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# Why does a gas fill its container?

Year 7 Chemistry · Particle theory

<!-- Speaker note: 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). About 25 minutes (Engage and Explore 8 min · Explain 10 min · Evaluate 3 min · Typical adds 6 min, Wider about 12 min). -->

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## Today

**We are learning to** describe the arrangement, motion and attraction of particles in a solid, a liquid and a gas, and use them to explain why solids keep their shape and gases fill their containers.

- 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.

<!-- Speaker note: 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". Teacher: In your own words: collect the juice and sugar cube paragraph; it is the best evidence of the node's checkpoint. -->

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## Why it matters

> Tip a sugar cube out of its box and it stays a cube. Tip out juice and it spreads across the table. Open perfume and soon the whole room smells. Same particles idea, three behaviours.

<!-- Speaker note: The particle model is not just a picture to label. In this lesson you use it to **explain** what solids, liquids and gases do. -->

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## Engage: predict

**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
- Mostly at the top of the box
- Mostly at the bottom of the box
- Still in a jar-shaped clump

<!-- Speaker note: What happens: Spread through the whole box: up, down and sideways. Its particles move fast in every direction. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many predicted 'the top'. That is the 'gases are light' misconception the jar targets. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry%2Fstates%2Fstep-2&code=widget%3A+jar%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry/states/?full=step-2#step-2)

<!-- Speaker note: Choose a state, then remove the jar. Do all three, and watch the particles each time. -->

<!-- Speaker note: Teacher: make sure students remove the jar for all three states, not just the gas. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* Spread through the whole box: up, down and sideways. Its particles move fast in every direction.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Describe each state

**Which state does each description belong to?**

Groups: Solid · Liquid · Gas

Particles close together in a fixed pattern · Particles close together with no pattern · Particles far apart · Particles vibrate in place · Particles slide past each other · Particles move fast in all directions · Strong attraction between particles · Weaker attraction: particles stay close but can move · Very weak attraction · Keeps its shape · Spreads across the bottom of its container · Fills its whole container

* Answer: 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

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the 12-item sort is the minimum description. Then the two questions turn description into explanation. -->

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## 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
- Air pushes it around
- Gas particles are light and float
- Its particles expand to fill the space

* Answer: Its particles move fast in all directions and barely attract each other

<!-- Speaker note: Nothing holds gas particles together, so they spread in every direction until they fill the space. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Why solids keep their shape

**Why does a solid keep its shape?**

- Its particles are bigger
- It is heavier than a liquid
- Its particles don't move
- Strong attraction holds its particles in fixed positions, where they vibrate

* Answer: Strong attraction holds its particles in fixed positions, where they vibrate

<!-- Speaker note: Each particle vibrates but stays in its place, so the whole shape stays. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Mix it up

**Looking back at the particle model: which statement about particles is true?**

- Particles get bigger when they are heated
- There is air between the particles of water
- The particles in a solid have stopped moving
- Particles are always moving, even in a solid

* Answer: Particles are always moving, even in a solid

<!-- Speaker note: Key idea 3: particles are always moving. In a solid they vibrate in place, held there by strong attraction. What this step is for: Bringing back the four key ideas of the particle model, which this lesson uses to explain solids, liquids and gases. -->

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## Elaborate: six substances

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry%2Fstates%2Fstep-5&code=widget%3A+windows%0Aset%3A+six%0Acaption%3A+Each+window+shows+the+same+tiny+volume+of+a+substance.%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry/states/?full=step-5#step-5)

<!-- Speaker note: What this step is for: Ranking substances by spacing, then checking real densities, shows what the model predicts well (the big gaps) and what it needs added (particle mass). -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Rank by spacing

**Using the spacing only, where does each substance belong?**

Groups: Densest pair · Middle pair · Least dense pair

Iron (real density 7.87 g/cm³) · Aluminium (2.70 g/cm³) · Water (1.00 g/cm³) · Cooking oil (0.92 g/cm³) · Carbon dioxide (0.0018 g/cm³) · Air (0.0012 g/cm³)

* Answer: 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

<!-- Speaker note: Spacing predicts the groups: solids and liquids close together, then a huge drop to the gases. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Inside a pair

**Iron and aluminium have the same spacing, but iron is almost three times denser. Why?**

- Each iron particle is heavier
- Aluminium has air gaps in it
- Iron particles are bigger

* Answer: Each iron particle is heavier

<!-- Speaker note: Spacing explains the big groups; the mass of each particle explains differences within a group. The same goes for carbon dioxide and air. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: heating one substance

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry%2Fstates%2Fstep-6&code=widget%3A+heat%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry/states/?full=step-6#step-6)

<!-- Speaker note: What this step is for: The same particles at different temperatures connect motion and energy to spacing. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Spacing

**When liquid water becomes steam, the space between the particles…**

- stays the same
- decreases
- increases a lot

* Answer: increases a lot

<!-- Speaker note: Steam takes up roughly 1,700 times the volume of the same mass of liquid water. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Energy

**What lets particles move further apart as water is heated?**

- They gain energy, so they can overcome the attraction
- The attraction disappears
- The particles shrink

* Answer: They gain energy, so they can overcome the attraction

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: comparing attraction

**Put the substances in order of attraction between their particles, strongest first.**

```science
Water (boils at 100 °C)
Oxygen (boils at −183 °C)
Table salt (boils at 1413 °C)
Ethanol (boils at 78 °C)
```

<!-- Speaker note: Higher boiling point means stronger attraction. Salt needs over 1400 °C; oxygen is already a gas at −183 °C. Correct order: Table salt (boils at 1413 °C) Water (boils at 100 °C) Ethanol (boils at 78 °C) Oxygen (boils at −183 °C) What this step is for: Boiling points are evidence for attraction strength: particles need more energy to pull apart when they attract more strongly. -->

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## Elaborate: the odd one out, ice

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry%2Fstates%2Fstep-8&code=widget%3A+windows%0Aset%3A+ice%0Acaption%3A+The+same+volume+of+ice+and+of+liquid+water.%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry/states/?full=step-8#step-8)

<!-- Speaker note: What this step is for: An anomaly is a chance to evaluate the model: water needs an extra detail, not a new model. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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?**

- Solid particles are always further apart than liquid particles
- Ice floats because of air bubbles trapped inside it
- The model is wrong and should be thrown out
- 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

* Answer: 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

<!-- Speaker note: 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.) What this step is for: An anomaly is a chance to evaluate the model: water needs an extra detail, not a new model. -->

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## Evaluate: look back

Where did you think the gas would be? Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- **Solid:** close together, fixed pattern, vibrating in place, strong attraction. It keeps its shape.
- **Liquid:** close together, no pattern, sliding past each other, weaker attraction. It flows and spreads across the bottom of its container.
- **Gas:** far apart, moving fast in all directions, very weak attraction. It spreads to fill its whole container.
- A gas spreads in every direction, including down. It is not floating up.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this 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. -->