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# How can you tell a reaction happened?

Year 8 Chemistry · Chemical change

<!-- Speaker note: 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). About 22 minutes (Engage and Explore 7 min · Explain 9 min · Evaluate 3 min · Typical adds 3 min, Wider about 2 min). -->

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## Today

**We are learning to** you can't see atoms rearrange, but you can see the signs: a colour change, a gas given off, a solid forming in a liquid, light, a smell, or a temperature change. Each is a clue that a new substance formed, not proof: boiling water bubbles too. The test is still a new substance.

- 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.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it is best paired with a circus of quick reactions (vinegar and bicarb, lead-free precipitates, steel wool burning). Set the width with `?width=minimum`, `typical` (default) or `wider`. It follows Is it a new substance? (node 2) and comes before Hotter or colder? (nodes 4 and 5). Not a hinge node. Students tend to treat any one sign as proof; the prediction and the "clue or trap" sort target it. Board notes: the board can't show colour, smell or a precipitate; it shows new particles and the temperature change. The dissolve board shows a case with no sign at all. Teacher: In your own words: five signs, each with a misleading example where possible. -->

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## Why it matters

> 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?

<!-- Speaker note: Last lesson: a chemical change makes a new substance. This lesson: how to spot one without seeing the atoms. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**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

<!-- Speaker 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. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. Any single sign chosen means 'sign = proof'. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fsigns%2Fstep-2&code=widget%3A+reaction%0Ashow%3A+CH4+x6%2C+O2+x12%0Areact%3A+CH4+%2B+2+O2+-%3E+CO2+%2B+2+H2O%0Aea%3A+1.8%0Arelease%3A+1%0Acaption%3A+Methane+burning.+Which+signs+of+a+reaction+can+you+see+in+the+panel%3F%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-8/signs/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask which observation from the burning board would also happen without a reaction. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* 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.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Signs

**Is it a sign of a chemical change?**

Groups: A sign · Not a sign

A colour change when two substances mix · Bubbles of a new gas · A solid forming when two clear liquids mix (a precipitate) · Light or heat given out, or the mixture getting colder · A new smell · The substance changing shape

* 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

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<!-- Speaker note: Teacher: the two sorts are the success criteria. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Clue or trap

**Chemical change, or a sign that misleads?**

Groups: Chemical change · Misleading sign: physical change

A cake browning in the oven · Vinegar and bicarb fizzing · Water boiling and bubbling · Blue and yellow paint mixing to green · A nail rusting orange · A hot drink cooling down

* 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

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: The white powder: a new substance with different properties from the shiny metal

<!-- Speaker note: A new substance with new properties is the test; the light and heat are signs that go with it. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Why each is a clue

**Why does each sign suggest a new substance has formed?**

| | |
|---|---|
| `A colour change` | A. A new substance has formed that doesn't dissolve in the liquid |
| `Bubbles of gas` | B. Atoms rearranging into new particles gives out or takes in energy |
| `A precipitate` | C. A new substance that is a gas has been made |
| `Heat or light given out, or getting colder` | D. Particles of a new substance are reaching your nose |
| `A new smell` | E. The new substance has a different colour from the starting ones |

* 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

<!-- Speaker note: 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. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: melting gives signs too

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fsigns%2Fstep-4&code=widget%3A+heat%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-8/signs/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Melting and boiling show a temperature change and bubbles with no new substance, the clearest misleading signs. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: The particles are still water: no new substance forms

<!-- Speaker note: The same particles, more spread out. Bubbles are a clue that misleads here. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: a precipitate

**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

* Answer: A new insoluble substance (a precipitate) formed; filter it out and test its properties

<!-- Speaker note: 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. What this step is for: A precipitate is a sign students meet less often; reasoning about it with particles checks the idea transfers. -->

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## 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.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Bicarb and vinegar fizz and the mixture gets colder. Name two signs of a chemical change, and say what would prove it.**

* Gas given off (fizzing) and a temperature change. A new substance would prove it (carbon dioxide, which turns limewater cloudy).

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: Hotter or colder? (exothermic and endothermic, nodes 4 and 5). Comparing physical and chemical changes with all the criteria (node 6) is a class task. In class: a reaction circus with an observation table (what you saw, which sign, new substance?). -->