Smart Study Wiki

1.7 Diffusion

4 min read

1.7 Diffusion

Summary: A teacher fills a gas jar with brown bromine gas. They put a lid on it and place a gas jar of air on top. Then the teacher removes the lid. Slowly, the bromine gas fills the top gas jar. You can use… Tags: science oxford-science student-book-7 chemistry Created: 2026-05-19 Last Updated: 2026-05-19

After this topic, you will be able to:

  • describe how the movement of the particles changes when a substance diffuses
  • use the particle model to explain how temperature, particle size, and state affect how quickly Diffusion happens
  • use the particle model to explain what you see in a diffusion experiment.

Think back

  1. What are the three states of matter?
  2. What are particles?
  3. How do particles move in the gas state?

Key idea In diffusion, particles move from an area of high concentration to an area of low concentration.

Key word concentration, diffusion

A teacher fills a gas jar with brown bromine gas. They put a lid on it and place a gas jar of air on top. Then the teacher removes the lid. Slowly, the bromine gas fills the top gas jar. You can use the particle model to explain why this happens. See Figures 1 and 2**.**

Two pairs of glass jars. Each pair of jars has one jar of brown gas on the bottom and one jar of clear air on top. In the jars on the left, the gases are separate. In the jars on the right, the brown gas has begun to fill the top glass jar.

Figure 1Bromine gas diffuses into the top gas jar on the right.

Two vertically stacked glass jars are separated by a removable lid. The lower jar is filled with bromine vapour, while the upper jar contains air. When the lid is removed, the bromine vapour and air mix.

Figure 2This particle diagram shows how bromine gas diffuses into the top gas jar. Not to scale.

A Describe the difference between the ‘before’ gas jars on the left of the photo and the ‘after’ gas jars on the right.

What is diffusion?

In the gas jars in Figure 1, bromine particles move from an area where there are many particles to an area where there are fewer particles. In other words, particles move from an area of high concentration to an area of low concentration. This is diffusion.

Diffusion happens in any substance in the liquid or gas state. It happens because the particles move around all the time. When they bump into each other, they change direction. Over time, the moving particles spread out. Diffusion happens by itself. You do not need to shake or mix the particles.

Diffusion explains why you can smell perfume across a room. Perfume particles evaporate from skin. The particles move around and bump into other perfume particles and air particles. Over time, they move from where there are many particles (near the skin) to where there are fewer particles.

B Name the two states of matter in which diffusion can happen.

What factors affect how quickly diffusion happens?

Several factors affect how quickly diffusion happens, including:

  • temperature
  • particle size
  • the state of the diffusing substance.

Diffusion happens more quickly at higher temperatures because particles move faster.

A teacher sets up the equipment in Figure 3 to show diffusion.

Hydrogen chloride particles evaporate from the cotton wool at the left end of the test tube. Ammonia particles evaporate from the cotton wool at the right. The particles diffuse along the tube. The two types of particle meet and form a ring of white solid. The solid is formed closer to the hydrogen chloride end. This shows that the hydrogen chloride particles diffuse more slowly. Hydrogen chloride particles are bigger and heavier than ammonia particles. Big, heavy particles diffuse more slowly than small, Light ones.

An experiment to show diffusion.

Figure 3This equipment shows that big, heavy particles diffuse more slowly than small, light particles. Full text alternative to the content

Diffusion happens more quickly in the gas state than in the liquid state. Diffusion does not happen in solids because the particles cannot move from place to place.

C Suggest why particles diffuse more slowly at lower temperatures.

Stretch zone

  1. Copy and complete the sentences. The movement of particles from an area where there are many to an area where there are few is called concentration/diffusion. Diffusion happens in the liquid and gas/solid states. Diffusion happens more quickly at high/low temperatures.
  2. Diffusion can be seen in an experiment. Describe an experiment that shows the diffusion of bromine.Explain what you see in your answer to 2a.

Stretch zone

  1. The air contains particles of argon, nitrogen, and other substances. Use the data below to predict which type of particle diffuses faster. Give a reason for your choice. Relative masses of particles: nitrogen = 28, argon = 40

Backlinks