Melting and boiling need energy put in: the particles move faster and pull free of each other. Freezing and condensing give energy out. While a substance is changing state, its temperature stays the same, because the energy goes into separating the particles, not speeding them up.
A pot of water on the stove climbs to 100 °C and then stops, even with the gas still on full. Where is all that heat going?
You know how particles are arranged in solids, liquids and gases. This lesson asks what energy does when one changes into another.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Practise: A few on your own, to check you have it.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: the stuck thermometer
ExploreTry it on the board and compare what happens with your prediction.
Explore: ice to steam
Drag the temperature slider and watch the particles. Where does the heat go?
Try it yourselfNothing is saved: start again as often as you like.
Heat the water from ice to steam.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: energy in, energy out
Move R with its slider or by clicking the graph. The curve is to scale: the stove gives the same energy every minute, so each part's length shows how much energy it takes.
In your book: sketch water's heating curve, label the solid, liquid and gas parts and the two flat parts. Beside each flat part, write what the particles are doing and why the temperature doesn't change.
Try it yourselfYour edits are saved in this browser.
This is a heating curve for 1 kg of ice, heated steadily until it is steam. Move the cursor R along the whole curve, to 74 minutes. Where is the line flat?
Use the board. This ticks when you get there.
Does the substance take energy in, or give energy out?
Melting (solid to liquid)
Boiling or evaporating (liquid to gas)
Freezing (liquid to solid)
Condensing (gas to liquid)Steam on your skin burns badly because it gives out energy as it condenses.
At what temperature (°C) is the second flat part of water's heating curve?
What happens to the particles in each change of state?
Melting
Freezing
Evaporating
Condensing
PractiseA few on your own, to check you have it.
Mix it up
Elaborate TypicalUse the same idea in a new context.
Elaborate: a cooling curve
Try it yourselfYour edits are saved in this browser.
Melted stearic acid (a waxy substance used in candles) is cooling. Move R to where it starts to freeze (the line goes flat).
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: sweating
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Melting, boiling and evaporating take energy in; freezing and condensing give energy out.
Adding energy to a substance makes its particles move faster (the temperature rises), until it reaches a change of state.
During a change of state the temperature stays the same: the energy goes into separating the particles (or comes out as they settle).
A heating curve shows this as flat parts at the melting and boiling points.
For teachers
Students explain changes of state with particle theory, identify energy input or output for each, and explain the flat parts of a heating or cooling curve (AC9S7U05, node 4). About 24 minutes.
I can say whether energy goes in or out when a substance melts, boils, freezes or condenses.
I can explain a change of state using particles.
I can explain why the temperature stays the same while a substance changes state.
Before class
A concept tutorial for introducing or revising the idea; a real heating curve (ice in a beaker with a thermometer and a timer) is the best partner. Set the width with ?width=minimum, typical (default) or wider. It follows Why can you squash air but not water? and comes before Pure substance or mixture?
Not a hinge node, but the plateau surprises almost everyone: students expect heating always to raise the temperature. The prediction tests that.
Board notes: the particle board changes state at 0 °C and 100 °C without showing the plateau (the slider sets the temperature directly); the heating curve on the graph board shows it. The curve is idealised: real ones curve at the corners, and the boiling plateau is longer than the melting one because boiling needs more energy.
Running the lesson
Engage: tally the predictions. 'It keeps rising' is the idea the curve challenges.
Explore: ask what the particles were doing just as the ice turned to water.
Explain: the energy in/out sort and the flat-part question are the success criteria.
In your own words: the labelled heating curve is the evidence.
The hard part (on purpose)
Explaining the flat part. When a student says 'the heat stopped', ask where the stove's energy is going if the particles aren't speeding up, and wait.
When students get stuck
A student says the temperature stops rising because the heat turned off. Ask: Was the stove still on? Fix: Yes. The energy is still going in, but into separating particles, not speeding them up.
A student thinks freezing takes energy in, because things get colder. Ask: When water freezes in a freezer, does it give energy to the freezer or take it? Fix: It gives energy out: the freezer has to remove that energy.
A student thinks steam is hotter than 100 °C as soon as it forms. Ask: What was the temperature on the flat part of the curve while it boiled? Fix: 100 °C. Steam only gets hotter once all the water has boiled and you keep heating.
Stretch and support
Stretch: Why is a burn from steam at 100 °C worse than one from water at 100 °C? (Steam gives out a lot of extra energy as it condenses on your skin.)
Support: Give the frame: 'When it melts, energy goes … . The particles … , so the temperature … .'
Where it leads
Next: Pure substance or mixture? (AC9S7U06). Particle theory's scope and limits (node 5) is a short lesson.
In class: a heating curve for ice or a cooling curve for stearic acid, with data loggers if you have them. Candle wax is a mixture, so it freezes over a range of temperatures, not at one: use it to show what a mixture's curve looks like. The water curve on the board is to scale for steady heating (1 kg: 42 kJ to warm the ice, 334 kJ to melt, 418 kJ to warm the water, 2260 kJ to boil); a real pot loses some heat, so times vary.
What each step is for
Engage: the stuck thermometer: Students expect temperature to keep rising while heating continues; the plateau is the observation that needs the particle explanation.
Explore: ice to steam: Watching particles change through ice, water and steam shows what the energy does at each change of state.
Explain: energy in, energy out: Reading a real heating curve, sorting the changes by energy in or out, then explaining a flat part of the curve with particles, is the node's success criterion.
Mix it up: Gas particles moving fast in every direction is what the heating curve builds on: energy in makes particles move faster and spread.
Elaborate: a cooling curve: Reading a cooling curve applies the same idea in reverse: a flat part while the substance freezes and gives out energy.
Elaborate: sweating: Evaporation taking energy from the skin connects 'energy in for a change of state' to everyday cooling.
Now you can
Say whether energy goes in or out when a substance melts, boils, freezes or condenses.
Explain a change of state using particles.
Explain why the temperature stays the same while a substance changes state.