The shape of the lesson
By the end of the lesson, learners can
- Three states, one model: Switch the simulator between solid, liquid and gas and watch how the particles are arranged and how they move.
- Changes of state: Each change of state has its own name.
- Gases, temperature and pressure: Extended.
- Solid: close, regular, vibrate on the spot.
- Liquid: close, irregular, move around each other.
- Gas: far apart, random, fast in all directions.
- Flat on a heating curve: energy overcomes forces, temperature constant.
One picture every idea returns to
In a solid the particles are close together in a regular arrangement and vibrate about fixed positions, so a solid has a fixed shape and volume. In a liquid they are close together but irregular and move around each other, so a liquid has a fixed volume but takes the shape of its container. In a gas they are far apart, randomly arranged and move quickly in all directions, so a gas has no fixed shape or volume and is easily compressed. Heating gives particles more kinetic energy; changes of state happen when the forces between particles are overcome or formed.
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, Same particles, different freedom.. Ice, water and steam are made of exactly the same particles. 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: Three states, one model | Switch the simulator between solid, liquid and gas and watch how the particles are arranged and how they move. Project tab 1 of the lab (Kinetic particle model lab, states of matter) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Changes of state | Each change of state has its own name. | Pairs |
| 21 to 30 min | Teach and check: Gases, temperature and pressure | Extended. Project tab 3 of the lab (Kinetic particle model lab, gas pressure and temperature) and let a student drive it while the class predicts. | 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, 18 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 Gases, temperature and pressure, 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 |
|---|---|
| A flat section on a heating curve does not mean the heating has stopped. | Energy is still going in, but it is being used to overcome the forces between particles, so the kinetic energy and the temperature stay the same until the change of state is complete. |
Solids, liquids and gases: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the arrangement and the motion of the particles in a solid, a liquid and a gas.
Name each change of state: (a) solid to liquid, (b) liquid to gas throughout the liquid at its boiling point, (c) gas to liquid, (d) liquid to solid.
The table gives the melting and boiling points of four substances.
| Substance | Melting point / °C | Boiling point / °C |
|---|---|---|
| P | −210 | −196 |
| Q | 801 | 1413 |
| R | −95 | 56 |
| S | 17 | 118 |
State the physical state of each substance at 25 °C.
(Extended) A sample of solid wax is heated steadily from room temperature until it is a liquid well above its melting point. Sketch and explain the shape of its heating curve.
Give two differences between boiling and evaporation.
(Extended) Use the kinetic particle theory to explain what happens to the volume of a gas when the pressure on it is increased at constant temperature.
Answer key and marking notes
- Solid: particles close together in a regular arrangement, vibrating about fixed positions.
- Liquid: particles close together but irregularly arranged, moving around each other.
- Gas: particles far apart and randomly arranged, moving quickly in all directions.
- (a) melting
- (b) boiling
- (c) condensation
- (d) freezing
- P: gas (25 °C is above its boiling point).
- Q: solid (25 °C is below its melting point).
- R: liquid (between its melting and boiling points).
- S: liquid (17 °C < 25 °C < 118 °C).
- The temperature rises while the wax is solid, as the particles gain kinetic energy and vibrate faster.
- At the melting point there is a flat section: the energy supplied is used to overcome the forces of attraction between particles, so the temperature stays constant while solid and liquid are both present.
- Once all the wax has melted the temperature rises again as the liquid particles move faster.
- Boiling happens only at the boiling point; evaporation happens at any temperature below the boiling point.
- Boiling happens throughout the liquid, with bubbles forming inside it; evaporation happens only at the surface.
- The volume decreases.
- There are large spaces between gas particles, so the extra pressure pushes the particles closer together.
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 statement explains why a solid has a fixed shape?
- AIts particles are held in fixed positions by strong forces and can only vibrate
- BIts particles do not move at all
- CIts particles are larger and harder than the particles of a liquid
- DThere are no spaces at all between its particles
Which property do liquids and gases share, but solids do not have?
- AThey can flow and take the shape of their container
- BThey have a fixed volume
- CThey are easily compressed
- DTheir particles are far apart
The diagram shows the particles in three substances, A, B and C. Which box shows a liquid?
- AB
- BA
- CC
- Dboth A and B
Four substances have these melting and boiling points. W: −7 °C and 59 °C. X: 44 °C and 280 °C. Y: 113 °C and 445 °C. Z: −259 °C and −253 °C. Which substance is a liquid at 100 °C?
