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# Hotter or colder?

Year 8 Chemistry · Chemical change

<!-- Speaker note: Students explain exothermic and endothermic reactions as energy released to or absorbed from the surroundings, and classify everyday reactions (AC9S8U07, nodes 4 and 5). About 24 minutes (Engage and Explore 8 min · Explain 9 min · Evaluate 3 min · Typical adds 3 min, Wider about 2 min). -->

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## Today

**We are learning to** some reactions give out energy to their surroundings, so the surroundings get hotter: they are exothermic (burning, respiration, hand warmers). Others take in energy, so the surroundings get colder: they are endothermic (instant ice packs, photosynthesis, cooking).

- I can explain what exothermic and endothermic mean.
- I can say which way energy moves in each, between the reaction and its surroundings.
- I can give an everyday example of each.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it is best beside a hands-on test with a thermometer (citric acid and bicarb gets colder; magnesium in acid gets hotter). Set the width with `?width=minimum`, `typical` (default) or `wider`. One lesson for two nodes, after How can you tell a reaction happened? Neither is a hinge node. The misconception: cold is a thing that flows ("cold energy"). The prediction targets it. Board notes: the reaction board warms or cools the gas as reactions give out or take in energy. The endothermic reaction (nitrogen + oxygen → nitrogen monoxide) is real but only happens at engine temperatures; it starts at 600 K and cools. Both reactions stop at their target. Teacher: In your own words: the two thermometers with energy arrows are the evidence. -->

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## Why it matters

> Snap an instant ice pack and it goes freezing cold. Shake a hand warmer and it gets hot. Neither is plugged in. Where does the heat go, and where does it come from?

<!-- Speaker note: A temperature change is one sign of a chemical change. This lesson asks which way, and why. -->

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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

**An instant ice pack contains two chemicals that react when you snap it. It goes cold. What is happening?**

- The reaction gives out energy, so the pack cools down
- The reaction takes in energy from the pack and your hand
- The chemicals were stored in a freezer
- The reaction makes 'cold', which flows into your hand

<!-- Speaker note: What happens: The reaction takes in energy from its surroundings (the pack, the air, your hand), so they get colder. A reaction that takes in energy is endothermic. There is no 'cold energy': cold just means less thermal energy. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. 'Makes cold' is the one to discuss. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fenergy-change%2Fstep-2&code=widget%3A+reaction%0Ashow%3A+H2+x12%2C+O2+x6%0Areact%3A+2+H2+%2B+O2+-%3E+2+H2O%0Aea%3A+1.5%0Arelease%3A+1%0Acaption%3A+Hydrogen+burning+in+oxygen.+Watch+the+temperature.%0A}}

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

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask which way the temperature moved on each board, and what that says about the energy. -->

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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?

* The reaction takes in energy from its surroundings (the pack, the air, your hand), so they get colder. A reaction that takes in energy is endothermic. There is no 'cold energy': cold just means less thermal energy.

<!-- 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>

## Energy out

**In an exothermic reaction, where does the energy go?**

- Into the reaction mixture, so it gets colder
- Out to the surroundings, so they get hotter
- It is destroyed

* Answer: Out to the surroundings, so they get hotter

<!-- Speaker note: Exo = out. Energy stored in the reactants is released to the surroundings. -->

<!-- Speaker note: Teacher: the definition and the sort 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>

## Sort them

**Exothermic or endothermic?**

Groups: Exothermic (gets hotter) · Endothermic (gets colder)

Wood burning · Respiration in your cells · A hand warmer (iron rusting quickly) · An instant ice pack · Photosynthesis (taking in light energy) · Bicarb soda reacting with vinegar and fizzing

* Answer: Wood burning → Exothermic (gets hotter); Respiration in your cells → Exothermic (gets hotter); A hand warmer (iron rusting quickly) → Exothermic (gets hotter); An instant ice pack → Endothermic (gets colder); Photosynthesis (taking in light energy) → Endothermic (gets colder); Bicarb soda reacting with vinegar and fizzing → Endothermic (gets colder)

<!-- 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>

## Stopping on its own

**On the board, the nitrogen and oxygen only reacted at 600 K, and the temperature fell. Why might an endothermic reaction stop on its own?**

- It runs out of atoms
- It takes in energy and cools its surroundings, so fewer collisions have enough energy to react
- Endothermic reactions are always slow

* Answer: It takes in energy and cools its surroundings, so fewer collisions have enough energy to react

<!-- Speaker note: Unless energy keeps coming in (a hot engine, sunlight for photosynthesis), an endothermic reaction cools itself down. -->

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## Where the energy comes from

**Burning gas in a stove gives out a lot of energy. Where was that energy before?**

- Stored in the gas and oxygen (chemical energy), released as their atoms rearrange into new particles
- In the match or spark that lit it
- In the air around the flame
- Nowhere: burning makes new energy

* Answer: Stored in the gas and oxygen (chemical energy), released as their atoms rearrange into new particles

<!-- Speaker note: A fuel and oxygen together store chemical energy. In an exothermic reaction their atoms rearrange into new particles that store less, and the difference is given out to the surroundings. -->

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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: hand warmers and ice packs

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fenergy-change%2Fstep-4&code=widget%3A+reaction%0Ashow%3A+H2+x12%2C+O2+x6%0Areact%3A+2+H2+%2B+O2+-%3E+2+H2O%0Aea%3A+1.5%0Arelease%3A+1%0Acaption%3A+Hydrogen+burning+in+oxygen.+Watch+the+temperature.%0A}}

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

<!-- Speaker note: What this step is for: Matching a reaction's energy direction to a product's job turns the idea into design reasoning. -->

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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>

## Which product

**A sports trainer needs something to reduce swelling on a sprained ankle, at a game far from a freezer. Which product, and why?**

- A hand warmer: its reaction is exothermic
- Either: both change temperature
- An instant ice pack: its endothermic reaction takes energy from the ankle

* Answer: An instant ice pack: its endothermic reaction takes energy from the ankle

<!-- Speaker note: Endothermic for cooling, exothermic for warming: the direction of energy transfer is the design choice. -->

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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: why burning keeps going

**Wood is exothermic when it burns, yet a log doesn't catch fire on its own. You need a match. Why, and why does it keep burning once lit?**

- The match adds fuel
- Wood is endothermic until it is hot
- Reactions need enough energy to start; once lit, the energy given out heats the next bit enough to react

* Answer: Reactions need enough energy to start; once lit, the energy given out heats the next bit enough to react

<!-- Speaker note: Collisions need enough energy to react (activation energy, Year 10). An exothermic reaction supplies that energy to its own next steps, so it keeps going. What this step is for: Activation energy explains why exothermic reactions need a start (a match) and then keep themselves going. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- **Exothermic** reactions give out energy to their surroundings, so the surroundings get hotter: burning, respiration, hand warmers.
- **Endothermic** reactions take in energy from their surroundings, so the surroundings get colder: instant ice packs, photosynthesis, bicarb and vinegar.
- Energy is never made or destroyed; it moves between the reaction and its surroundings.
- There is no "cold energy": colder means less thermal energy.

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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

**Burning candle wax: exothermic or endothermic? Explain using the word surroundings.**

* Exothermic: the reaction gives out energy to the surroundings, which get hotter (and light is given out).

<!-- 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: comparing physical and chemical changes with every criterion (node 6) is a class task. Year 9 energy conservation and Year 10 collision theory build on this. In class: thermometer readings for citric acid + bicarb (endothermic) and magnesium + dilute acid (exothermic). -->