The shape of the lesson
By the end of the lesson, learners can
- Watch the fast ones leave: See the most energetic molecules break free from the surface, leaving slower, cooler ones behind.
- Why it cools, and what speeds it up: Removing the fastest molecules drains energy from the liquid, and several factors change how quickly this happens.
- Where: evaporation happens at the surface, at any temperature below boiling.
- Cooling: the most energetic particles escape, lowering the average energy left behind.
- Faster when: higher temperature, larger surface area, more airflow over the surface.
One picture every idea returns to
Evaporation is the escape of the more energetic molecules from the surface of a liquid, at any temperature below boiling. Because the fastest molecules leave, the average energy of those remaining falls, so the liquid cools.
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, Escape that leaves it cooler.. Step out of a pool on a breezy day and you shiver, even in warm air. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Watch the fast ones leave | See the most energetic molecules break free from the surface, leaving slower, cooler ones behind. Project tab 3 of the lab (Evaporation) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Why it cools, and what speeds it up | Removing the fastest molecules drains energy from the liquid, and several factors change how quickly this happens. | Think-Pair-Share |
| 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 Why it cools, and what speeds it up, 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.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
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 |
|---|---|
| Evaporation cools the liquid, it does not warm it. | The fastest, highest-energy molecules are the ones that escape, so the average energy of the molecules left behind drops. This is why sweating cools the body. |
Evaporation and cooling: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
State what evaporation is, including where in the liquid it occurs and at what temperatures it can happen.
Explain, in terms of particles, why a liquid cools down as it evaporates.
A patient with a high fever is cooled by dabbing their forehead with a damp cloth. Explain how this helps to cool them.
State three changes that would increase the rate of evaporation of water from an open dish.
State two ways in which evaporation differs from boiling.
Wet washing on a line dries much faster on a warm, windy day than on a cool, still day. Explain why, referring to two factors.
Answer key and marking notes
- Evaporation is a liquid changing to a gas at the surface of the liquid. ✓
- It can happen at any temperature below the boiling point. ✓
- The fastest-moving (most energetic) particles at the surface escape from the liquid. ✓
- This leaves behind particles with a lower average kinetic energy. ✓
- A lower average kinetic energy means a lower temperature, so the liquid cools. ✓
- Water from the cloth evaporates, with its fastest particles escaping and carrying energy away. ✓
- This cools the cloth and skin, drawing energy from the patient, lowering their temperature. ✓
- Increase the temperature. ✓
- Increase the surface area of the liquid. ✓
- Increase the airflow (draught) over the surface. ✓
- Evaporation happens only at the surface; boiling happens throughout the liquid. ✓
- Evaporation happens at any temperature below boiling; boiling happens only at the boiling point. ✓
- The warmth gives more water particles enough energy to escape, so evaporation is faster. ✓
- The wind (airflow) carries the escaped particles away from the surface, stopping them returning. ✓
- Both effects increase the rate of evaporation, so the washing dries faster. ✓
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.
Which change would increase the rate of evaporation of water from an open dish?
- AIncreasing the temperature and the surface area of the water.
- BSealing the dish with a lid.
- CLowering the temperature of the water.
- DReducing the surface area of the water.
Evaporation causes the remaining liquid to cool. Why?
- AThe most energetic particles escape, lowering the average kinetic energy of those left behind.
- BThe slowest particles escape first.
- CEnergy is created as particles leave.
- DThe liquid gains particles from the air.
How does evaporation differ from boiling?
- AEvaporation occurs only at the surface and at any temperature, while boiling occurs throughout the liquid at a fixed temperature.
- BEvaporation needs a higher temperature than boiling.
- CEvaporation and boiling are exactly the same process.
- DBoiling happens only at the surface.
Quiz key
A higher temperature gives more particles enough energy to escape, and a larger surface area gives more of them the chance to leave, so both raise the rate. Sealing, cooling or reducing the area would all slow it down.
Only the fastest, most energetic particles can escape from the surface, so the average kinetic energy of the particles left behind falls, which means a lower temperature. If the slowest left (B), the liquid would warm up, which is not observed.
Evaporation takes place only at the surface and can happen at any temperature, whereas boiling happens throughout the liquid and only at the boiling point. Option D swaps the two, since it is boiling that occurs throughout.
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.