1.2 Diffusion

2026 Syllabus Objectives

Core:

  • Describe and explain diffusion in terms of kinetic particle theory

Supplement:

  • Describe and explain the effect of relative molecular mass on the rate of diffusion of gases

What is Diffusion? 🔑

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Diffusion is the process by which different fluids mix as a result of the random motions of their particles.

Key Characteristics of Diffusion

Main Ideas:

  • Particles move from a region of higher concentration towards a region of lower concentration
  • Eventually, the particles are evenly spread throughout the available space
  • Their concentration becomes the same throughout

Important Points:

  • Diffusion occurs in both liquids and gases (collectively called fluids)
  • The rate of diffusion in liquids is much slower than in gases
  • Diffusion does not take place in solids as the particles cannot move from place to place

Diffusion and Kinetic Particle Theory ⚡

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Diffusion provides some of the earliest evidence for the kinetic model of states of matter. The kinetic particle theory explains diffusion through the following principles:

Key Principles

1. Particle Motion:

  • Particles in fluids are in constant, random motion
  • This random motion causes particles to spread out naturally

2. Temperature Dependence:

  • The average speed of particles increases with an increase in temperature
  • Higher temperatures lead to faster diffusion rates

3. Collisions:

  • Atoms or molecules in gases move at high speeds (approximately 1800 km/hour for nitrogen and oxygen in air)
  • Particles undergo millions of collisions per second with other particles
  • These frequent collisions slow down the overall rate of diffusion from one place to another

The pressure of a gas is the result of collisions of the fast-moving particles with the walls of the container.

Diffusion in Liquids 💧

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Example: Potassium Manganate(VII) in Water

When a crystal of potassium manganate(VII) is placed at the bottom of a beaker of water:

Step 1: The water around the crystal becomes purple as the solid begins to dissolve

Step 2: Particles move off from the surface of the crystal into the water

Step 3: Eventually, the crystal dissolves completely and the purple color spreads through the liquid

Step 4: The whole solution becomes uniformly purple - particles from the solid become evenly spread throughout the water

Practical Demonstration: Silver Nitrate and Potassium Iodide

This experiment demonstrates diffusion in liquids through the formation of a precipitate:

Method:

  1. Place a Petri dish on a white tile filled nearly to the top with deionised water
  2. Using tweezers, place a crystal of silver nitrate at one side and a crystal of potassium iodide at the other side
  3. As the crystals dissolve, ions diffuse through the solution
  4. Where the ions meet, they react to form a yellow precipitate of silver iodide
  5. The precipitate line forms where the diffusing ions encounter each other

Importance of Diffusion in Solution

Diffusion in solution is especially important when the solute is a gas. This plays a critical role in breathing:

  • Diffusion contributes to the movement of oxygen from the lungs to the blood
  • Diffusion enables the movement of carbon dioxide from the blood to the lungs

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