---
title: "How do you balance an equation?"
subtitle: "Years 9 and 10 Chemistry · Equations and conservation of mass · Lesson 8"
---

# How do you balance an equation?

**Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}}

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


:::teacher-note

**In one sentence:** 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).

**Students should be able to say:** 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.

**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.

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## Engage


:::keep-together

**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

My prediction, and why:

{{lines-3}}

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:::teacher-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.

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## Explore

Try it on the board in the tutorial. Was your prediction right? What was different?

{{lines-2}}

## Explain: why only the coefficients


:::keep-together

### 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

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:::teacher-note

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

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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:::keep-together

### Counting atoms

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

Answer: {{field-15}}

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:::teacher-note

**Answer:** 4

Each H₂O has 2 hydrogen atoms; two of them have 4.

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:::keep-together

### Oxygen in methane

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

Answer: {{field-15}}

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:::teacher-note

**Answer:** 4

CO₂ has 2 oxygen atoms; 2 H₂O have 2 more: 4 in all, matching 2 O₂ on the left.

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:::keep-together

### 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

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:::teacher-note

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

Same atoms in, same atoms out, so the same total mass.

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## three more


:::wider

## Elaborate: the heavier ash


:::keep-together

**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

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:::teacher-note

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

In an open system, gases can enter or leave. Weigh everything, including the oxygen used, and the mass is the same before and after.

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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.

## Exit ticket

:::keep-together

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

{{lines-3}}

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:::teacher-note

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

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