Lesson overview
States and changes of state: Why does ice melt but stay water?
Predict what the thermometer will do, then heat one block of ice until it melts and boils away. Read the thermometer every 30 s, plot your own heating curve, and watch the same molecules change their arrangement.
Suggested sequence
- Lesson 1 · Same water, three states. Properties and particles of solids, liquids and gases. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Where does the heat go? Melting, boiling, evaporating and the heating curve. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Cooling it back down. Condensing, freezing and the cooling curve. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 3 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. 3 questions, 15 min, lesson closed.
Learning goals
Part 1: Same water, three states
- State the distinguishing properties of solids, liquids and gases
- Describe the separation, arrangement and motion of particles in solids, liquids and gases
Part 2: Where does the heat go?
- Describe melting, boiling and evaporating, and distinguish evaporation from boiling
- Interpret a heating curve and explain its flat sections using kinetic particle theory
Part 3: Cooling it back down
- Describe freezing and condensing, and interpret a cooling curve using kinetic particle theory
- Use melting and boiling points to decide the state of a substance at a given temperature
Misconceptions to listen for
- “It changes shape, so it must be a gas.” — Liquids and gases both change shape. A gas would also change its volume: it would fill a closed container and squash in a syringe.
- “Particles in a solid don't move.” — They vibrate all the time about fixed positions; they just can't move past each other.
- “Anything that can be squashed is a gas.” — Check all the properties. A sponge keeps its own shape and doesn't fill its container, so it's a solid. Squeezing bends its solid parts and pushes most of the air out of its holes; the solid parts themselves still take up almost the same space.
- “Squeezing pushes the sponge's own particles closer together, or makes them smaller.” — The particles of a solid already touch, and squeezing doesn't change their size. The solid parts only bend. The space that disappears is the holes, and most of their air has been pushed out.
- “Liquid particles are far apart because liquids flow.” — A liquid flows because its particles can slide past each other, not because they are far apart. They stay close together, which is why a liquid keeps its volume.
- “Z must be a solid because it can't be squashed.” — Solids and liquids both hardly squash. Z settles at the bottom with a level surface, taking the container's shape: it is a liquid.
- “Gas particles are bigger than liquid particles.” — The particles are the same size. The spaces between them are much larger in a gas.
- “Melting makes new substances.” — Melting is a change of state. The same H2O molecules are there before and after; only their arrangement and motion change.
- “Particles shrink when a gas is compressed.” — The particles keep their size. Compressing a gas reduces the empty space between them.
- “The melting point is where heating starts, at 20 °C.” — At A the solid is simply being heated. The melting point is the temperature of the first flat section, where the temperature stops rising while the solid melts.
Scope
Distinguishing properties and particle structure of solids, liquids and gases; melting, boiling, evaporating, freezing and condensing; heating and cooling curves explained with kinetic particle theory (1.1.1–1.1.3, Supplement 1.1.5). The experiment heats one ideal 50 g ice sample; freezing and condensing are taught with figures and practice. Gas volume with temperature and pressure (1.1.4, 1.1.6) comes in a later module.