Evidence for Particles: Diffusion

Introduction

When someone opens a bottle of perfume at the front of a classroom, the smell reaches the back a short time later, even if nobody moves and there is no draught. A drop of ink in still water slowly colours the whole glass without stirring. This spontaneous spreading is called diffusion , and it provides direct evidence that particles are constantly moving.

Core explanation

Definition. Diffusion is the net movement of particles from a region where they are at a higher concentration to a region where they are at a lower concentration, as a result of their random motion. It continues until the particles are evenly spread.

How random motion causes spreading. Each particle moves in a straight line until it collides with another particle, then changes direction unpredictably. No particle "knows" where to go. However, where particles are crowded, more of them happen to move outwards than move inwards, simply because there are more of them. The overall (net) result is spreading from high to low concentration. Once the particles are evenly spread, they keep moving, but equal numbers move in every direction, so there is no further net change.

Diffusion in gases. Diffusion in gases is relatively fast because gas particles move quickly and have lots of space between them. Smells spread across a room within minutes (helped in real rooms by air currents).

Diffusion in liquids. Diffusion in liquids is much slower, because the particles are close together and constantly bump into neighbours, travelling only a tiny distance before changing direction. A drop of food colouring in still water may take hours to spread evenly.

Diffusion in solids. Particles in solids only vibrate about fixed positions, so diffusion in solids is extremely slow — though over years, metals pressed tightly together can show some mixing at the surface.

Factors affecting the rate. - Temperature: higher temperature → faster particles → faster diffusion. - Mass of the particles: lighter particles move faster at the same temperature, so they diffuse faster. - Concentration difference: a larger difference produces faster net spreading at first.

The ammonia and hydrogen chloride experiment. In a classic teacher demonstration, cotton wool soaked in ammonia solution is placed at one end of a long glass tube and cotton wool soaked in hydrochloric acid at the other, and the ends are sealed. The two gases diffuse towards each other and react where they meet, forming a white ring of ammonium chloride. The ring forms closer to the hydrochloric acid end, because ammonia molecules (mass 17) are lighter than hydrogen chloride molecules (mass 36.5) and so diffuse faster.

Step-by-step reasoning

To explain the position of the white ring:

1. Both gases diffuse along the tube towards each other. 2. At the same temperature, lighter particles move faster on average. 3. Ammonia particles are lighter than hydrogen chloride particles. 4. So ammonia travels further in the same time. 5. The ring forms where they meet — nearer the hydrogen chloride end.

Visual explanation

In the diffusion simulation, coloured particles start crowded on the left of a box and spread randomly to the right until they are evenly spread; a graph of concentration across the box flattens out over time. A diagram of the long tube shows the white ring about 60% of the way from the ammonia end.

Real-world analogy

When a class is let out of a crowded hall into a big empty playground, students wander randomly — nobody is told to spread out — but soon they are scattered all over the playground. There were simply more students moving away from the crowded area than into it.

Real-world example

Oxygen enters your blood by diffusion. In the lungs, air in the tiny air sacs (alveoli) has a higher concentration of oxygen than the blood arriving in the surrounding capillaries, so oxygen molecules diffuse into the blood. Carbon dioxide diffuses in the opposite direction. The huge surface area of the lungs makes this diffusion fast enough to keep us alive.

Why?

Why does diffusion stop changing things once the particles are evenly spread, even though the particles keep moving? At even concentration, as many particles move one way as the other. Movement continues, but there is no net change — diffusion is a statistical result of random movement.

Common misconception

"Particles diffuse because they want to spread out" or "they move towards emptier regions". Particles do not choose a direction; their motion is random. The spreading happens because there are more particles in the crowded region to move out of it.

Worked example

Question: Predict whether a smell will spread faster in a warm room or a cold room, and explain.

Reasoning: In a warm room, the particles have more kinetic energy and move faster, so they spread out more quickly.

Answer: Faster in the warm room, because the particles move faster at higher temperature.

Quick check

1. Why is diffusion faster in gases than in liquids? Answer: Gas particles move faster and have much more space between them, so they travel further between collisions.

Exam focus

Use the phrase "from a higher concentration to a lower concentration" and mention random movement. For the ammonia–hydrogen chloride tube, explain that the lighter particles diffuse faster, so the ring forms nearer the heavier gas. Link higher temperature to faster diffusion.

Advanced insight

Graham's law states that the rate of diffusion of a gas is inversely proportional to the square root of its molar mass. For ammonia (17) and hydrogen chloride (36.5), the ratio of rates is √(36.5 ÷ 17) ≈ 1.47, which matches the position of the white ring reasonably well.

Summary

Diffusion is the net spreading of particles from higher to lower concentration due to random motion. It is fastest in gases, slower in liquids and extremely slow in solids. Higher temperature and lighter particles speed it up. Diffusion experiments, such as perfume spreading and the ammonia–hydrogen chloride tube, provide strong evidence that particles are always moving.

Practice questions

1. Define diffusion. Answer: The net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. 2. Two gases, X (mass 4) and Y (mass 44), are released at the same time. Which diffuses faster? Why? Answer: X, because lighter particles move faster at the same temperature. 3. Why does food colouring spread faster in hot water than in cold water? Answer: Particles move faster in hot water, so they spread out more quickly. 4. Does diffusion stop when particles are evenly spread? Explain. Answer: The particles keep moving, but there is no net change because equal numbers move in all directions.