- AX
- BW
- CY
- DZ
A puddle of water dries up on a warm day, although the water never reaches 100 °C. Which explanation is correct?
- AThe fastest-moving water particles at the surface escape into the air as a gas
- BThe water boils because the sun heats it
- CThe water particles break down into hydrogen and oxygen
- DAll the water particles move down into the ground
The graph is a cooling curve for a substance that starts as a gas. During which section is the substance freezing?
- AS
- BQ
- CR
- DT
Liquid naphthalene freezes at 80 °C. While it is freezing it keeps losing energy to the surroundings, yet its temperature stays at 80 °C. Why?
- AEnergy is released as forces of attraction form between the particles, balancing the energy lost
- BThe particles have stopped moving, so no more energy can be lost
- CThe particles gain kinetic energy as they freeze
- DEnergy is taken in to break the forces between the particles
A sealed balloon full of air is moved from a warm room into a freezer. What happens to its volume, and why?
- AIt decreases, because the particles move more slowly and hit the inside of the balloon less often and less hard
- BIt decreases, because the particles become smaller when they are cooled
- CIt stays the same, because the number of particles has not changed
- DIt increases, because cold particles push harder on the balloon
The end of a gas syringe is sealed and the plunger is pushed in slowly, keeping the temperature constant. Which statement about the gas inside is correct?
- AThe particles are closer together, so they hit the walls more often and the pressure increases
- BThe particles become smaller, so they fit into a smaller volume
- CThe particles move faster because they are squeezed
- DSome particles are destroyed, so the volume decreases
Which change of state happens when a liquid loses energy and its particles become fixed in a regular arrangement?
- Afreezing
- Bcondensation
- Cmelting
- Devaporation
Quiz key
In a solid the particles are held in a regular arrangement by strong forces of attraction. They vibrate about fixed positions but cannot move past each other, so the shape cannot change. The particles do still move (they vibrate), they are the same particles as in the liquid (not bigger), and although they are close together it is the fixed positions, not a total lack of space, that fixes the shape.
Particles in both liquids and gases can move past one another, so both flow and take the shape of their container. A fixed volume belongs to solids and liquids, not gases. Being easily compressed and having particles far apart are true only of gases.
In a liquid the particles are close together but irregularly arranged, which is box B. Box A shows particles close together in a regular pattern, a solid. Box C shows particles far apart and randomly arranged, a gas. A and B both have particles touching, but only B is irregular.
X melts at 44 °C and boils at 280 °C, and 100 °C lies between them, so X is a liquid. W is above its melting point but also above its boiling point (59 °C), so it is a gas. Y has not reached its melting point (113 °C), so it is still a solid. Z is far above its boiling point, so it is a gas.
This is evaporation. Particles in a liquid have a range of speeds, and the fastest ones at the surface have enough energy to escape as a gas, even well below 100 °C. It is not boiling, because the water never reaches its boiling point. Water particles do not split into elements on a warm day; that would be a chemical change.
The temperature stays constant during a change of state, so the flat sections Q and S are changes of state. Cooling a gas, the first change is condensation (gas to liquid), section Q. The second is freezing (liquid to solid), section S. R is the liquid cooling and T is the solid cooling; in those sections the temperature is falling, so no change of state is happening.
As the liquid freezes, forces of attraction form between the particles and energy is released. This replaces the energy lost to the surroundings, so the average kinetic energy, and the temperature, stay the same until all the liquid has become solid. The particles still vibrate, they do not gain kinetic energy, and breaking forces takes energy in, which is what happens in melting, not freezing.
Cooling lowers the kinetic energy of the particles, so they move more slowly and collide with the inside of the balloon less often and with less force. The gas pressure inside drops and the balloon shrinks until the pressures balance. The particles themselves do not shrink, and although the number of particles is unchanged, the volume of a gas depends on temperature as well.
There are large spaces between gas particles, so pushing the plunger moves the particles closer together. The same number of particles now collides with a smaller area of wall more often, so the pressure rises. At constant temperature the particles keep the same average speed, they do not change size, and none are destroyed.
Particles fixed in a regular arrangement describe a solid, so this is liquid to solid: freezing. Condensation is gas to liquid, melting is solid to liquid (energy taken in), and evaporation is liquid to gas.
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.