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We cannot see individual particles, so how do we know they exist and are moving? The answer is that the particle model explains things we can observe — most importantly diffusion and dissolving. At the same time, the simple model used at GCSE is a simplification: it pictures particles as solid spheres with no forces between them, which is not quite true of real atoms, ions and molecules. A good scientist can do both — use the model where it works and recognise where it falls short. This lesson, part of Topic C1 of OCR Gateway Science A, examines the evidence for the particle model and its main limitations.
By the end of this lesson you should be able to define diffusion, explain diffusion and dissolving using the particle model, describe how temperature and state affect the rate of diffusion, state the limitations of the simple particle model, and explain why the model is still useful despite those limitations.
Diffusion is the net movement of particles from a region of high concentration to a region of low concentration, caused by the random motion of the particles. The particles spread out and mix without anything stirring or pushing them.
A familiar example is smell: when perfume is sprayed in one corner of a room, the scent gradually reaches the far side. The perfume particles, mixed with the air particles, move randomly in all directions; over time this random movement carries them throughout the room, from where they are concentrated to where they are not. Diffusion is strong evidence that gas particles are moving and that there are spaces between them for the perfume particles to spread into.
Diffusion can be shown clearly in the laboratory:
Exam Tip: A complete definition of diffusion has three parts: it is the net movement of particles, from high to low concentration, by random motion. Saying simply "particles spread out" is not enough for full marks.
Dissolving is more evidence for moving particles. When salt dissolves in water, the salt seems to disappear — but it is still there. The particle explanation is that the water particles, which are moving, surround and separate the salt particles and mix them evenly throughout the liquid. The salt particles fit into the spaces between the moving water particles, which is why the volume does not simply add up and why the solution is clear: the particles are too small to see and are spread evenly.
There is a neat demonstration of this hiding in plain sight. If you dissolve a teaspoon of sugar in a full glass of water, the water does not overflow as much as you might expect — and the same mass that went in can be recovered if the water is evaporated away. Nothing has been created or destroyed; the sugar particles have simply slotted into the gaps between the moving water particles. The fact that the solution tastes sweet throughout, not just where the sugar was added, is itself evidence that the dissolved particles have spread out by their own movement, just as in diffusion. Dissolving and diffusion are therefore two sides of the same coin: both rely on particles that are constantly moving and are small enough to fit into the spaces around them.
Two factors change how fast diffusion happens, and both make sense from the particle model:
1. State — diffusion is faster in gases than in liquids. In a gas the particles are far apart and move quickly, with lots of space to spread into, so mixing is fast. In a liquid the particles are closer together and move more slowly, getting in each other's way, so diffusion is much slower. (In a solid, diffusion is so slow as to be effectively negligible, because the particles only vibrate in place.)
2. Temperature — diffusion is faster when hotter. Heating gives the particles more kinetic energy, so they move faster. Faster-moving particles spread and mix more quickly, so diffusion speeds up. This is why a smell spreads more quickly in a warm room, and why sugar dissolves and spreads faster in hot tea than in iced tea.
| Factor | Faster diffusion when… | Why (particle model) |
|---|---|---|
| State | The substance is a gas rather than a liquid | Gas particles are far apart and move fast, with space to spread |
| Temperature | The temperature is higher | Particles have more kinetic energy and move faster |
Exam Tip: If asked why diffusion is faster at a higher temperature, the marking point is that the particles gain kinetic energy and so move faster. Always connect "hotter" to "faster-moving particles".
Explain, in terms of particles, why the smell of cooking spreads from the kitchen to the rest of the house, and why it spreads faster on a warm day.
Step 1 — identify the process: the smell spreads by diffusion.
Step 2 — explain the spreading: the smell particles, mixed with air particles, move randomly in all directions; this net movement carries them from where they are concentrated (the kitchen) to where they are less concentrated (the rest of the house).
Step 3 — explain the temperature effect: on a warm day the particles have more kinetic energy and move faster, so they diffuse and spread more quickly.
Answer: the smell spreads by the random motion of diffusing particles, faster when warm because the particles move faster.
A drop of food colouring is added to a glass of cold water, and an identical drop is sprayed into the air as a fine mist. In which does the colour spread faster, and why?
Step 1 — identify the two states: the water is a liquid, the air is a gas.
Step 2 — recall the rule: diffusion is faster in gases than in liquids.
Step 3 — give the reason from the particle model: in the gas the particles are far apart and move quickly, with plenty of space to spread into, whereas in the liquid the particles are closer together and move more slowly, getting in each other's way.
Answer: the colour spreads faster in the air (the gas), because gas particles are further apart and move faster, so the colouring diffuses more quickly.
The model used at GCSE — small circles for particles — is extremely useful, but it is a simplification of reality. You should be able to state its main limitations:
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