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# How do you balance an equation?

Years 9 and 10 Chemistry · Equations and conservation of mass

<!-- Speaker note: Students balance simple symbolic equations by changing coefficients (never subscripts), verify by counting atoms, and explain that a balanced equation shows conservation of mass (AC9S9U07, node 4, a hinge node). About 28 minutes (Engage and Explore 7 min · Explain 9 min · Practise 8 min · Evaluate 3 min · Wider about 3 min). -->

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## Today

**We are learning to** a balanced equation has the same number of each kind of atom on both sides, because atoms are only rearranged. You balance it by changing the coefficients in front of formulas, never the formulas themselves.

- I can balance a simple equation by changing only the coefficients.
- I can check an equation by counting the atoms of each element on both sides.
- I can explain why a balanced equation shows conservation of mass.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea, after Where do the atoms go? (node 2). Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge errors are systematic: changing subscripts (H₂O to H₂O₂) and miscounting atoms in a coefficient (2 H₂O as 2 hydrogen). The board makes both visible: formulas can't be edited, and every molecule is drawn, so students can count rather than compute. Board notes: formulas without brackets only; coefficients 1 to 6. The board accepts a balance only with the smallest whole numbers (4 H₂ + 2 O₂ → 4 H₂O balances but is not accepted). Word equations (node 3) and open and closed systems (node 5) are class work; the Wider step is one open-system case. -->

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## Why it matters

> Chemists write reactions in a shorthand that has to obey one rule: no atom may appear or vanish.

<!-- Speaker note: Last lesson: atoms are rearranged, never made or destroyed. This lesson writes that down as an equation. -->

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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

**Hydrogen burns in oxygen to make water: H₂ + O₂ → H₂O. There are 2 oxygen atoms on the left but only 1 on the right. What do you predict fixes it?**

- Write H₂O₂ instead of H₂O
- Nothing: equations don't need to match
- Put a 2 in front of H₂O, then a 2 in front of H₂
- Take one oxygen atom away

<!-- Speaker note: What happens: 2 H₂ + O₂ → 2 H₂O. Only the numbers in front (coefficients) may change. H₂O₂ is hydrogen peroxide, a different substance, and atoms can't be taken away. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count who chose H₂O₂. Ask them what H₂O₂ is. -->

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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>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fbalancing%2Fstep-2&code=widget%3A+equation%0Aequation%3A+H2+%2B+O2+-%3E+H2O%0A&fresh=1}}

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

<!-- Speaker note: Use **+** and **−** to change how many of each molecule there are. Watch the drawing and the atom tally: balanced means every row says "same". -->

<!-- Speaker note: Teacher: ask what changed in the drawing when they pressed + on H₂O (a whole molecule, two H and one O). -->

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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?

* 2 H₂ + O₂ → 2 H₂O. Only the numbers in front (coefficients) may change. H₂O₂ is hydrogen peroxide, a different substance, and atoms can't be taken away.

<!-- 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>

## Formulas are fixed

**Why can't you balance it by changing H₂O to H₂O₂?**

- H₂O₂ has too many atoms to draw
- You can: it balances the oxygen
- H₂O₂ is a different substance (hydrogen peroxide), not water

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

<!-- Speaker note: The small numbers (subscripts) say what the substance is. Coefficients say how many molecules there are, which is the only thing you can choose. -->

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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>

## Counting atoms

**How many hydrogen atoms are in 2 H₂O?**

* Answer: 4

<!-- Speaker note: Each H₂O has 2 hydrogen atoms; two of them have 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>

## Oxygen in methane

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

* Answer: 4

<!-- Speaker note: CO₂ has 2 oxygen atoms; 2 H₂O have 2 more: 4 in all, matching 2 O₂ on the left. -->

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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>

## Conservation of mass

**Why does a balanced equation show that mass is conserved?**

- The same molecules are on both sides
- The same atoms are on both sides, and each atom keeps its mass
- Products always weigh more than reactants

* Answer: The same atoms are on both sides, and each atom keeps its mass

<!-- Speaker note: Same atoms in, same atoms out, so the same total mass. -->

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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>

## three more

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fbalancing%2Fstep-4&code=widget%3A+equation%0Aequation%3A+CH4+%2B+O2+-%3E+CO2+%2B+H2O%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/balancing/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Balancing is a precision skill (practise here): three equations with different starting moves build fluency with the same check each time. -->

<!-- Speaker note: Teacher: the three equations are the node's evidence; methane is the hardest, so give the 'atom that appears once' hint 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>

## Elaborate: the heavier ash

**Magnesium ribbon burns in an open crucible, and the white ash weighs more than the ribbon did. Has mass been created?**

- No: the scale must be wrong
- No: oxygen atoms from the air joined the magnesium (2 Mg + O₂ → 2 MgO), so the product includes their mass
- Yes: burning makes extra mass

* Answer: No: oxygen atoms from the air joined the magnesium (2 Mg + O₂ → 2 MgO), so the product includes their mass

<!-- Speaker note: In an open system, gases can enter or leave. Weigh everything, including the oxygen used, and the mass is the same before and after. What this step is for: An open-system result that seems to break conservation of mass is explained by the balanced equation: oxygen came in from the air. -->

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## Evaluate: look back

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

**Revision summary**

- A **balanced equation** has the same number of each kind of atom on both sides.
- Balance by changing **coefficients** (the numbers in front). Never change **subscripts**: they define the substance.
- Use the smallest whole numbers, and check by counting atoms of each element.
- A balanced equation shows **conservation of mass**: the same atoms, so the same 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>

## Exit ticket

**Balance: H₂ + Cl₂ → HCl. How do you know it's balanced?**

* H₂ + Cl₂ → 2 HCl. Two hydrogen and two chlorine atoms on each side.

<!-- 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: Before: Where do the atoms go? (node 2). Node 5 (conservation of mass in open and closed systems) is a class task: weigh a reaction in a sealed bag, then open it. In class: molecular model kits to build both sides of 2 H₂ + O₂ → 2 H₂O. -->