--- title: "Why does a gas fill its container?" subtitle: "Year 7 Chemistry · Particle theory · Lesson 2" --- # Why does a gas fill its container? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** 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. :::teacher-note **In one sentence:** 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). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage The box has no air in it. Decide before you look. :::keep-together **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 My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Spread through the whole box: up, down and sideways. Its particles move fast in every direction. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: arrangement, motion, attraction :::keep-together ### Describe each state **Which state does each description belong to?** Groups: Solid, Liquid, Gas - Particles close together in a fixed pattern: {{field-15}} - Particles close together with no pattern: {{field-15}} - Particles far apart: {{field-15}} - Particles vibrate in place: {{field-15}} - Particles slide past each other: {{field-15}} - Particles move fast in all directions: {{field-15}} - Strong attraction between particles: {{field-15}} - Weaker attraction: particles stay close but can move: {{field-15}} - Very weak attraction: {{field-15}} - Keeps its shape: {{field-15}} - Spreads across the bottom of its container: {{field-15}} - Fills its whole container: {{field-15}} ::: :::teacher-note **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 ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** 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. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** 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 :::keep-together **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 ::: :::teacher-note **Answer:** 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. ::: :::typical ## Elaborate: six substances :::keep-together ### 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): {{field-15}} - Aluminium (2.70 g/cm³): {{field-15}} - Water (1.00 g/cm³): {{field-15}} - Cooking oil (0.92 g/cm³): {{field-15}} - Carbon dioxide (0.0018 g/cm³): {{field-15}} - Air (0.0012 g/cm³): {{field-15}} ::: :::teacher-note **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 Spacing predicts the groups: solids and liquids close together, then a huge drop to the gases. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** 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. ::: ::: :::wider ## Elaborate: heating one substance :::keep-together ### Spacing **When liquid water becomes steam, the space between the particles…** ☐ stays the same ☐ decreases ☐ increases a lot ::: :::teacher-note **Answer:** increases a lot Steam takes up roughly 1,700 times the volume of the same mass of liquid water. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** They gain energy, so they can overcome the attraction ::: ::: :::wider ## Elaborate: comparing attraction To boil, a substance's particles need enough energy to pull completely away from each other. The stronger the attraction, the hotter it has to get. :::keep-together **Put the substances in order of attraction between their particles, strongest first.** A. `Water (boils at 100 °C)` B. `Oxygen (boils at −183 °C)` C. `Table salt (boils at 1413 °C)` D. `Ethanol (boils at 78 °C)` Correct order (letters): {{field-24}} ::: :::teacher-note **Answer:** see the program below Higher boiling point means stronger attraction. Salt needs over 1400 °C; oxygen is already a gas at −183 °C. ```science Table salt (boils at 1413 °C) Water (boils at 100 °C) Ethanol (boils at 78 °C) Oxygen (boils at −183 °C) ``` ::: ::: :::wider ## Elaborate: the odd one out, ice :::keep-together **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 ::: :::teacher-note **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 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.) ::: ::: ## 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. ## Exit ticket :::keep-together **A solid keeps its shape but a gas does not. Use arrangement, motion and attraction to explain the difference.** {{lines-3}} ::: :::teacher-note **Answer:** 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. :::