Lesson overview
Melting, boiling and evaporation: Why does sweating cool you down?
Wrap two tubes of hot water in paper towels, wet one towel and weigh it as the water evaporates, while a magnifier shows the fastest particles escaping from its surface. Then test how a fan, a smaller surface and a cooler start change the rate of evaporation, and compare evaporation with boiling.
Suggested sequence
- Lesson 1 · Melting and boiling. Melting and boiling at fixed temperatures; melting and boiling points of water; heating and cooling curves, and condensation and solidification. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 2 · Evaporation and cooling. Evaporation: the more energetic particles escape from the surface, so the liquid left cools; how an object in contact with an evaporating liquid cools. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · What changes the rate of evaporation. What changes the rate of evaporation: temperature, surface area and moving air; how evaporation differs from boiling. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Melting and boiling
- Describe melting and boiling in terms of energy input without a change in temperature (2.2.3.1)
- Know the melting and boiling temperatures for water at standard atmospheric pressure (2.2.3.2)
- Describe condensation and solidification in terms of particles (2.2.3.3)
Part 2: Evaporation and cooling
- Describe evaporation in terms of the escape of more-energetic particles from the surface of a liquid (2.2.3.4)
- Know that evaporation causes cooling of a liquid (2.2.3.5)
- Explain the cooling of an object in contact with an evaporating liquid (2.2.3.8)
Part 3: What changes the rate of evaporation
- Describe evaporation in terms of the escape of more-energetic particles from the surface of a liquid (2.2.3.4)
- Know that evaporation causes cooling of a liquid (2.2.3.5)
- Explain the cooling of an object in contact with an evaporating liquid (2.2.3.8)
- Describe the differences between boiling and evaporation (2.2.3.6)
- Describe how temperature, surface area and air movement over a surface affect evaporation (2.2.3.7)
Misconceptions to listen for
- “No energy goes into the beaker until all the ice has melted.” — The heater is on the whole time, so energy goes in the whole time. Along the flat part, the energy separates the particles instead of making them move faster.
- “The ice cancels out the energy from the heater.” — Ice does not destroy energy. The energy goes into the ice and breaks its particles out of their fixed positions: that is melting.
- “(5) hotter: turning the hob up makes boiling water hotter” — Boiling water stays at its boiling point, 100 °C, however high the hob is turned up. More energy each second makes it boil away faster.
- “(2) 90: pure water boils at 90 °C” — Water boils at about 90 °C high on a mountain, where the air pressure is lower. At standard atmospheric pressure, pure water boils at 100 °C.
- “Water freezes below 0 °C, for example at −10 °C.” — Pure water freezes at its melting point, 0 °C. While it freezes, the temperature stays at 0 °C; only when it is all ice can it cool further, to −10 °C.
- “Water boils at a higher temperature if you heat it harder.” — The boiling point does not depend on the heater. At standard atmospheric pressure pure water boils at 100 °C; heating it harder makes it boil away faster.
- “The temperature falls steadily the whole time, from 110 °C to −10 °C.” — It stops falling twice: at 100 °C while the steam condenses, and at 0 °C while the water freezes. Energy given out as the particles come together keeps the temperature steady until each change is complete.
- “In ice the particles stop moving.” — They still vibrate about their fixed positions. As the ice cools further, they vibrate more slowly.
- “The particles get smaller when a gas condenses.” — Each particle stays the same size. What changes is how far apart they are: in a liquid they are close together.
- “The particles settle into fixed positions.” — That is solidification, a liquid turning into a solid. In a liquid the particles are close together but can still move past each other.
Scope
Melting and boiling, with the melting and boiling points of water at standard atmospheric pressure (2.2.3.1, 2.2.3.2); condensation and solidification in terms of particles (2.2.3.3); evaporation as the escape of more energetic particles from the surface of a liquid (2.2.3.4); how evaporation causes cooling of a liquid (2.2.3.5); the differences between boiling and evaporation (2.2.3.6); how temperature, surface area and air movement over a surface affect evaporation (2.2.3.7); how the cooling of an object in contact with an evaporating liquid happens (2.2.3.8). Specific heat capacity is in SHC01; conduction, convection and radiation are in TRANS01.