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To balance an equation, count the atoms of each element on both sides and change the coefficients until they match. Free notes and 12 questions answered.
Balancing Chemical Equations is part of Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations in GCSE AQA Higher Chemistry.
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To balance an equation, count the atoms of each element on both sides and change the coefficients until they match. Free notes and 12 questions answered.
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Use this lesson to stay inside Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations while connecting the detail back to the wider Topic 3: Quantitative chemistry topic.
Read the core explanations from Balancing Chemical Equations before testing yourself from memory.
Imagine you're making sandwiches. If you start with 2 slices of bread and 1 slice of cheese, you can't magically end up with 3 slices of bread. Atoms work the same way in chemical reactions.
Chemical reactions rearrange atoms, but they never create or destroy them. This is called conservation of mass. Every atom that goes into a reaction must come out somewhere - just like every ingredient you put into cooking must end up in the final dish or the waste.
This means chemical equations must be balanced. If you have 4 hydrogen atoms on the left side of an equation, you must have exactly 4 hydrogen atoms on the right side. No exceptions.
Before we balance equations, you need to understand the two types of numbers in chemical formulae. They do completely different jobs, and mixing them up is the fastest way to get balancing wrong.
Think of it like this: $H_2O$ means one water molecule containing 2 hydrogen atoms and 1 oxygen atom. But $2H_2O$ means you have 2 water molecules, giving you 4 hydrogen atoms and 2 oxygen atoms total.
Crucial rule: You can change coefficients when balancing equations, but you must never change subscripts. Changing subscripts changes the chemical identity - $H_2O$ is water, but $H_2O_2$ is hydrogen peroxide, a completely different substance.
Balancing equations feels overwhelming at first, but there's a systematic method that works every time. Let's walk through it using a reaction students often struggle with.
Worked example: Balancing the Combustion of Methane
Balance this equation: CH₄ + O₂ → CO₂ + H₂O
Answer: CH₄ + 2O₂ → CO₂ + 2H₂O
Pro tip: Always balance oxygen last because it often appears in multiple compounds. Start with metals, then non-metals (except oxygen), then oxygen.
Some equations look more intimidating than others, but the method stays the same. Let's tackle a reaction that catches many students out because it involves polyatomic ions.
Worked example: Balancing Equations with Polyatomic Ions
Balance: Ca(OH)₂ + HCl → CaCl₂ + H₂O
Answer: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O
Trap alert: Many students forget to count all the hydrogen atoms. In Ca(OH)₂, there are 2 hydrogen atoms from the OH groups. In 2HCl, there are 2 more hydrogen atoms. That's 4 hydrogen atoms total on the left side.
Balancing equations appears in every chemistry exam, often worth 2-3 marks per question. Here's how to maximise your marks and avoid the traps examiners set.
Common exam mistakes to avoid: Don't change subscripts (changes the chemical), don't leave coefficients as fractions, and don't forget to balance all elements including hydrogen and oxygen.
Exam precision note: In Balancing Chemical Equations - Conservation of mass and balanced chemical equations - Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
Balancing equations is about making sure the number of each type of atom is the same on both sides of the equation. Coefficients change how many particles are involved, but they do not change the formula itself. This is why balancing links directly to conservation of mass.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
To balance a chemical equation, count the atoms of each element on both sides, then change the coefficients in front of the formulae until every element has the same number of atoms on each side. Balance metals first, then non-metals, and oxygen last.
The balanced equation for the combustion of methane is CH₄ + 2O₂ → CO₂ + 2H₂O. Both sides then have 1 carbon atom, 4 hydrogen atoms and 4 oxygen atoms.
A coefficient is the normal-sized number in front of a formula showing how many molecules or formula units there are, like the 2 in 2H₂O. A subscript is the small number after an element symbol showing how many atoms of that element are in one molecule.
H₂O is water, but H₂O₂ is hydrogen peroxide, a completely different substance. Changing a subscript changes the chemical identity, which is why subscripts must never be changed when balancing equations; only the coefficients in front of formulae can change.
Subscripts must never be changed when balancing because changing them changes the chemical identity of the substance. For example, H₂O is water but H₂O₂ is hydrogen peroxide, a completely different substance, so only coefficients can be changed.
To balance equations with polyatomic ions, count every atom in each group, such as both H atoms in the OH groups of Ca(OH)₂. Ca(OH)₂ + HCl → CaCl₂ + H₂O balances as Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O, with 4 hydrogen atoms on each side.
The large 2 in front of 2H₂O is a coefficient meaning there are 2 water molecules. Each H₂O molecule has 2 hydrogen atoms and 1 oxygen atom, so 2H₂O contains 4 hydrogen atoms and 2 oxygen atoms in total.
When balancing an equation, metals are balanced first, then non-metals except oxygen, and oxygen is balanced last because it often appears in multiple compounds. In CH₄ + O₂ → CO₂ + H₂O, carbon is balanced first, then hydrogen, then oxygen.
Chemical equations need to be balanced because of conservation of mass: atoms cannot be created or destroyed in chemical reactions, only rearranged. The number of each type of atom must therefore be the same on both sides, so 4 hydrogen atoms on the left means exactly 4 on the right.
The balanced equation is Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O. Two HCl are needed to give the 2 Cl atoms in CaCl₂, and two water molecules balance the 4 hydrogen atoms and 2 oxygen atoms on the left.
Coefficients should not be left as fractions in a balanced equation. The other common mistakes are changing subscripts, which changes the chemical, and not balancing all the elements, including hydrogen and oxygen.
An equation is checked by counting the atoms of each element on both sides. For CH₄ + 2O₂ → CO₂ + 2H₂O, the left has C = 1, H = 4, O = 4 and the right has C = 1, H = 4, O = 4, so it is balanced.