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).
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Students will be able to
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.
Timing
Engage and Explore 6 min · Explain 11 min · Evaluate 3 min · Typical adds 3 min, Wider about 2 min
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
What you will see
A question to ask
The fix
A student says the temperature stops rising because the heat turned off.
Was the stove still on?
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.
When water freezes in a freezer, does it give energy to the freezer or take it?
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.
What was the temperature on the flat part of the curve while it boiled?
100 °C. Steam only gets hotter once all the water has boiled and you keep heating.
Answers to the checks
Engage: the stuck thermometer: A pot of water is boiling on a stove. The stove stays on full. What does a thermometer in the water show over the next five minutes? It stays at about 100 °C. The energy from the stove is going into separating the water particles so they escape as steam, not into making them move faster, so the temperature stays the same until the water has all boiled away.
Explore: ice to steam: Heat the water from ice to steam.
The heating curve: 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?
In or out: Does the substance take energy in, or give energy out? Melting (solid to liquid) → Energy in; Boiling or evaporating (liquid to gas) → Energy in; Freezing (liquid to solid) → Energy out; Condensing (gas to liquid) → Energy out
The flat part: On the heating curve, the line is flat from 1 to 9 minutes. What is happening? The ice is melting: energy goes into freeing the particles from their fixed places, so the temperature doesn't rise During a change of state, the energy goes into overcoming the attraction between particles, not into making them move faster, so the temperature stays constant.
The second flat part: At what temperature (°C) is the second flat part of water's heating curve? 100 100 °C: water's boiling point (at sea level). The first flat part, at 0 °C, is its melting point.
Why boiling takes longer: The curve is drawn to scale: the stove gives the same energy every minute. Why is the flat part at 100 °C about seven times longer than the one at 0 °C? Boiling pulls the particles completely apart, which takes far more energy than loosening them from their fixed places Melting only frees the particles to slide past each other. Boiling has to pull them far apart against their attraction, so it needs about seven times as much energy (2260 kJ for each kilogram, against 334 kJ to melt it).
Before melting: What does adding energy do to the particles in a solid, before it melts? They vibrate faster, so the temperature rises Temperature is a measure of how fast the particles move on average. Energy in, faster particles, higher temperature, until a change of state begins.
What the particles do: What happens to the particles in each change of state? Melting → D. Particles vibrate hard enough to break free of their fixed places and start sliding past each other; Freezing → B. Particles slow down and settle into fixed places, giving out energy; Evaporating → C. Fast particles at the surface pull free of the others and escape as a gas; Condensing → A. Gas particles slow down and are pulled close together into a liquid, giving out energy Energy in frees the particles from each other's attraction; energy out lets the attraction pull them back together.
Mix it up: Looking back at states of matter: a small jar of gas is opened in the middle of a large, sealed room. A few minutes later, where are its particles? Spread through the whole room Gas particles move fast in every direction and barely attract each other, so they spread to fill the room.
Freezing stearic acid: Melted stearic acid (a waxy substance used in candles) is cooling. Move R to where it starts to freeze (the line goes flat).
Why it stays flat: Why does the stearic acid's temperature stay at 69 °C for 8 minutes while it freezes? As particles settle into fixed places they give out energy, which keeps the temperature steady Freezing gives energy out. Until every particle has settled, that energy balances what is lost to the room.
Elaborate: sweating: Why does sweating cool you down, even on a hot day? Evaporating sweat takes energy in, and it takes it from your skin Changing liquid to gas needs energy. The fastest particles escape from the sweat, carrying energy away from your skin.
Solutions
One possible solution for each task. Other correct programs are fine.
Elaborate: a cooling curve › Freezing stearic acid
Open task: any program that does what the step asks.
Elaborate: a cooling curve › Why it stays flat
Open task: any program that does what the step asks.
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 … .'
Coach prompt (optional)
For a student who has done the lesson and is still stuck. They paste it into an AI tool your school allows; it asks questions about this lesson's one idea and doesn't give answers. Students see it on the course recap page only through a link ending in ?coach=1: recap with coach prompts.
You are a patient science coach for a Year 7 student in Australia. Help me understand ONE idea only, from the lesson "Where does the heat go?".
The idea: Changes of state and energy: I can explain changes of state using particle theory and correctly identify energy input or output for each change.
What understanding looks like:
- Explains melting, freezing, evaporation and condensation using particle theory
- Correctly identifies energy input or output for each change
- Does not confuse temperature change with state change during plateau regions
Mistakes students often make here:
- A student says the temperature stops rising because the heat turned off.
- A student thinks freezing takes energy in, because things get colder.
- A student thinks steam is hotter than 100 °C as soon as it forms.
How to coach me (rules from the Waypoints Kit coach):
- Ask me one short question at a time (2 to 4 sentences) and wait for my answer. Never give me the answer before you have heard my thinking, and don't hide the answer inside your question.
- Start in Discover: ask me to explain the idea in my own words, or to predict what happens in a simple example. I can switch any time: "teach it back" (you play a beginner and I explain), "spot the flaw" (you show me an answer with one common mistake and I find it), or "check" (I explain, and you tell me what is solid and what to revisit).
- If I am stuck or say "just tell me", ask what I think first, then give a small hint or a simpler example. Explain only after I have had a real go.
- If I seem to be missing the idea that comes before this one (Explaining physical properties using particle theory), check it quickly, then come back.
- Stay in this idea: don't teach later ideas or add extra content. If I ask about something else, say it is for another lesson and bring me back.
- After you help, ask me one reflective question, such as how I know my answer is right or what I would check.
- Praise my effort and method, not my ability.
- You can be wrong: if I rely on something you said, remind me to check it against my notes or the lesson. If you don't know, say so.
- Use simple words.
- Stop when I can explain the idea in my own words and get one new example right. Then tell me to go back to the lesson "Where does the heat go?".
Exit ticket
Water vapour condenses on a cold window. Is energy taken in or given out, and what are the particles doing?
Given out. The water particles slow down and come close together, forming a liquid on the glass.
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.
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.