1.2 Diffusion

Cambridge (CIE) IGCSE Chemistry Revision Notes

📚 Part of Topic 1 📝 Exam code: 0620 & 0971

📌 Topic 1: States of Matter

1.1 Solids, liquids and gases

1 State the distinguishing properties of solids, liquids and gases ▼

Property	Solid	Liquid	Gas
Shape	Fixed shape	Takes shape of container	Takes shape of container
Volume	Fixed volume	Fixed volume	Fills container completely
Compressibility	Very difficult to compress	Difficult to compress	Easy to compress
Flow	Does not flow	Flows freely	Flows freely
Density	High density	High density	Very low density

🔑 Key Understanding: Solids keep their shape because particles are held tightly in fixed positions. Liquids flow because particles can slide past each other. Gases spread out to fill any container because particles move rapidly in all directions.

2 Describe the structures of solids, liquids and gases in terms of particle separation, arrangement and motion ▼

Property	Solid	Liquid	Gas
Particle separation	Very close (touching)	Close (touching)	Far apart (large spaces)
Arrangement	Regular, repeating pattern	Random, irregular	Random, irregular
Motion	Vibrate in fixed positions	Move around each other	Move rapidly in all directions

SOLID                    LIQUID                    GAS
  ●   ●   ●               ●    ●                  ●        ●
    ●   ●                   ●    ●                       ●
  ●   ●   ●               ●    ●                  ●        ●
(Regular pattern)      (Random arrangement)   (Far apart, random)

3 Describe changes of state in terms of melting, boiling, evaporating, freezing and condensing ▼

Change	From → To	Energy Change
Melting	Solid → Liquid	Heat absorbed (endothermic)
Freezing	Liquid → Solid	Heat released (exothermic)
Boiling	Liquid → Gas	Heat absorbed (endothermic)
Evaporation	Liquid → Gas	Heat absorbed (endothermic)
Condensation	Gas → Liquid	Heat released (exothermic)
Sublimation	Solid → Gas	Heat absorbed (endothermic)

     Melting ↑      Boiling/Evaporation ↑
SOLID ──────────► LIQUID ──────────────► GAS
      ←──────────          ←─────────────
     Freezing ↓           Condensation ↓

💡 Important Notes:
• Melting point = Freezing point (same temperature)
• Boiling point is specific for each substance
• Evaporation occurs at any temperature, only at the surface
• Boiling occurs throughout the liquid at a specific temperature

4 Describe the effects of temperature and pressure on the volume of a gas ▼

Condition	Effect	Explanation
Temperature ↑	Volume increases	Particles gain kinetic energy, move faster, collide with walls more forcefully, pushing outward
Temperature ↓	Volume decreases	Particles lose energy, move slower, collisions become weaker, volume contracts
Pressure ↑	Volume decreases	External force pushes particles closer together, reducing space between them
Pressure ↓	Volume increases	Particles are allowed to spread apart into larger space

⚠️ Common Mistake Alert: Particles do NOT expand when heated. The particles themselves stay the same size; they just move more and take up more space.

5 Explain changes of state in terms of kinetic particle theory, including the interpretation of heating and cooling curves Supplement ▼

How Energy Affects Particles:

Process	What Happens to Particles
Melting	Particles absorb energy → vibrate more → forces between particles weaken → particles can move past each other
Boiling	Particles gain enough energy to overcome all attractive forces → escape as gas
Freezing	Particles lose energy → move slower → forces pull them into fixed positions
Condensing	Particles lose energy → slow down → forces pull them together into liquid

HEATING CURVE:
Temperature
    ↑
    │                                    Gas
    │                                  ─────── (Boiling point)
    │                              Liquid + Gas
    │                            ─────── (Melting point)
    │                        Solid + Liquid
    │      ───────────────────────────────────→ Time
    │      Solid

🔑 Key Points:
• Flat sections (plateaus) = change of state occurring
• During a change of state, temperature remains constant
• All energy goes into breaking or forming bonds between particles

6 Explain, in terms of kinetic particle theory, the effects of temperature and pressure on the volume of a gas Supplement ▼

Temperature Effect:
Increasing temperature → particles gain kinetic energy → particles move faster and collide with container walls more frequently → each collision has more force → volume increases (if container can expand)

Pressure Effect:
Increasing external pressure → particles are forced closer together → same number of particles in smaller space → volume decreases → particles hit walls more frequently but over smaller area

1.2 Diffusion

1 Describe and explain diffusion in terms of kinetic particle theory ▼

Definition: Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration until evenly spread.

Explanation Using Kinetic Theory:
• Particles are in constant random motion
• They move from areas where there are many particles to areas where there are fewer
• This continues until particles are evenly distributed (equilibrium)

Examples:
• Perfume spreading across a room
• Food coloring dispersing in water
• Oxygen entering blood in lungs

💡 Key Fact: Diffusion happens without any energy input. Higher temperatures speed up diffusion because particles have more kinetic energy.

2 Describe and explain the effect of relative molecular mass on the rate of diffusion of gases Supplement ▼

Gas	Relative Molecular Mass (Mr)	Diffusion Rate
Hydrogen (H₂)	2	Fastest
Helium (He)	4	Very fast
Water vapour (H₂O)	18	Moderate
Oxygen (O₂)	32	Slow
Carbon dioxide (CO₂)	44	Slower
Chlorine (Cl₂)	71	Very slow

Explanation:
• At the same temperature, all gas particles have the same average kinetic energy
• Kinetic energy = ½ × mass × velocity²
• Therefore: velocity² ∝ 1/mass
• Lighter particles (lower Mr) move faster
• Faster particles → faster diffusion

🧪 Classic Experiment: Ammonia (Mr = 17) and hydrogen chloride (Mr = 36.5) diffuse in opposite directions. Ammonia travels faster, so the white ring of ammonium chloride forms closer to the hydrogen chloride end.

⚠️ Common Mistakes to Avoid:

• ❌ "Particles expand when heated" → ✅ Particles gain energy and move more, but they do NOT expand
• ❌ "Space between particles is filled with air" → ✅ Space between particles is empty (vacuum)
• ❌ "Particles change state" → ✅ The SUBSTANCE changes state due to particle energy changes
• ❌ "Diffusion requires energy" → ✅ Diffusion happens naturally due to random motion
• ❌ "Gases with higher Mr diffuse faster" → ✅ Gases with LOWER Mr diffuse faster

❓ Did this page help you?

Written by: ChemPassion Team | Reviewed by: IGCSE Chemistry Examiners

Updated on: March 2026