Balanced Equations | GCSE Chemistry

Quantitative chemistry

Lesson overview

This lesson introduces the core chemistry idea, the useful equipment and the calculation or data skills used on this page.

Focusbalanced equations, conservation of mass and reacting masses

Time45-60 minutes

EquipmentPeriodic table, calculator and notebook.

Practical linkmass changes in practical reactions need interpretation

Maths tagsbalancing, ratios and mass conservation

What you will learn

Describe the key chemistry ideas behind balanced equations and reacting masses.
Use precise GCSE command-word language in explanations.
Apply the idea to unfamiliar substances, data or practical contexts.
Check answers using units, state symbols, observations or balanced equations where relevant.

Core knowledge

Equations are balanced because atoms are conserved in chemical reactions.
Do not change the small numbers in formulae; change only coefficients in front.
Balanced equations show mole ratios, not mass ratios.
Reacting-mass calculations use the balanced ratio, then convert between moles and mass.
Measured mass can change in open systems if a gas leaves or enters.

Balanced equations and reacting masses infographic

Illustrated GCSE Chemistry infographic showing conservation of mass, balanced equations, atom counts, coefficients, reacting mass steps and answer checks. — Use this infographic to connect conservation of mass with atom counting, coefficients, reacting mass calculations and unit checks.Download visual

Balanced Equations practice set

Use the worked examples and practice questions on this page as a complete study task: learn the coefficient and conservation of mass meanings, summarise the infographic in your own words, then answer the questions using the data, equations and observations given here. Check every answer for balancing, ratios and mass conservation.

Clear explanation

Balanced equations and reacting masses is built around balanced equations, conservation of mass and reacting masses. Start by learning what coefficient and conservation of mass mean, then connect them to the sequence in the infographic.

The key GCSE skill is to move from evidence to explanation: name the observation, data trend, particle model, structure or reaction, then explain why it supports the conclusion.

For calculation or data work, show balancing, ratios and mass conservation. For written explanations, include the specific chemistry reason rather than a memorised topic sentence.

Worked examples

Core example: methane combustion

Start: CH4 + O2 -> CO2 + H2O.

Carbon is balanced.

Put 2 before H2O to balance hydrogen.

Products now contain 4 oxygen atoms, so put 2 before O2.

Answer: CH4 + 2O2 -> CO2 + 2H2O.

Extension example: magnesium oxide mass

Equation: 2Mg + O2 -> 2MgO.

Mole ratio Mg:MgO is 1:1.

0.20 mol Mg makes 0.20 mol MgO.

Mr MgO = 40, so mass = 0.20 x 40 = 8.0 g.

Answer: 0.20 mol magnesium makes 8.0 g magnesium oxide.

Quick checks

Choose an answer, then check your thinking.

1. Why must equations be balanced?

Products must be heavier All reactions release gas Atoms are conserved

2. What can you change when balancing equations?

Coefficients Formulae Element symbols

Practice questions

Question 1

Balance: H2 + O2 -> H2O.

Reveal answer and marking guidance

Answer: 2H2 + O2 -> 2H2O.

Marking: Credit correct coefficients.

Question 2

Balance: Al + O2 -> Al2O3.

Reveal answer and marking guidance

Answer: 4Al + 3O2 -> 2Al2O3.

Marking: Credit correct atom counts.

Question 3

Why does mass seem to decrease when calcium carbonate reacts with acid in an open flask?

Reveal answer and marking guidance

Answer: Carbon dioxide gas escapes, so measured mass decreases even though atoms are conserved.

Marking: Credit gas escape and conservation.

Question 4

Stretch: In N2 + 3H2 -> 2NH3, how many mol NH3 form from 6 mol H2?

Reveal answer and marking guidance

Answer: 3 mol H2 makes 2 mol NH3, so 6 mol H2 makes 4 mol NH3.

Marking: Credit the 3:2 ratio.

Practice ladder

FluencyRecall the key definition, symbol, structure, equation or observation.

ApplicationApply balanced equations and reacting masses to unfamiliar substances, practicals or data.

Practical interpretationUse evidence, method quality, uncertainty or conclusion wording where asked to evaluate.

Maths skillUse units, ratios, graphs and significant figures accurately.

Answers and marking guidance

The exact practice answers are hidden under each question so you can try first. Marks come from using the correct chemistry model, choosing the right equation where needed, keeping units with values, and explaining changes with precise words such as particles, ions, bonds, energy, rate, evidence and uncertainty.

Common mistakes

Changing formulae to balance equations.
Treating mole ratios as mass ratios.
Ignoring gases in mass-change questions.
Leaving an equation unbalanced before calculating.

Extension challenge

Create a six-line revision card for balanced equations and reacting masses: two definitions, one infographic detail or equation line, one equation or data check, one common mistake, and one complete explanation sentence using coefficient, conservation of mass, limiting reactant.

Reveal answer

Example answer: A complete response names the chemistry model, uses accurate units or observations, and explains why the evidence supports the conclusion.