The shape of the lesson
By the end of the lesson, learners can
- Random motion: Watch a large particle being knocked about by particles too small to see.
- What changes the rate?: Every factor comes back to one question: how fast are the particles moving, and how often do they collide?
- Heavier particles are slower: Extended.
- Diffusion: net movement from higher to lower concentration.
- Gases diffuse faster than liquids; solids hardly at all.
- Hotter means faster particles and faster diffusion.
- Extended: lower Mr means faster diffusion.
One picture every idea returns to
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. No stirring is needed. Diffusion is fast in gases, much slower in liquids, and faster at higher temperatures because the particles move faster. Extended: at the same temperature, particles with a lower relative molecular mass move faster, so a gas with a lower Mr diffuses faster.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Particles never keep still.. Open a bottle of perfume and within a minute someone across the room can smell it. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Random motion | Watch a large particle being knocked about by particles too small to see. Project tab 2 of the lab (Kinetic particle model lab, Brownian motion) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: What changes the rate? | Every factor comes back to one question: how fast are the particles moving, and how often do they collide? | Pairs |
| 21 to 30 min | Teach and check: Heavier particles are slower | Extended. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Heavier particles are slower, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The white ring forms closer to the hydrochloric acid end, not in the middle and not near the ammonia. | Explain it with relative molecular mass and speed of particles: saying the ammonia is "stronger" or "more concentrated" earns no marks. |
Diffusion: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Define diffusion.
A crystal of potassium manganate(VII) is placed at the bottom of a beaker of water and left without stirring. Describe what is seen over several hours and explain it using the kinetic particle theory.
(Extended) In the ammonia and hydrogen chloride tube experiment, write the equation, with state symbols, for the reaction that forms the white ring, state where the ring forms and explain why.
(Extended) Put these gases in order of rate of diffusion, fastest first: carbon dioxide (Mr 44), helium (Mr 4), oxygen (Mr 32), methane (Mr 16). Explain your order.
Explain why a smell spreads across a room faster on a hot day than on a cold day.
Explain why diffusion does not noticeably happen in solids.
Answer key and marking notes
- The net movement of particles from a region of higher concentration to a region of lower concentration,
- as a result of their random motion.
- The purple colour spreads slowly upwards and outwards until all the water is evenly purple.
- The crystal dissolves and its particles, and the water particles, move randomly and collide.
- There is a net movement of manganate(VII) particles from high to low concentration until the concentration is the same everywhere.
- NH3(g) + HCl(g) → NH4Cl(s).
- The ring forms nearer the hydrochloric acid end of the tube.
- Ammonia has a lower relative molecular mass (17) than hydrogen chloride (36.5).
- So ammonia particles move faster and diffuse further in the same time.
- helium, methane, oxygen, carbon dioxide.
- At the same temperature, particles with a lower relative molecular mass move faster, so they diffuse faster.
- At a higher temperature the particles have more kinetic energy and move faster.
- So they spread out, and diffuse through the air, more quickly.
- Particles in a solid are held in fixed positions by strong forces of attraction.
- They only vibrate about those positions, so they cannot move past one another to mix.
10 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which observation is evidence that the particles in a liquid are moving?
- AA drop of ink spreads through a beaker of still water
- BIce floats on water
- CWater expands when it freezes
- DSalt crystals have a regular shape
A bottle of perfume is opened at the front of a still room. Perfume particles move at hundreds of metres per second, yet the smell takes about a minute to reach the back. Why?
- AThe perfume particles collide with air particles many times, so they follow random zigzag paths
- BThe perfume particles must first turn back into a liquid
- CThe air particles push the perfume particles back towards the bottle
- DThe perfume particles only move when there is a draught
A drop of food colouring is added to each of two beakers of still water, one at 20 °C and one at 60 °C. What is observed?
- AThe colour spreads faster at 60 °C, because the particles move faster
- BThe colour spreads faster at 20 °C, because the particles are closer together
- CThe colour spreads at the same rate, because the same amount of colouring was added
- DThe colour spreads only in the 60 °C beaker, because diffusion needs heat
A purple crystal dissolves in a beaker of still water. After two days the colour is the same everywhere. Which statement is correct now?
- AThe particles are still moving, but there is no longer any net movement in one direction
- BThe particles have stopped moving because diffusion has finished
- CThe purple particles have all sunk to the bottom
- DOnly the water particles are still moving
In which example does diffusion happen most slowly?
