The shape of the lesson
By the end of the lesson, learners can
- Heat it through the states: Add energy steadily and watch the temperature climb, then pause at each change of state.
- The flat parts of the curve: Each named change happens at a fixed temperature, shown as a flat section on a heating graph.
- During a change of state the temperature stays constant; energy breaks the bonds between particles.
- Boiling is throughout the liquid at the boiling point; evaporation is at the surface, at any temperature.
- Freezing and condensing release energy.
One picture every idea returns to
A substance changes state at fixed temperatures: its melting point and its boiling point. During a change of state, energy is transferred but the temperature stays constant, because the energy is used to change the arrangement of the particles rather than to speed them up.
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, Melting, boiling, and back again.. Hold a thermometer in melting ice and it sticks stubbornly at zero, even as you keep heating. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Heat it through the states | Add energy steadily and watch the temperature climb, then pause at each change of state. Project tab 2 of the lab (Changes of state) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: The flat parts of the curve | Each named change happens at a fixed temperature, shown as a flat section on a heating graph. | Cooperative Concept Mapping |
| 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 The flat parts of the curve, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 |
|---|---|
| During a change of state the temperature stays constant. | While ice melts or water boils you keep supplying energy, yet the thermometer does not rise, because the energy is changing the arrangement of particles, not their speed. |
Changes of state: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Name the change of state in each case:
(a) a solid turning into a liquid, [1] (b) a gas turning into a liquid, [1] (c) a liquid turning into a solid. [1]While a pure solid is melting, energy is still being supplied to it, yet its temperature does not change. Explain why.
A heating curve for a substance shows the temperature rising, then a flat horizontal section, then rising again. State what is happening during the flat section, and what stays constant.
State two ways in which boiling differs from evaporation.
On a cold day, droplets of water form on the inside of a window in a warm, steamy kitchen. Name the change of state taking place, and explain how it happens.
When water freezes into ice, energy is transferred. State whether energy is taken in or given out, and explain in terms of the particles.
Answer key and marking notes
(a) Melting. ✓
(b) Condensation (condensing). ✓
(c) Freezing (solidifying). ✓
- The energy supplied is used to break the bonds holding the particles in fixed positions, separating them. ✓
- It is not used to make the particles move faster, so the temperature stays constant. ✓
- During the flat section the substance is changing state (melting or boiling). ✓
- The temperature stays constant throughout the flat section. ✓
- Boiling happens throughout the liquid; evaporation happens only at the surface. ✓
- Boiling happens only at the boiling point; evaporation happens at any temperature below boiling. ✓
- Condensation. ✓
- Warm water vapour in the air touches the cold glass, loses energy, and changes back into liquid water droplets. ✓
- Energy is given out (released) to the surroundings. ✓
- The particles slow down and form bonds into a fixed solid arrangement, releasing energy as they do so. ✓
6 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 change of state releases energy to the surroundings?
- Amelting
- Bboiling
- Ccondensation
- Dsublimation of a solid to a gas
While a pure solid is melting, energy is supplied but the temperature stays constant. Where does this energy go?
- AInto weakening the forces between the particles, changing the arrangement.
- BInto making the particles move faster.
- CIt is destroyed.
- DInto increasing the mass of the substance.
How much energy is needed to melt 0.20 kg of ice at 0 °C? The specific latent heat of fusion of ice is 3.4 × 105 J/kg.
- A6800 J
- B68 000 J
- C340 000 J
- D1 700 000 J
51 000 J of energy completely melts 0.15 kg of a solid at its melting point. What is the specific latent heat of fusion of the solid?
- A3.4 × 104 J/kg
- B3.4 × 105 J/kg
- C3.4 × 106 J/kg
- D7650 J/kg
While a pure solid melts, energy is supplied steadily but the temperature does not change. Why not?
- AThe energy is used to break the bonds between particles rather than to raise their kinetic energy.
- BThe thermometer stops working at the melting point.
- CThe energy is destroyed during melting.
- DThe energy increases the mass of the solid.
The temperature-time graph shows a solid being heated steadily until it becomes a gas. What is happening during the flat section labelled F?
- AThe substance is melting at constant temperature.
- BThe substance is cooling down.
- CThe heater has been switched off.
- DThe substance is already a gas.
Quiz key
Condensation (gas to liquid) releases energy as the particles slow and form bonds. Melting and boiling both absorb energy from the surroundings.
During melting the energy goes into breaking down the rigid arrangement of particles, not into raising their kinetic energy, so the temperature stays constant. Energy is never destroyed (ruling out C).
E = mL = 0.20 × 3.4 × 105 = 68 000 J. Option C forgets to multiply by the mass; A is ten times too small.
L = E ÷ m = 51 000 ÷ 0.15 = 340 000 = 3.4 × 105 J/kg. Option D multiplies instead of dividing (51 000 × 0.15).
During a change of state the energy (the latent heat) goes into overcoming the forces between particles, not into faster motion, so the temperature stays constant. Energy is conserved, not destroyed (ruling out C).
The first flat section is melting: energy goes into the change of state (latent heat) so the temperature holds steady while solid turns to liquid. The graph is still being heated, so it is not cooling (ruling out B).
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.