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# Where do the atoms go?

Years 9 and 10 Chemistry · Equations and conservation of mass

<!-- Speaker note: Students model a reaction as atoms rearranging, count atoms on both sides, and connect the unchanged atoms to conservation of mass (AC9S9U07, node 2, a hinge node). About 25 minutes (Engage and Explore 6 min · Explain 8 min · Practise 5 min · Evaluate 3 min · Typical adds 4 min, Wider about 4 min). -->

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## Today

**We are learning to** in a chemical reaction atoms are rearranged into new particles, never made or destroyed. Counting atoms before and after shows why mass is conserved and why equations must balance.

- I can use a model to show a reaction with the same atoms before and after.
- I can count the atoms on each side of an equation and check they match.
- I can explain that atoms are rearranged, not made or destroyed, so mass is conserved.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace a closed-system mass experiment. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge risk is balancing as a procedure without meaning. Keep returning to the atom tally on the board: balancing describes what the atoms actually do. Model note: in the simulation, CH₄ + 2O₂ happens as one event, taking the second oxygen from nearby. Real combustion goes through several quick steps; the atoms and energy are the same. -->

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## Why it matters

> A log burns down to a handful of ash. Where did the rest of the log go?

<!-- Speaker note: You know a chemical change makes a new substance. This lesson follows 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

**Methane (natural gas, CH₄) burns in a gas stove. What happens to the carbon and hydrogen atoms in the methane?**

- They turn into heat and light
- They float away as smoke, still as methane
- They join oxygen atoms to make new particles
- They are destroyed by the flame

<!-- Speaker note: What happens: They join oxygen atoms: carbon ends up in carbon dioxide (CO₂) and hydrogen in water (H₂O). Every atom is still there. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: ask who chose 'turn into heat and light'. The tally answers them. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Frearranging-atoms%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+1.5%0Arelease%3A+1%0Acaption%3A+Methane+burning+in+oxygen.%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/rearranging-atoms/?full=step-2#step-2)

<!-- Speaker note: Watch the line under the counts: **atoms at the start** and **now**. Do the numbers ever change? -->

<!-- Speaker note: Teacher: pause the board at any moment and count one atom type on screen against the panel. -->

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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?

* They join oxygen atoms: carbon ends up in carbon dioxide (CO₂) and hydrogen in water (H₂O). Every atom is still there.

<!-- 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>

## The atom count

**What happened to the number of each type of atom during the reaction?**

- It stayed the same
- It went up as new particles formed
- It went down as the methane was used up

* Answer: It stayed the same

<!-- Speaker note: Particles change; atoms don't. Bonds break and new bonds form. -->

<!-- Speaker note: Teacher: the two number checks are the counting skill; the last question links it to mass. -->

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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>

## Count one side

**In CH₄ + 2O₂ → CO₂ + 2H₂O, how many oxygen atoms are on the left side?**

* Answer: 4

<!-- Speaker note: 2O₂ is two oxygen particles, each with 2 atoms: 4 oxygen atoms. -->

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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>

## Count the other side

**How many oxygen atoms are on the right side?**

* Answer: 4

<!-- Speaker note: CO₂ has 2, and 2H₂O has 2 (one in each water): 4. -->

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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 mass is conserved

**Why must the mass of the products equal the mass of the reactants?**

- The same atoms are there, just rearranged, and each atom keeps its mass
- The products are the same substances as the reactants
- Energy is conserved

* Answer: The same atoms are there, just rearranged, and each atom keeps its mass

<!-- Speaker note: Same atoms, same mass. This is the law of conservation of mass. -->

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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>

## Balanced or not

**Is each equation balanced (the same atoms on both sides)?**

Groups: Balanced · Not balanced

H₂ + Cl₂ → 2HCl · H₂ + O₂ → H₂O · 2H₂ + O₂ → 2H₂O · C + O₂ → CO₂ · N₂ + H₂ → NH₃ · CH₄ + O₂ → CO₂ + H₂O

* Answer: H₂ + Cl₂ → 2HCl → Balanced; H₂ + O₂ → H₂O → Not balanced; 2H₂ + O₂ → 2H₂O → Balanced; C + O₂ → CO₂ → Balanced; N₂ + H₂ → NH₃ → Not balanced; CH₄ + O₂ → CO₂ + H₂O → Not balanced

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the H₂O₂ question is the commonest balancing error; discuss it aloud. -->

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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>

## Changing the formula

**To balance H₂ + O₂ → H₂O, a student changes it to H₂ + O₂ → H₂O₂. What is wrong?**

- Nothing: it balances now
- You can't put a 2 after O
- H₂O₂ is a different substance (hydrogen peroxide), not water

* Answer: H₂O₂ is a different substance (hydrogen peroxide), not water

<!-- Speaker note: Balance by changing how many particles (the big numbers in front), never the formulas: 2H₂ + O₂ → 2H₂O. -->

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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: making water

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Frearranging-atoms%2Fstep-5&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+and+oxygen+making+water.%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/rearranging-atoms/?full=step-5#step-5)

<!-- Speaker note: What this step is for: A second reaction, with a ratio to reason about, extends counting to predicting how much product forms. -->

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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>

## Predict the amount

**In 2H₂ + O₂ → 2H₂O, 12 hydrogen particles react with 6 oxygen particles. How many water particles can form?**

* Answer: 12

<!-- Speaker note: Every 2 H₂ and 1 O₂ make 2 H₂O. Six times over: 12 H₂O, with 24 H and 12 O atoms, the same as at the start. -->

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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>

## An open fire

**Wood burned on an open fire leaves ash that weighs much less than the wood. Does that break conservation of mass?**

- Yes: the energy given out had mass
- Yes: the burnt atoms were destroyed
- No: carbon dioxide and water vapour went into the air, carrying atoms away

* Answer: No: carbon dioxide and water vapour went into the air, carrying atoms away

<!-- Speaker note: In an open container, gaseous products escape. In a sealed one, the mass doesn't change. -->

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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>

## A rusting nail

**An iron nail gets heavier as it rusts. Where does the extra mass come from?**

- Water makes it heavier
- Rust is a heavier kind of iron
- Oxygen atoms from the air join the iron

* Answer: Oxygen atoms from the air join the iron

<!-- Speaker note: Iron + oxygen → iron oxide. The mass gained is the mass of the oxygen atoms. -->

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## Evaluate: look back

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

**Revision summary**

- In a chemical reaction, **atoms are rearranged**: bonds break and new bonds form. No atom is made or destroyed.
- The same atoms means the **same mass**: the law of **conservation of mass**.
- A **balanced equation** has the same number of each type of atom on both sides. Balance it by changing the numbers in front, never the formulas.
- Mass seems to change only when a gas escapes or joins from the air.

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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

**When charcoal burns in a sealed jar, does the total mass change? Why?**

* No: the carbon and oxygen atoms are rearranged into carbon dioxide, which stays in the jar. Same atoms, same mass.

<!-- 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: balanced symbolic equations (node 4) and conservation of mass with evidence (node 5). In class: steel wool burned on a balance (mass goes up), and vinegar with baking soda in a sealed bottle with a balloon, weighed before and after. -->