The shape of the lesson
By the end of the lesson, learners can
- Three states, one model: Heat the solid in the simulator and watch the particles.
- Vapour pressure: Seal a liquid in a container.
- When a liquid boils: Boiling happens throughout the liquid, when bubbles of vapour can form because the vapour pressure matches the pressure pushing down on the surface.
- Liquid: close, random, mobile particles.
- ΔHvap > ΔHfus: vaporising separates molecules completely.
- Vapour pressure: dynamic equilibrium, rises with T.
- Weaker forces: higher vapour pressure, lower b.p.
One picture every idea returns to
In a liquid the particles are close together but disordered, moving randomly and sliding past each other while intermolecular forces still hold them. Melting overcomes only some of those forces, but vaporisation separates the particles completely, so the enthalpy change of vaporisation is much larger than that of fusion (water: 41 against 6.0 kJ mol−1). In a closed container, evaporation and condensation reach a dynamic equilibrium; the pressure of the vapour then is the vapour pressure. It rises with temperature, and a liquid boils when its vapour pressure equals the external pressure.
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, Close, but free to move.. A liquid sits between the order of a solid and the chaos of a gas. 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 | Heat the solid in the simulator and watch the particles. Project tab 1 of the lab (States of matter lab, kinetic model) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Vapour pressure | Seal a liquid in a container. | Pairs |
| 21 to 30 min | Teach and check: When a liquid boils | Boiling happens throughout the liquid, when bubbles of vapour can form because the vapour pressure matches the pressure pushing down on the surface. | 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 When a liquid boils, 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 |
|---|---|
| Evaporation happens at any temperature, from the surface only; boiling happens at one temperature for a given pressure, throughout the liquid. | And when explaining vapour pressure, say the equilibrium is dynamic: both processes continue at equal rates, they do not stop. |
The liquid state: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the arrangement and motion of the particles in a liquid.
For water, ΔHfus = +6.0 kJ mol−1 and ΔHvap = +41 kJ mol−1. Explain the difference.
Some water is placed in a sealed flask at 25 °C. Explain how a constant vapour pressure is established.
Use Fig. 1 to explain why ethanol boils at a lower temperature than water.
Explain why food cooks faster in a pressure cooker.
State two differences between evaporation and boiling.
Answer key and marking notes
- The particles are close together and randomly arranged.
- They move randomly and slide past one another.
- On melting the molecules stay close together, so only some hydrogen bonds are overcome.
- On vaporisation the molecules are separated completely, so all the intermolecular forces must be overcome, which needs much more energy.
- Molecules with enough energy escape from the surface of the liquid (evaporation).
- As vapour builds up, more vapour molecules return to the liquid (condensation).
- Eventually the rates of evaporation and condensation are equal: a dynamic equilibrium, and the vapour pressure stays constant.
- Ethanol has weaker intermolecular forces overall, so it has a higher vapour pressure at any temperature.
- Its vapour pressure reaches 101 kPa at 78 °C, whereas water's reaches it at 100 °C.
- The pressure inside is above atmospheric, so the vapour pressure of the water must be higher to equal it.
- Water therefore boils at a higher temperature (above 100 °C), and the food is heated more.
- Evaporation happens only at the surface; boiling happens throughout the liquid.
- Evaporation happens at any temperature; boiling happens only when the vapour pressure equals the external pressure.
8 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 describes the particles in a liquid?
- Aclose together in a random arrangement, and able to move past one another
- Bclose together in a regular arrangement, vibrating about fixed positions
- Cfar apart in a random arrangement, moving rapidly in straight lines
- Dclose together in a regular arrangement, and able to move past one another
For water, the enthalpy change of vaporisation (+41 kJ mol−1) is much larger than the enthalpy change of fusion (+6.0 kJ mol−1). Why?
- Aon boiling the molecules are separated completely, so all the intermolecular forces must be overcome; on melting only some are overcome
- Bon boiling the covalent O–H bonds inside the molecules are broken, but on melting they are not
- Cboiling happens at a higher temperature, and enthalpy changes always increase with temperature
- Don melting the molecules move further apart than they do on boiling
A liquid is placed in a sealed container at constant temperature. Once the vapour pressure is constant, which statement is correct?
- Athe rate of evaporation equals the rate of condensation, so the system is in dynamic equilibrium
- Bevaporation has stopped because the space above the liquid is full
- Ccondensation has stopped and only evaporation continues
- Dall of the liquid has changed into vapour
Water boils at a lower temperature at the top of a high mountain than at sea level. Why?
- Athe atmospheric pressure is lower, so the vapour pressure of the water equals the external pressure at a lower temperature
- Bthe air is colder, so less energy is needed to boil the water
- Cthere is less oxygen, so the hydrogen bonds between water molecules are weaker
- Dthe vapour pressure of water at any given temperature is higher on a mountain
Why does the vapour pressure of a liquid increase as its temperature rises?
