does it make sense?

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Worked Examples

Example 1: Explaining Diffusion

Explain why you can smell perfume from across a room.

Solution:

1. Perfume contains volatile particles that evaporate into the air
2. These particles have high kinetic energy and move rapidly (gas state)
3. Particles diffuse from the area of high concentration (near the perfume) to areas of low concentration (across the room)
4. The random movement of particles causes them to spread out evenly
5. Eventually, perfume particles reach your nose, allowing you to smell it

Example 2: Factors Affecting Rate

Explain why diffusion is faster in hot water than cold water.

Solution:

1. At higher temperatures, water particles have more kinetic energy
2. They move faster and collide more frequently
3. Dissolved particles (e.g., food coloring) also move faster
4. This increases the rate at which particles spread from high to low concentration areas
5. Therefore, diffusion occurs faster in hot water

Example 3: Comparing Diffusion Rates

Explain why hydrogen gas diffuses faster than chlorine gas.

Solution:

1. Hydrogen (H₂) has a molecular mass of 2
2. Chlorine (Cl₂) has a molecular mass of 71
3. According to Graham’s Law, lighter particles diffuse faster
4. At the same temperature, hydrogen particles move faster than chlorine particles
5. Therefore, hydrogen diffuses faster than chlorine

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Common Examiner Traps (and How to Dodge Them)

• Confusing diffusion direction - Always from high to low concentration, never the reverse
• Forgetting about random movement - Diffusion is due to random particle motion, not directed movement
• Ignoring temperature - Temperature significantly affects diffusion rate
• Not mentioning particle size - Smaller/lighter particles diffuse faster
• Confusing diffusion with other processes - Diffusion is passive, down a gradient
• Forgetting state of matter - Gases diffuse fastest, solids slowest

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IGCSE Diffusion Practice Questions

Question 1: Basic Explanation

What is diffusion? Explain why it occurs.

Solution: Diffusion is the movement of particles from an area of high concentration to an area of low concentration. It occurs because particles are in constant random motion due to their kinetic energy. There are more particles in high concentration areas, so more particles move from these areas to low concentration areas until evenly distributed.

Question 2: Factors Affecting Rate

State three factors that affect the rate of diffusion and explain how each affects the rate.

Solution:

1. Temperature: Higher temperature increases particle kinetic energy and speed, making diffusion faster
2. Particle size: Smaller/lighter particles move faster, so diffusion is faster
3. Concentration gradient: Steeper gradient (larger difference) provides stronger driving force, making diffusion faster

Question 3: Comparing States

Explain why diffusion occurs faster in gases than in liquids.

Solution:

• Gas particles have higher kinetic energy and move much faster than liquid particles
• Gas particles are widely spaced with large gaps between them, allowing easier movement
• Liquid particles have lower kinetic energy, move slower, and are closer together, making diffusion slower

Question 4: Real-World Application

Explain how diffusion is involved in the process of smelling food being cooked.

Solution:

1. Cooking releases volatile particles (aromas) from the food
2. These particles evaporate and enter the air as gas particles
3. Gas particles have high kinetic energy and move rapidly
4. Particles diffuse from the area of high concentration (near the cooking food) to areas of low concentration (throughout the room)
5. When particles reach your nose, you can smell the food

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Frequently Asked Questions About IGCSE Diffusion

What is diffusion?

Diffusion is the movement of particles from an area of high concentration to an area of low concentration until they are evenly distributed.

Why does diffusion occur?

Diffusion occurs because particles are in constant random motion due to their kinetic energy. More particles naturally move from areas of high concentration to areas of low concentration.

What factors affect the rate of diffusion?

The main factors are: temperature (higher = faster), particle size (smaller = faster), concentration gradient (steeper = faster), and state of matter (gases > liquids > solids).

Why is diffusion faster in gases than liquids?

Gas particles have higher kinetic energy, move faster, and are more widely spaced than liquid particles, allowing faster movement and spreading.

What is a concentration gradient?

A concentration gradient is the difference in concentration between two areas. Diffusion occurs down the concentration gradient (from high to low).

Does diffusion require energy?

No, diffusion is a passive process that occurs spontaneously without requiring external energy. It happens due to the natural random movement of particles.

What is Brownian motion?

Brownian motion is the random, erratic movement of small particles suspended in a fluid, caused by collisions with rapidly moving fluid particles. It provides visible evidence that particles are in constant motion.

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Related IGCSE Chemistry Resources

Strengthen your IGCSE Chemistry preparation with these comprehensive guides:

• IGCSE Solids, Liquids and Gases: Complete Guide - Master states of matter and particle theory
• IGCSE Elements, Compounds and Mixtures: Complete Guide - Master classification of matter
• IGCSE Chemistry Revision Notes, Syllabus and Preparation Tips - Complete syllabus overview, topic breakdown, and revision strategies
• IGCSE Past Papers Guide - Access free IGCSE past papers and exam resources

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