--- title: "How can you tell a reaction happened?" subtitle: "Year 8 Chemistry · Chemical change · Lesson 8" --- # How can you tell a reaction happened? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Bake a cake and it rises, browns and smells wonderful. Leave a nail outside and it turns orange. In each case, how do you know something new was made? :::teacher-note **In one sentence:** Students list observable signs of chemical change and explain that each is a clue to a new substance, not proof, distinguishing misleading signs in physical changes (AC9S8U07, node 3). **Students should be able to say:** I can list the signs of a chemical change. I can explain why a sign is a clue, not proof. I can spot a sign that misleads in 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 **Which of these shows a chemical change has happened for certain?** ☐ It changes colour ☐ Bubbles appear ☐ It gets warmer ☐ None of them on its own: each is a clue, and the test is whether a new substance formed My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** None of them on its own. Bubbles, warmth and colour change are all signs, but boiling water bubbles, a radiator warms things, and mixing paints changes colour, with no new substance. Signs are clues; the test is a new substance. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: signs and the test :::keep-together ### Signs **Is it a sign of a chemical change?** Groups: A sign, Not a sign - A colour change when two substances mix: {{field-15}} - Bubbles of a new gas: {{field-15}} - A solid forming when two clear liquids mix (a precipitate): {{field-15}} - Light or heat given out, or the mixture getting colder: {{field-15}} - A new smell: {{field-15}} - The substance changing shape: {{field-15}} ::: :::teacher-note **Answer:** A colour change when two substances mix → A sign; Bubbles of a new gas → A sign; A solid forming when two clear liquids mix (a precipitate) → A sign; Light or heat given out, or the mixture getting colder → A sign; A new smell → A sign; The substance changing shape → Not a sign ::: :::keep-together ### Clue or trap **Chemical change, or a sign that misleads?** Groups: Chemical change, Misleading sign: physical change - A cake browning in the oven: {{field-15}} - Vinegar and bicarb fizzing: {{field-15}} - Water boiling and bubbling: {{field-15}} - Blue and yellow paint mixing to green: {{field-15}} - A nail rusting orange: {{field-15}} - A hot drink cooling down: {{field-15}} ::: :::teacher-note **Answer:** A cake browning in the oven → Chemical change; Vinegar and bicarb fizzing → Chemical change; Water boiling and bubbling → Misleading sign: physical change; Blue and yellow paint mixing to green → Misleading sign: physical change; A nail rusting orange → Chemical change; A hot drink cooling down → Misleading sign: physical change ::: :::keep-together ### The strongest evidence **When magnesium burns there is a bright light, and a white powder is left. Which is the strongest evidence of a chemical change?** ☐ The white powder: a new substance with different properties from the shiny metal ☐ The magnesium getting hot ☐ The bright light ::: :::teacher-note **Answer:** The white powder: a new substance with different properties from the shiny metal A new substance with new properties is the test; the light and heat are signs that go with it. ::: :::keep-together ### Why each is a clue **Why does each sign suggest a new substance has formed?** A. A new substance has formed that doesn't dissolve in the liquid B. Atoms rearranging into new particles gives out or takes in energy C. A new substance that is a gas has been made D. Particles of a new substance are reaching your nose E. The new substance has a different colour from the starting ones {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `A colour change` | | | `Bubbles of gas` | | | `A precipitate` | | | `Heat or light given out, or getting colder` | | | `A new smell` | | ::: :::teacher-note **Answer:** A colour change → E. The new substance has a different colour from the starting ones; Bubbles of gas → C. A new substance that is a gas has been made; A precipitate → A. A new substance has formed that doesn't dissolve in the liquid; Heat or light given out, or getting colder → B. Atoms rearranging into new particles gives out or takes in energy; A new smell → D. Particles of a new substance are reaching your nose Each sign is a property of a new substance (colour, state, whether it dissolves, smell) or the energy of atoms rearranging. A physical change can show the same sign, which is why a sign is a clue, not proof. ::: :::typical ## Elaborate: melting gives signs too :::keep-together ### Not a reaction **As the water boils, it bubbles. Why isn't this a chemical change?** ☐ Because it can be reversed ☐ The particles are still water: no new substance forms ☐ Because no light is given out ::: :::teacher-note **Answer:** The particles are still water: no new substance forms The same particles, more spread out. Bubbles are a clue that misleads here. ::: ::: :::wider ## Elaborate: a precipitate :::keep-together **Two clear solutions are mixed and the liquid goes cloudy with a fine yellow solid that settles. What does this suggest, and how could you check?** ☐ A new insoluble substance (a precipitate) formed; filter it out and test its properties ☐ One of the solutions has frozen ☐ Nothing new: the two solutions have just mixed, like two paints ::: :::teacher-note **Answer:** A new insoluble substance (a precipitate) formed; filter it out and test its properties A solid forming from two clear solutions is a strong sign of a new substance. Filtering it out and testing it is how a chemist would confirm it. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - Signs of a chemical change: a colour change, a gas given off, a precipitate, light, a smell, a temperature change. - Each sign is a **clue**, not proof: boiling water bubbles and paint mixing changes colour, with no new substance. - The **test** is whether a new substance formed, with new properties. ## Exit ticket :::keep-together **Bicarb and vinegar fizz and the mixture gets colder. Name two signs of a chemical change, and say what would prove it.** {{lines-3}} ::: :::teacher-note **Answer:** Gas given off (fizzing) and a temperature change. A new substance would prove it (carbon dioxide, which turns limewater cloudy). :::