Experiment: Diffusion of particles in matter

Diffusion is the everyday evidence that particles move on their own, and in the Matter and the Atomic Structure chapter it is used to establish two ideas at once: that particles are in constant random motion, and that there are empty spaces between them. This experiment shows diffusion in a liquid, where a coloured solid spreads through water, and in a gas, where two vapours meet and react. This guide sets out the method, the observations, and the Paper 3 skills the practical assessment rewards.

Aim

To show that the particles of matter are in constant random motion by observing the diffusion of a coloured substance through a liquid, and the diffusion of gases towards each other, and to relate the observations to the kinetic theory of matter.

Apparatus and materials

• Beaker of water and a small crystal of potassium manganate(VII)
• Forceps or a drinking straw to place the crystal
• Long dry glass tube with two rubber bungs
• Cotton wool
• Concentrated ammonia solution and concentrated hydrochloric acid
• Two droppers
• Retort stand and clamp
• Access to a fume cupboard

Procedure

Diffusion in a liquid

1. Half fill a beaker with water and leave it to stand until the water is completely still.
2. Using forceps or a straw, gently place a single small crystal of potassium manganate(VII) at the bottom of the beaker without stirring.
3. Leave the beaker undisturbed and observe the colour around the crystal over time.
4. Record how the purple colour spreads through the water.

Diffusion in a gas

5. Set up a long, dry glass tube horizontally on a retort stand, working in a fume cupboard.
6. Soak one piece of cotton wool with concentrated ammonia solution and a second piece with concentrated hydrochloric acid.
7. At the same moment, place the ammonia-soaked cotton wool in one end of the tube and the acid-soaked cotton wool in the other end, then close both ends with bungs.
8. Observe where a white ring of solid forms inside the tube and note its position relative to the two ends.

Expected observations

In the liquid, the purple colour of the potassium manganate(VII) slowly spreads outwards from the crystal, colouring the water around it and eventually turning the whole beaker an even, pale purple, even though the water is not stirred. In the gas tube, a white ring of solid (ammonium chloride) forms inside the tube; it forms nearer the end that held the hydrochloric acid rather than at the middle.

Inference and conclusion

The purple colour spreads because the particles of potassium manganate(VII) move at random and travel into the spaces between the water particles, moving from where they are concentrated to where they are less concentrated until they are evenly spread. This shows that particles are in constant motion and that matter contains spaces between particles. In the gas tube, both gases diffuse and meet to form a white solid. The ring forms nearer the hydrochloric acid because ammonia particles have a smaller mass than hydrogen chloride particles and so diffuse faster, travelling further before the two gases meet. Diffusion is faster in gases than in liquids because gas particles are further apart and move more freely.

Science process skills (Paper 3 style)

Making a hypothesis. The higher the temperature of the water, the faster the coloured substance diffuses through it.

Identifying variables. In an investigation of how temperature affects the rate of diffusion, the manipulated variable is the temperature of the water, the responding variable is the time taken for the colour to spread a fixed distance, and the controlled variables are the volume of water, the size of the crystal and the type of substance.

Tabulating data. Draw a table with a column for the manipulated variable (for example, temperature) and a column for the responding variable (time taken for the colour to reach a mark), each recorded to a consistent precision.

Making inferences. From the position of the white ring, infer that the lighter gas diffuses faster; from the even colour of the water, infer that particles move without being pushed.

Operational definition. Diffusion is operationally defined as the spreading of one substance through another until the mixture is uniform, without stirring.

Safety precautions

• Handle concentrated ammonia and concentrated hydrochloric acid in a fume cupboard, because both give off choking, corrosive fumes.
• Wear safety goggles, because the fumes and solutions can irritate or damage the eyes.
• Use forceps to handle the acid- and ammonia-soaked cotton wool, so the corrosive liquids do not touch the skin.
• Avoid staining, because potassium manganate(VII) leaves a stubborn brown stain on skin and clothing.

Common errors

• Stirring or knocking the beaker. Any disturbance mixes the water mechanically and hides the diffusion; leave the beaker completely still.
• Using too large a crystal. A large crystal makes the colour too dark to judge the spread; use a single small crystal.
• Not adding the two cotton wools at the same time. If one gas has a head start, the position of the white ring no longer compares the diffusion rates fairly.
• Working in the open with the gases. The fumes are harmful, so the gas part must be done in a fume cupboard, not on the open bench.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we use diffusion to lock in the kinetic theory: a student who can explain both the spreading colour and the off-centre white ring can answer the particle-model questions that recur across SPM Chemistry. Because explaining diffusion in terms of particle motion and mass is a recurring Paper 3 idea (Paper 3 is a practical test assessing science process skills), a clear grasp of this simple experiment supports the whole of Form 4 theory.