- AGold atoms moving into a block of lead pressed tightly against it
- BInk spreading through water
- CThe smell of cooking spreading through a kitchen
- DCarbon dioxide escaping from a fizzy drink into the air
A student says that diffusion only happens in gases. Which observation shows that the student is wrong?
- ASugar at the bottom of an unstirred cup of tea slowly sweetens all of the tea
- BThe smell of perfume spreads across a room
- CSteam from a shower spreads across a bathroom
- DSmoke from a candle spreads through the air
At room temperature, which gas diffuses more slowly than oxygen, O2? (Ar: H 1, C 12, N 14, O 16)
- Acarbon dioxide, CO2
- Bnitrogen, N2
- Cmethane, CH4
- Dammonia, NH3
Gases X and Y are released at opposite ends of a tube. They react to form a white solid where they meet, as shown. Which pair could X and Y be?
- AX is ammonia (Mr 17) and Y is hydrogen bromide (Mr 81)
- BX is hydrogen bromide (Mr 81) and Y is ammonia (Mr 17)
- CX and Y are two different gases with the same Mr
- DX is hydrogen chloride (Mr 36.5) and Y is hydrogen bromide (Mr 81)
Hydrogen (Mr 2) and oxygen (Mr 32) are at the same temperature. Which statement is correct?
- AHydrogen molecules move faster on average than oxygen molecules
- BBoth kinds of molecule move at the same speed because the temperature is the same
- COxygen molecules move faster because they are heavier
- DHydrogen molecules move faster because they are larger
Two identical rubber balloons are filled to the same size, one with helium (Mr 4) and one with air (average Mr about 29). After a day the helium balloon has shrunk much more. Which explanation is best?
- AHelium particles move faster, so they escape through tiny gaps in the rubber more quickly
- BHelium particles are lighter, so they float up and out of the balloon
- CAir particles are smaller than helium particles, so they stay trapped
- DHelium particles are larger, so they stretch the rubber and break it
Quiz key
Ink spreading through still water is diffusion: the ink and water particles move randomly and mix without any stirring, which can only happen if the particles are moving. Ice floating and water expanding on freezing are about density and arrangement, and the shape of a salt crystal tells us about the regular arrangement in a solid, not about movement in a liquid.
Each perfume particle collides with air particles billions of times a second, changing direction each time. Its path is a random zigzag, so its overall progress across the room is much slower than its actual speed. The particles stay as a gas, air particles do not push them in any particular direction, and diffusion needs no draught.
At a higher temperature the particles have more kinetic energy and move faster, so they spread out and mix more quickly. Diffusion still happens at 20 °C, just more slowly, so it does not need heating. The amount of colouring does not decide the rate, and the particles being slightly closer together at 20 °C does not speed diffusion up.
Diffusion is the net movement from higher to lower concentration. Once the concentration is even, particles still move randomly, but as many move one way as the other, so there is no net change. Particles in a liquid never stop moving, the purple particles do not settle out (the colour is even), and both kinds of particle keep moving.
In a solid the particles are held in fixed positions and only vibrate, so diffusion between two metal blocks is extremely slow, taking years to be noticeable. Ink in water is diffusion in a liquid, which is faster, and the cooking smell and carbon dioxide are diffusion in gases, which is fastest of all.
The sugar dissolves and its particles spread through the liquid tea with no stirring, so diffusion also happens in liquids. Perfume, steam and smoke all spread through air, a gas, so they agree with the student instead of disproving the claim.
O2 has Mr 32. The gas that diffuses more slowly must have a larger Mr: CO2 = 12 + 2 × 16 = 44. N2 (28), CH4 (16) and NH3 (17) all have smaller Mr than oxygen, so their particles move faster and they diffuse faster.
X travelled 70 cm while Y travelled only 30 cm, so X diffuses faster and must have the lower Mr. Ammonia (17) and hydrogen bromide (81) fit, and they react to form solid ammonium bromide. Swapping them would put the ring near the X end. Equal Mr would put the ring in the middle. Hydrogen chloride and hydrogen bromide are both acidic gases, so they would not react to form a white solid at all.
At the same temperature, lighter particles move faster than heavier ones. Hydrogen has a much lower Mr, so its molecules move faster and hydrogen diffuses faster. The same temperature does not give the same speed, heavier particles move more slowly, not faster, and hydrogen molecules are smaller, not larger, than oxygen molecules.
Helium has a much lower Mr than air, so at the same temperature its particles move faster. They reach and pass through the tiny gaps in the rubber more often, so helium escapes more quickly. Particles do not "float" out of a sealed balloon, and the answer depends on speed linked to Mr, not on helium particles being larger or air particles being smaller.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.