--- title: "Physical or chemical? Use all the clues" subtitle: "Year 8 Chemistry · Chemical change · Lesson 10" --- # Physical or chemical? Use all the clues **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A pot of water boils: bubbles, steam, heat. A pot of milk 'boils' and then turns to a skin. Both look dramatic. Only one is a chemical change. How do you tell? :::teacher-note **In one sentence:** Students apply the new-substance criterion, observable clues and energy change together to classify unfamiliar changes, and explain why several clues are better than one (AC9S8U07, node 6, an integration node). **Students should be able to say:** I can classify an unfamiliar change using all the criteria. I can say why several clues are better than one. I can give an example where one clue misleads. **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 in a pot is heated and bubbles all through it. An antacid tablet dropped into water also bubbles. Which is a chemical change?** ☐ Both: bubbles always mean a chemical change ☐ Only the antacid: it makes a new gas, while the bubbles in boiling water are just water vapour ☐ Only the water: it takes in heat ☐ Neither My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Only the antacid. The bubbles in boiling water are water as a gas (a physical change). The tablet reacts to make carbon dioxide, a new substance. A clue like bubbles only counts if it points to a new substance. ::: ## Explain: classify with evidence :::keep-together ### Sort **Physical or chemical change?** Groups: Physical, Chemical - Butter melting in a pan: {{field-15}} - An egg cooking and turning solid and white: {{field-15}} - Sugar dissolving in tea: {{field-15}} - Iron nails going orange-brown and flaky in damp air: {{field-15}} - Dry ice (solid carbon dioxide) turning straight into a gas: {{field-15}} - Milk going sour: {{field-15}} ::: :::teacher-note **Answer:** Butter melting in a pan → Physical; An egg cooking and turning solid and white → Chemical; Sugar dissolving in tea → Physical; Iron nails going orange-brown and flaky in damp air → Chemical; Dry ice (solid carbon dioxide) turning straight into a gas → Physical; Milk going sour → Chemical ::: :::keep-together ### Colder **Ice melts and the water around it gets colder. Does a temperature change show a chemical change here?** ☐ No: melting is a physical change that takes in energy, so a temperature change alone does not prove a new substance ☐ Yes, because ice is a different substance from water ☐ Yes: any temperature change means a chemical change ::: :::teacher-note **Answer:** No: melting is a physical change that takes in energy, so a temperature change alone does not prove a new substance Temperature changes happen in both kinds of change, so you need the new-substance test too. ::: :::keep-together ### Many clues **A liquid in a test tube changes colour, fizzes, gets hot and leaves a solid at the bottom. Why is this more convincing than a colour change alone?** ☐ Fizzing is always a chemical change ☐ More clues make it physical ☐ Several different clues point the same way, so a new substance is very likely ::: :::teacher-note **Answer:** Several different clues point the same way, so a new substance is very likely No single clue is certain. Several agreeing clues make a classification much more reliable. ::: :::keep-together ### A cold reaction **Pouring vinegar on baking soda fizzes and gets cold, the mixture is left as a different liquid, and the gas puts out a flame. How should you classify it?** ☐ Chemical: a new gas and new substances were made, even though it got colder ☐ Physical, because it looks like a liquid before and after ☐ Physical, because it got colder ::: :::teacher-note **Answer:** Chemical: a new gas and new substances were made, even though it got colder A gas that behaves differently from air, and new substances, show a chemical change. Taking in energy fits, because some reactions are endothermic. ::: :::typical ## Elaborate: a whole recipe :::keep-together **Bread making: flour and water are mixed, the yeast makes gas, the dough rises, and the loaf is baked brown. Which stages are chemical?** ☐ Only the baking ☐ None of it ☐ All of it, because it ends up as bread ☐ Yeast making gas, and baking to a brown crust; mixing is physical ::: :::teacher-note **Answer:** Yeast making gas, and baking to a brown crust; mixing is physical Mixing is physical, while the yeast making carbon dioxide and the baking and browning are chemical changes. Most real processes have both. ::: ::: :::wider ## Elaborate: testing :::keep-together **A water test kit changes colour to show chlorine. Why might a good kit use a clue that is hard to fake, such as a very specific colour change?** ☐ A colour change is always chemical, so one is enough ☐ Colour doesn't matter ☐ Other substances might cause a weaker or different clue, so a specific one reduces false positives ::: :::teacher-note **Answer:** Other substances might cause a weaker or different clue, so a specific one reduces false positives A test is reliable when its clue points to one substance and nothing else. That is the same idea as using several clues together. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - First ask: **is a new substance made?** That is what makes a change chemical. - The clues (colour, gas, solid, light, temperature or energy change) support the answer, but **one clue can mislead**. - **Several clues pointing the same way** give more confidence. - Real processes, such as cooking, often include both physical and chemical changes. ## Exit ticket :::keep-together **A student says 'it fizzed, so it was chemical'. Give one case where fizzing is not chemical and say what to check instead.** {{lines-3}} ::: :::teacher-note **Answer:** Boiling water fizzes with steam, a physical change. Check whether a new substance was made. :::