Students describe arrangement, motion and attraction for each state and use them to explain why solids keep their shape and gases fill containers (AC9S7U05, node 2, a hinge node).
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Students will be able to
I can describe the arrangement, motion and attraction of particles in a solid, a liquid and a gas.
I can explain why a solid keeps its shape and a gas fills its container.
I can use a particle diagram as evidence in an explanation.
Timing
Engage and Explore 8 min · Explain 10 min · Evaluate 3 min · Typical adds 6 min, Wider about 12 min
Before class
A concept tutorial for introducing or revising the idea; it does not replace practical work. Set the width with ?width=minimum, typical (default) or wider.
The hinge risk for this node is decorative diagrams: students who label but do not reason with diagrams cannot explain properties in node 3. Push every answer back to a diagram: "show me where that is in the particles".
Running the lesson
Engage: count how many predicted 'the top'. That is the 'gases are light' misconception the jar targets.
Explore: make sure students remove the jar for all three states, not just the gas.
Explain: the 12-item sort is the minimum description. Then the two questions turn description into explanation.
In your own words: collect the juice and sugar cube paragraph; it is the best evidence of the node's checkpoint.
The hard part (on purpose)
Explaining a property from what the particles do, not the state's name. When a student says gas rises because it is light, ask which way the particles moved on the board.
When students get stuck
What you will see
A question to ask
The fix
A student says the gas floats up because it is light.
Which way did the gas particles move on the board?
They moved in every direction, including down. Fast motion and weak attraction spread a gas out.
A student says solid particles don't move.
Look at the solid in the jar for a few seconds. What do the particles do?
They vibrate in fixed positions. Attraction holds them in place; it does not stop them moving.
A student answers 'because it is a liquid' when asked why juice spreads.
What are the liquid's particles doing that makes it spread?
Use the three words: arrangement (close, no pattern), motion (sliding), attraction (weaker).
Answers to the checks
Engage: a jar of gas: A small jar of gas sits in the middle of a large, empty, sealed box. The jar is removed. A few seconds later, where is the gas? Spread through the whole box: up, down and sideways. Its particles move fast in every direction.
Explore: remove the jar: Remove the jar for the solid, the liquid and the gas.
Describe each state: Which state does each description belong to? Particles close together in a fixed pattern → Solid; Particles close together with no pattern → Liquid; Particles far apart → Gas; Particles vibrate in place → Solid; Particles slide past each other → Liquid; Particles move fast in all directions → Gas; Strong attraction between particles → Solid; Weaker attraction: particles stay close but can move → Liquid; Very weak attraction → Gas; Keeps its shape → Solid; Spreads across the bottom of its container → Liquid; Fills its whole container → Gas
Why gases fill: Why does a gas spread to fill its whole container? Its particles move fast in all directions and barely attract each other Nothing holds gas particles together, so they spread in every direction until they fill the space.
Why solids keep their shape: Why does a solid keep its shape? Strong attraction holds its particles in fixed positions, where they vibrate Each particle vibrates but stays in its place, so the whole shape stays.
Mix it up: Looking back at the particle model: which statement about particles is true? Particles are always moving, even in a solid Key idea 3: particles are always moving. In a solid they vibrate in place, held there by strong attraction.
Rank by spacing: Using the spacing only, where does each substance belong? Iron (real density 7.87 g/cm³) → Densest pair; Aluminium (2.70 g/cm³) → Densest pair; Water (1.00 g/cm³) → Middle pair; Cooking oil (0.92 g/cm³) → Middle pair; Carbon dioxide (0.0018 g/cm³) → Least dense pair; Air (0.0012 g/cm³) → Least dense pair Spacing predicts the groups: solids and liquids close together, then a huge drop to the gases.
Inside a pair: Iron and aluminium have the same spacing, but iron is almost three times denser. Why? Each iron particle is heavier Spacing explains the big groups; the mass of each particle explains differences within a group. The same goes for carbon dioxide and air.
