The shape of the lesson
By the end of the lesson, learners can
- Heat the bar, watch it stretch: Raise the temperature of a solid and watch how its length grows as the particles vibrate more.
- Least for solids, most for gases: The same effect appears in all three states, but to very different degrees, with important everyday uses.
- Mechanism: heating gives particles energy, so they vibrate more and spread apart.
- Order: gases expand most, then liquids, then solids.
- Never say the particles themselves get bigger. Only the spacing increases.
One picture every idea returns to
Most substances expand when heated, because the particles vibrate or move more and take up slightly more space on average. For the same temperature rise, solids expand least and gases expand most.
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, Warm it, and it grows.. Bridges are built with gaps, power lines sag in summer, and a tight metal lid loosens under hot water. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Heat the bar, watch it stretch | Raise the temperature of a solid and watch how its length grows as the particles vibrate more. Project tab 5 of the lab (Thermal expansion) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Least for solids, most for gases | The same effect appears in all three states, but to very different degrees, with important everyday uses. | Connect-Extend-Challenge |
| 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, 13 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 Least for solids, most for gases, 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.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
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 |
|---|---|
| Heating does not make the individual atoms or molecules larger. | They stay the same size; the substance expands because the particles move more energetically and the average gap between them grows. |
Thermal expansion: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Explain, in terms of its particles, why a metal bar gets slightly longer when it is heated.
For the same rise in temperature, place a solid, a liquid, and a gas in order of how much they expand, from most to least, and state which one expands the most.
A long steel bridge is built resting on rollers at one end, with a toothed expansion joint in the road surface. Explain why these features are needed.
A bimetallic strip is made of brass joined to iron. Brass expands more than iron for the same temperature rise. Explain what happens to the strip when it is heated.
A student explains expansion by saying that the particles of the metal grow larger when heated. Explain why this is wrong and give the correct reason.
A glass jar has a metal lid that is screwed on too tightly to open. Running the lid under hot water makes it easier to unscrew. Explain why, given that metal expands more than glass.
Answer key and marking notes
- The particles gain energy and vibrate more strongly (with a bigger amplitude). ✓
- This pushes them slightly further apart, so the average spacing increases and the bar gets longer. ✓
- Most to least: gas, then liquid, then solid. ✓
- The gas expands the most. ✓
- When the temperature rises, the steel bridge expands and becomes longer. ✓
- The rollers and expansion joint let it lengthen and shorten freely. ✓
- Without them the forces from expansion could crack or buckle the structure. ✓
- When heated, the brass side expands more than the iron side. ✓
- The strip bends, curving towards the iron side (the brass forms the outside of the curve). ✓
- The particles do not change size. ✓
- They vibrate more and move slightly further apart, so it is the spacing between them that increases, not the particles themselves. ✓
- The hot water warms the metal lid, which expands more than the glass. ✓
- The lid becomes slightly larger and looser on the jar, so it is easier to unscrew. ✓
3 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.
For the same rise in temperature, which state of matter expands the most?
- Asolid
- Bliquid
- Cgas
- Dall expand equally
A bimetallic strip is made of brass and iron joined together. Brass expands more than iron for the same temperature rise. Which way does the strip bend when heated?
- AIt bends with the brass on the outside of the curve.
- BIt bends with the brass on the inside of the curve.
- CIt stays straight.
- DIt breaks apart.
Small gaps are left between sections of a steel railway line. Why?
- ATo allow the rails to expand on hot days without buckling.
- BTo save steel and reduce cost.
- CTo let rainwater drain away.
- DTo make the train quieter.
Quiz key
Gases expand the most for a given temperature rise, then liquids, then solids, because gas particles have the most freedom to move apart. Solids expand least as their particles are held in fixed positions.
Brass expands more, so it becomes the longer outer edge and the strip curves toward the iron side. Option B would require iron to expand more, which is the reverse of what is given.
On hot days the steel expands; the gaps give the rails room to lengthen so they do not push against each other and buckle. The gaps are not for drainage or cost.
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.