--- title: "Same particles, new look?" subtitle: "Year 8 Chemistry · Chemical change · Lesson 6" --- # Same particles, new look? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A puddle disappears on a hot day. A sugar cube vanishes in tea. Has anything actually gone? :::teacher-note **In one sentence:** Students explain physical change as the same particles rearranged, spread out or packed together, with no new substance formed, and represent it with the particle model (AC9S8U07, node 1). **Students should be able to say:** I can explain a physical change at the particle level. I can say what changes and what stays the same in a physical change. I can draw particle diagrams of a physical change. **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 :::keep-together **Water boils in a kettle and bubbles rise. What are the bubbles made of?** ☐ Water particles that have grown bigger ☐ Air that was hiding in the water ☐ Hydrogen and oxygen, split off from the water ☐ Water particles (steam), far apart My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Water particles, as steam. Boiling only gives the particles enough energy to escape each other and spread out; each one is still H₂O. Nothing new is made, so boiling is a physical change. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: what a physical change is :::keep-together ### The particles **In melting, boiling and dissolving, what happens to the particles?** ☐ They change how they are arranged and how they move, but stay the same particles ☐ They turn into new particles ☐ They get bigger as they warm up ::: :::teacher-note **Answer:** They change how they are arranged and how they move, but stay the same particles A physical change rearranges the same particles. Nothing new is made. ::: :::keep-together ### What changes **Physical change: what changes, and what stays the same?** Groups: Changes, Stays the same - How the particles are arranged: {{field-15}} - How fast the particles move: {{field-15}} - The spaces between the particles: {{field-15}} - The atoms inside each particle: {{field-15}} - The number of particles: {{field-15}} - The substance: {{field-15}} ::: :::teacher-note **Answer:** How the particles are arranged → Changes; How fast the particles move → Changes; The spaces between the particles → Changes; The atoms inside each particle → Stays the same; The number of particles → Stays the same; The substance → Stays the same ::: :::keep-together ### Physical or not **Physical change or not?** Groups: Physical change, Not physical: a new substance forms - Butter melting: {{field-15}} - Water vapour forming drops on a cold window: {{field-15}} - Sugar dissolving in tea: {{field-15}} - Cutting paper into pieces: {{field-15}} - Toast burning: {{field-15}} - An iron nail rusting: {{field-15}} ::: :::teacher-note **Answer:** Butter melting → Physical change; Water vapour forming drops on a cold window → Physical change; Sugar dissolving in tea → Physical change; Cutting paper into pieces → Physical change; Toast burning → Not physical: a new substance forms; An iron nail rusting → Not physical: a new substance forms ::: :::keep-together ### Where the salt went **Salt dissolved in water seems to disappear. How could you show it is still there?** ☐ Filter it out with filter paper ☐ You can't: dissolved salt is destroyed ☐ Evaporate the water: the salt is left behind ::: :::teacher-note **Answer:** Evaporate the water: the salt is left behind The salt particles are spread between the water particles. Evaporating the water leaves them behind as crystals. ::: :::keep-together ### Why it can be reversed **Melting, boiling and dissolving can usually be reversed. Why?** ☐ Being reversible is what makes a change physical ☐ Any change can be reversed if you try hard enough ☐ No energy goes in or out, so nothing is lost ☐ The same particles are still there, so they can be brought back to how they were ::: :::teacher-note **Answer:** The same particles are still there, so they can be brought back to how they were No new substance is made, so the original particles are all still there to rearrange back. Being reversible is a result of a physical change, not what defines it. ::: :::typical ## Elaborate: inside the steam :::keep-together ### What's inside **What is in each steam particle?** ☐ Air ☐ Separate hydrogen and oxygen atoms ☐ Two hydrogen atoms joined to one oxygen atom: H₂O, the same as liquid water ::: :::teacher-note **Answer:** Two hydrogen atoms joined to one oxygen atom: H₂O, the same as liquid water Steam is water as a gas: the same H₂O particles, moving fast and far apart. ::: ::: :::wider ## Elaborate: does mass change? :::keep-together ### The mass **100 g of ice melts in a sealed bag. What is the mass of the water?** ☐ 100 g: the same particles, so the same mass ☐ Less than 100 g: water takes up less space ☐ More than 100 g: liquids are heavier ::: :::teacher-note **Answer:** 100 g: the same particles, so the same mass No particles are made or lost in a physical change, so the mass stays the same. (Ice is actually a little less dense than water, which is why it floats.) ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - In a **physical change** (melting, freezing, boiling, condensing, dissolving, cutting) the particles change how they are arranged, how far apart they are and how fast they move. - The particles themselves stay the same, so **no new substance** is made. Steam is still water; dissolved salt is still salt. - The number of particles stays the same, so the **mass** stays the same. - Next lesson: a **chemical change**, where atoms rearrange into new particles. ## Exit ticket :::keep-together **Wax melts on a candle and drips. Use particles to explain why this is a physical change.** {{lines-3}} ::: :::teacher-note **Answer:** The wax particles move from fixed positions to sliding past each other, but each particle is still wax: no new substance, so physical. :::