--- marp: true theme: default html: true --- # Physical or chemical? Use all the clues Year 8 Chemistry · Chemical change --- ## Today **We are learning to** to classify an unfamiliar change, ask the main question first: is a new substance made? Then check the clues: colour, gas, solid forming, light, and temperature or energy change. One clue can mislead (boiling makes bubbles and ice melting takes in heat, yet both are physical), so several clues pointing the same way give more confidence. - 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. --- ## Why it matters > 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? --- 💬 Discussion ## Engage: predict **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 --- ✅ Check ## What happened? * 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. --- ✅ Check ## Sort **Physical or chemical change?** Groups: Physical · Chemical Butter melting in a pan · An egg cooking and turning solid and white · Sugar dissolving in tea · Iron nails going orange-brown and flaky in damp air · Dry ice (solid carbon dioxide) turning straight into a gas · Milk going sour * 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 --- ✅ Check ## 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 * Answer: No: melting is a physical change that takes in energy, so a temperature change alone does not prove a new substance --- ✅ Check ## 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 * Answer: Several different clues point the same way, so a new substance is very likely --- ✅ Check ## 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 * Answer: Chemical: a new gas and new substances were made, even though it got colder --- ✅ Check ## Elaborate: a whole recipe **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 * Answer: Yeast making gas, and baking to a brown crust; mixing is physical --- ✅ Check ## Elaborate: testing **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 * Answer: Other substances might cause a weaker or different clue, so a specific one reduces false positives --- ## 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. --- ✅ Check ## Exit ticket **A student says 'it fizzed, so it was chemical'. Give one case where fizzing is not chemical and say what to check instead.** * Boiling water fizzes with steam, a physical change. Check whether a new substance was made.