- Aa greater proportion of molecules have enough kinetic energy to overcome the intermolecular forces and escape from the surface
- Bthe intermolecular forces become stronger as the temperature rises
- Cthe liquid expands, so its surface area increases
- Dthe covalent bonds in the molecules break, producing more gas particles
The graph shows how the vapour pressure of water changes with temperature. What is the boiling point of water at a place where the atmospheric pressure is 70 kPa?
- Aabout 90 °C
- B100 °C
- Cabout 110 °C
- Dabout 70 °C
Which of these liquids has the lowest vapour pressure at 20 °C?
- Awater, H2O
- Bethanol, C2H5OH
- Cpropanone, CH3COCH3
- Dethoxyethane, C2H5OC2H5
A sealed container holds liquid hexane in equilibrium with its vapour at 25 °C. The volume of the container is doubled at constant temperature, and some liquid still remains. What is the vapour pressure once equilibrium is re-established?
- Athe same as before
- Bhalf of the original value
- Cdouble the original value
- Dzero, because all of the vapour condenses
Quiz key
In a liquid the particles are still close together (liquids are hardly compressible), but the arrangement is random with no long-range order, and the particles have enough energy to slide past one another, so the liquid flows. A regular arrangement with vibration about fixed positions describes a solid. Particles far apart moving rapidly in straight lines describes a gas. A regular arrangement is a feature of the solid lattice; it is lost on melting.
Melting only loosens the solid lattice: the molecules in the liquid are still close together and many hydrogen bonds remain. Vaporisation separates the molecules completely, so essentially all the remaining intermolecular forces must be overcome, which needs much more energy. Boiling water produces H2O molecules; no covalent bonds within the molecules are broken. The difference is not simply because of the temperature; it depends on how much attraction is overcome. The molecules move apart far more on boiling than on melting, not less.
Molecules continue to escape from the surface and vapour molecules continue to return to the liquid; when the two rates are equal the number of vapour molecules, and so the vapour pressure, stays constant. This is a dynamic equilibrium. Evaporation does not stop; the space is never "full", and molecules with enough energy still escape. If only evaporation continued, the vapour pressure would keep rising. Liquid remains in the container, which is why a steady vapour pressure is set up.
Boiling occurs when the vapour pressure of the liquid equals the external pressure. At altitude the atmospheric pressure is lower, so the vapour pressure of the water reaches it at a lower temperature, and the boiling point falls. The temperature of the air does not decide the boiling point; it only affects how long it takes to heat the water. The amount of oxygen in the air has no effect on the hydrogen bonding between water molecules. The vapour pressure of water depends only on its temperature, so at a given temperature it is the same on the mountain as at sea level.
At a higher temperature the average kinetic energy of the molecules is greater, so a larger fraction have enough energy to overcome the attractions of their neighbours and escape; at equilibrium there are more molecules in the vapour, so the vapour pressure is higher. Intermolecular forces do not get stronger on heating; stronger forces would lower the vapour pressure. The equilibrium vapour pressure does not depend on the surface area, which only changes how fast equilibrium is reached. Evaporation does not break covalent bonds within molecules.
Boiling happens when the vapour pressure equals the external pressure, so the boiling point is the temperature at which the curve reaches 70 kPa, which is about 90 °C. 100 °C is the boiling point only at 101 kPa (standard atmospheric pressure). 110 °C would need an external pressure above 101 kPa, as in a pressure cooker; lowering the pressure lowers the boiling point. 70 °C mixes up the pressure scale with the temperature scale; the vapour pressure of water at 70 °C is only about 31 kPa.
Each water molecule can form up to four hydrogen bonds (two lone pairs and two O–H hydrogens), so water has the strongest intermolecular forces for its size and the lowest vapour pressure (about 2.3 kPa at 20 °C). Ethanol can hydrogen bond through only one O–H group, so its forces are weaker and its vapour pressure is higher (about 5.9 kPa). Propanone has no O–H group, so it cannot hydrogen bond between its own molecules and relies on permanent dipole-permanent dipole forces (about 25 kPa). Ethoxyethane is only weakly polar and has the highest vapour pressure of the four (about 59 kPa).
Straight after the expansion the vapour pressure falls, so condensation slows while evaporation continues at the same rate; more liquid evaporates until the rates are equal again. Because the rate of escape per unit surface depends only on temperature and the liquid, the equilibrium vapour pressure returns to its original value. Halving would be true for a fixed amount of gas (Boyle's law), but here extra liquid evaporates to make up the difference. Doubling reverses the effect of the change. Increasing the volume lowers the pressure momentarily, which favours evaporation, not condensation.
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.