What Is Diffusion?

Introduction

Someone peels an orange at one end of a classroom and soon the whole room smells of it. A drop of food colouring in a glass of still water slowly tints all of the water. A sugar cube left in tea eventually sweetens every mouthful. In each case, particles spread from where they are crowded to where they are scarce, without anyone stirring or blowing. This spontaneous spreading is called diffusion , and it is a direct consequence of particles being in constant random motion.

Core explanation

Definition. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration , as a result of their random motion. It happens in gases and liquids, and needs no stirring, pumping or outside energy.

Concentration. Concentration describes how crowded particles of a substance are: how many are packed into a given volume. Where a perfume has just been sprayed, its concentration is high; across the room it is low. A difference in concentration between two places is called a concentration gradient . Diffusion happens down a concentration gradient — from high to low.

How random motion produces diffusion. Individual particles do not "know" where the low concentration is. Each one moves randomly, in any direction. But consider the boundary between a crowded region and an empty one. Because there are many more particles on the crowded side, many more of them happen to cross the boundary into the empty side than the other way round. Some particles do move back, but fewer. The overall or net result is a flow from high to low concentration.

Why "net" matters. Particles move in both directions all the time. Diffusion describes the balance: more particles travel down the gradient than up it. The word "net" reminds us that this is an overall effect of huge numbers of random movements, not a directed journey by each particle.

When diffusion ends. As diffusion continues, the concentrations on the two sides become closer. Eventually the particles are spread evenly and the concentration is the same everywhere. The particles keep moving randomly, but now as many cross any boundary one way as the other. There is no further net movement , so nothing appears to change. The substances are fully mixed.

Diffusion is evidence for moving particles. If particles were stationary, a coloured substance would stay exactly where it was placed. The fact that substances mix by themselves supports the idea that particles are always moving.

Where diffusion happens. Diffusion is fast in gases, slower in liquids, and effectively absent in solids, because of differences in particle spacing and movement.

Step-by-step reasoning

To explain how diffusion occurs:

1. Particles are in constant, random motion. 2. There are more particles in the region of higher concentration. 3. So more particles move from high to low concentration than from low to high. 4. There is a net movement from high to low until the concentration is even.

Visual explanation

In the simulation, remove the barrier between a box packed with red particles and a box of blue particles. Each particle wanders randomly, yet the red ones steadily spread into the blue side and the blue into the red. A concentration bar underneath levels out, then stays flat while the particles keep moving.

Real-world analogy

Imagine a crowded school hall next to an empty playground, with the doors open. Pupils wander randomly. Because the hall is packed, many more wander out than wander back in, so the crowd spreads until both spaces are equally busy — and then people keep wandering, but the numbers stay balanced.

Real-world example

Air fresheners and scented candles rely on diffusion. Fragrance particles escape into the air near the source, where their concentration is high, and diffuse into the rest of the room where their concentration is low. In a still room, the smell gradually reaches every corner without any fan.

Why?

Why does diffusion appear to stop even though particles never stop moving? Once the particles are evenly spread, random motion carries equal numbers across any boundary in each direction. Movement continues, but the net movement is zero, so the concentration no longer changes.

Common misconception

"In diffusion, particles deliberately move towards the area with fewer particles." Particles move randomly and have no direction preference. The net flow from high to low concentration is purely a result of there being more particles in the crowded region to move.

Worked example

Question: A crystal of purple potassium manganate(VII) is placed at the bottom of a beaker of water and left undisturbed. After a day, the whole of the water is pale purple. Explain this observation.

Reasoning: The crystal dissolves, giving a high concentration of purple particles at the bottom. Water and purple particles move randomly, so there is net movement upwards, where the concentration is lower.

Answer: The purple particles diffuse from high concentration near the crystal to low concentration elsewhere, through random motion, until they are spread evenly and the colour is uniform.

Quick check

1. In which direction is the net movement of particles in diffusion? Answer: From a region of higher concentration to a region of lower concentration.

Exam focus

Learn the definition word for word: "net movement of particles from a region of higher concentration to a region of lower concentration". Always include "random motion" in explanations and the word "net". Say that diffusion continues until the concentration is uniform.

Advanced insight

Diffusion increases the spreading out, or disorder, of a system — chemists call this an increase in entropy . Unmixing never happens by itself because there are vastly more ways for particles to be spread out than gathered in one place. Diffusion is one everyday example of this deep principle.

Summary

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by random particle motion. More particles cross from the crowded side simply because there are more of them. It continues until the concentration is uniform, after which particles still move but there is no net change. Diffusion is evidence that particles move.

Practice questions

1. Define diffusion. Answer: The net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their random motion. 2. Explain why diffusion happens from high to low concentration even though particles move randomly. Answer: There are more particles in the high-concentration region, so more of them happen to move towards the low-concentration region than move the other way. 3. Explain why a drop of ink in still water eventually colours the water evenly and then stops changing. Answer: Ink particles diffuse until they are evenly spread; after that, equal numbers move in every direction, so there is no net movement and the colour stays the same. 4. Why is diffusion considered evidence for the particle model? Answer: Substances mix without stirring, which can only happen if they are made of particles that are constantly moving.