Edexcel IGCSE Chemistry · Spec 1.3-1.4
Diffusion & Key Terms
Diffusion and the key chemistry terms you need, explained with the particle model.
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Diffusion & Key Terms
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Explained
Diffusion, and the words that go with solutions
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Drop purple dye into water and it spreads until the colour is even, with nobody stirring it.
Nothing is pushing the particles. They are already moving randomly, and random movement from a crowded region into an empty one happens more often than the reverse. The word net matters: particles keep moving in both directions, but more travel one way than the other until the concentrations are equal.
Why some particles diffuse faster
At the same temperature, lighter particles move faster than heavier ones, so they diffuse faster. Relative molecular mass is what decides it.
The classic demonstration puts a cotton wool plug soaked in ammonia solution at one end of a long glass tube and one soaked in concentrated hydrochloric acid at the other. The two gases travel towards each other and a white ring of ammonium chloride forms where they meet.
The ring is not in the middle. Ammonia has a relative molecular mass of 17 and hydrogen chloride 36.5, so the ammonia moves faster and travels further, and the ring forms nearer the hydrochloric acid end.
Raising the temperature speeds diffusion up, because the particles gain kinetic energy and move faster.
What the mark schemes accept and reject
Edexcel International GCSE mark schemes are precise about the definition. One awards the mark for the movement of particles, atoms, molecules or ions from a region of high concentration to a region of lower concentration, then adds two restrictions: ignore substance, and not just movement alone down a concentration gradient. Naming the particles is required, and saying that something moves down a concentration gradient is not enough on its own.
On the glass tube question, an International GCSE Chemistry mark scheme awards marks for saying the ring should be closer to the hydrochloric acid, because hydrogen chloride has the higher relative molecular mass, so it moves and diffuses more slowly and travels a shorter distance. It explicitly rejects the answer that the ring should be in the centre.
The examiner report on that paper adds that some learners missed a mark because they did not refer to particles or molecules at all.
The key terms
- A solute is the substance that dissolves, such as the salt.
- A solvent is the liquid it dissolves in, such as the water.
- A solution is the two of them mixed together, and it is a mixture rather than a compound.
- Soluble means it dissolves in a given solvent. Insoluble means it does not.
- A saturated solution is one that has dissolved as much solute as it can at that temperature, so any more added simply sits at the bottom.
The phrase at that temperature is part of the definition of saturated. Warm the solution and more will usually dissolve, which is why crystals appear again when a saturated solution cools.
Dissolving is not the same as diffusing
Drop a crystal of potassium manganate into still water and two things happen in order. The crystal dissolves, so its particles separate and mix with the water. Then those particles diffuse through the water until the purple colour is spread evenly.
A mark scheme for that question awards one mark for dissolving and one for diffusion and accepts them in either order, which tells you both words are wanted. Using only one of them describes half the process.
Spec 1.3-1.4
What you need to know
- Explain what diffusion is using particle theory
- Describe how the mass of a particle affects how fast it diffuses
- Use the terms solute, solvent, solution and saturated solution correctly
Active recall
Quick check
Answer each question before opening the answer.
What is diffusion?
The net movement of particles from a region of high concentration to a region of low concentration.
Why do lighter gas molecules diffuse faster than heavier ones?
At the same temperature they move at higher speeds, so they spread out more quickly.
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