From ice to steam: Heat the water from ice, through liquid, to steam.
Spacing: When liquid water becomes steam, the space between the particles… increases a lot Steam takes up roughly 1,700 times the volume of the same mass of liquid water.
Energy: What lets particles move further apart as water is heated? They gain energy, so they can overcome the attraction
Elaborate: comparing attraction: Put the substances in order of attraction between their particles, strongest first. see the program below Higher boiling point means stronger attraction. Salt needs over 1400 °C; oxygen is already a gas at −183 °C.
Table salt (boils at 1413 °C)
Water (boils at 100 °C)
Ethanol (boils at 78 °C)
Oxygen (boils at −183 °C)
Elaborate: the odd one out, ice: Most solids sink in their own liquid (solid wax sinks in melted wax), but ice floats on water. What does this mean for the particle model? Water is a special case: frozen water particles lock into an open pattern with more space than in the liquid, so the model needs this extra detail The model keeps its core ideas and gains a detail. (The diagram exaggerates the gap: ice takes up about 9% more space than the same mass of water.)
Solutions
One possible solution for each task. Other correct programs are fine.
Elaborate: six substances › Rank by spacing
Open task: any program that does what the step asks.
Elaborate: six substances › Inside a pair
Open task: any program that does what the step asks.
Elaborate: heating one substance › From ice to steam
Open task: any program that does what the step asks.
Elaborate: heating one substance › Spacing
Open task: any program that does what the step asks.
Elaborate: heating one substance › Energy
Open task: any program that does what the step asks.
Elaborate: the odd one out, ice
Open task: any program that does what the step asks.
Stretch and support
Stretch: Air is a gas, yet a room of air doesn't collapse to the floor. Why don't air particles all settle at the bottom like the liquid did?
Support: Give a three-column table (arrangement, motion, attraction) to fill in while watching the board, before the sort.
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 "Why does a gas fill its container?".
The idea: Particle arrangement, motion and attraction in each state: I can describe particle arrangement, motion and attraction for each state and explain observable properties using this model.
What understanding looks like:
- Describes particle arrangement, motion and attraction for each state
- Explains why solids hold shape and gases fill containers using particle theory
- Does not treat particle diagrams as decorative — uses them to support reasoning
Mistakes students often make here:
- A student says the gas floats up because it is light.
- A student says solid particles don't move.
- A student answers 'because it is a liquid' when asked why juice spreads.
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 (Introducing the particle model), 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 "Why does a gas fill its container?".
Exit ticket
A solid keeps its shape but a gas does not. Use arrangement, motion and attraction to explain the difference.
In a solid the particles are close, held in fixed places by strong attraction, and only vibrate, so the shape stays. In a gas they are far apart, move fast in all directions and barely attract, so they spread out.
What each step is for
Engage: a jar of gas: Students who think gases rise because they are light will predict the top; the jar shows spreading in every direction.
Explore: remove the jar: Releasing each state from the same jar makes the three behaviours directly comparable.
Explain: arrangement, motion, attraction: Describing each state by all three features, then explaining behaviour from them, is what stops diagrams being decorative.
Mix it up: Bringing back the four key ideas of the particle model, which this lesson uses to explain solids, liquids and gases.
Elaborate: six substances: Ranking substances by spacing, then checking real densities, shows what the model predicts well (the big gaps) and what it needs added (particle mass).
Elaborate: heating one substance: The same particles at different temperatures connect motion and energy to spacing.
Elaborate: comparing attraction: Boiling points are evidence for attraction strength: particles need more energy to pull apart when they attract more strongly.
Elaborate: the odd one out, ice: An anomaly is a chance to evaluate the model: water needs an extra detail, not a new model.
Where it leads
Next: node 3 explains properties such as compressibility, density and viscosity with the same three features.
In class: a sealed syringe of air, and a perfume or deodorant spray released at the front of the room, timed to when students at the back smell it.