Mini Chemistry
Part of Mini EducationSwitch subject

Writing and balancing chemical equations

Chemical equations: write reactants → products, balance using coefficients, and add state symbols (s/l/g/aq) without changing subscripts.

  • SEC G3 Pure Chemistry 2027

Reading settings

On this page

A chemical reaction rearranges atoms into new substances. Balancing an equation records that rearrangement while keeping the number of each kind of atom the same on both sides.

What a chemical equation represents

A chemical equation is a symbolic representation of a reaction that shows the reactants, products, and their relative amounts using coefficients. A correct equation must obey the law of conservation of mass (same number of each type of atom on both sides).

Conserve atoms and preserve formulae

  • Reactants on the left, products on the right; arrow shows “forms”: reactants → products.
  • Balance equations by changing coefficients only (numbers in front), never subscripts inside formulae.
  • Conserve atoms and total charge. For equations containing only neutral substances, charge is already zero on both sides. Ionic equations also need an explicit charge check.
  • Use the smallest whole-number coefficients (simplest ratio).
  • Add state symbols when required: (s), (l), (g), (aq).

Construct the equation

Quick Recall (balancing mindset)
  • Reactants on the left, products on the right; arrow means “forms”, e.g. 2H₂ + O₂ → 2H₂O.
  • Balance by changing coefficients only (never subscripts).
  • Watch diatomic elements when they appear: H₂, O₂, N₂, F₂, Cl₂, Br₂, I₂.
  • Add correct state symbols when asked: (s), (l), (g), (aq).

Reactants, products and coefficients

  • Reactants: substances that react (left side).
  • Products: substances formed (right side).
  • Coefficient: number in front of a formula (e.g., 2 in 2H₂O).
  • State symbol: physical state or an aqueous solution, e.g. H₂O(l).

From a word equation to a symbol equation

  1. Write the word equation.
  2. Convert each substance to its correct formula (see Chemical symbols and formulae).
  3. Balance using coefficients so each type of atom matches.
  4. Add state symbols (when asked / when it helps).

Balance with coefficients

  • Do not change subscripts (e.g., do not turn H₂O into H₂O₂).
  • Balance one element at a time; leave hydrogen and oxygen for last in combustion questions.
  • Check your final coefficients are the simplest whole numbers.
Why subscripts must stay fixed

If you change a subscript, you change the substance and the equation is no longer chemically correct, even if the atom count appears to work.

State symbols depend on the conditions

State symbols describe the substances under the reaction or reporting conditions:

  • (s) solid
  • (l) liquid
  • (g) gas
  • (aq) dissolved in water (aqueous)
Recall: (aq) means solution

(aq) means the substance is dissolved in water. It is not the same as “liquid”. See What Is Matter?.

State symbols depend on temperature. Water formed in a hot flame is H₂O(g); after the products cool to room temperature, it may be recorded as H₂O(l). The atom balance stays the same. Do not choose a state only from the substance’s name.

Balancing an equation using coefficientsThe unbalanced skeleton equation Fe plus Cl2 forms FeCl3 is balanced by placing coefficients 2, 3 and 2 in front of the formulae. State symbols are then added. Subscripts never change.1 · Write the skeleton equation (not the final answer)Fe + Cl₂ → FeCl₃2 · Change coefficients only; use the simplest whole-number ratio2Fe + 3Cl₂ → 2FeCl₃3 · Check atoms; use states for cooled products2Fe(s) + 3Cl₂(g) → 2FeCl₃(s)
Write correct formulae first, balance with coefficients only, then add state symbols. Here the iron(III) chloride product is shown after cooling to room temperature.

Check the completed equation

  • Balancing by changing subscripts instead of coefficients.
  • Forgetting that Cl₂, O₂, N₂ are diatomic (common at O-Level).
  • Leaving coefficients as 2, 4, 6 when they can be simplified to 1, 2, 3.
  • Using state symbols incorrectly (e.g., writing (aq) for pure water).
  • Writing unbalanced “skeleton equations” as the final answer.

Strategies for checking balance

Always check atom counts at the end

Count each element on both sides. If any element differs, the equation is not balanced.

Combustion shortcut

For hydrocarbons: balance C, then H, then O last.

Worked examples

Modelled example 1

Balancing the formation of iron(III) chloride

Core

Problem

Balance the formation of iron(III) chloride from iron and chlorine gas, including state symbols for the products after cooling to room temperature.

Study the worked solution

  1. Write correct formulae

    Method

    Construct the skeleton equation without altering compound identities.

    Reason

    Elemental chlorine is diatomic and iron(III) chloride is FeCl₃.

    Working

    Fe + Cl₂ → FeCl₃.

  2. Balance chlorine

    Method

    Use the lowest common multiple of 2 and 3.

    Reason

    Six chlorine atoms can be represented by 3Cl₂ and 2FeCl₃.

    Working

    Fe + 3Cl₂ → 2FeCl₃.

  3. Balance iron and add states

    Method

    Place coefficient 2 before iron and label chlorine gas and the cooled solid product.

    Reason

    This conserves two iron atoms and six chlorine atoms.

    Working

    2Fe(s) + 3Cl₂(g) → 2FeCl₃(s)

Guided practice 2

Balancing methane combustion

About 6 min

Problem

Balance the complete combustion of methane. Use state symbols for products collected after cooling to room temperature: CH₄ + O₂ → CO₂ + H₂O.

Balance C, then H, then O

Hints

Hint 1: balance hydrogen before oxygen

Four H atoms on the left require two water molecules.

Hint 2: count product oxygen last

One CO₂ plus two H₂O contains four O atoms.

View solution step by step

  1. Balance carbon and hydrogen

    Method

    Keep one carbon dioxide and place 2 before water.

    Reason

    Carbon is already 1:1; two water molecules contain four H atoms.

    Working

    CH₄ + O₂ → CO₂ + 2H₂O.

  2. Balance oxygen

    Method

    Place 2 before oxygen gas.

    Reason

    The products now contain four O atoms.

    Working

    CH₄ + 2O₂ → CO₂ + 2H₂O.

  3. Use the stated reporting conditions

    Reason

    The products are cooled to room temperature, so the water is recorded as liquid rather than steam.

    Working

    CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

Common misconception 3

Check the simplest coefficient ratio

Find and correct the mistake

Learner response

A student gives 4H₂ + 2O₂ → 4H₂O as the final balanced equation. Decide what is correct, locate the remaining issue and correct it.

Check conservation and simplest ratio separately

Atom balance
Simplest equation

View solution step by step

  1. Check conservation

    Method

    Count H and O atoms on both sides.

    Reason

    Eight H atoms and four O atoms appear on each side.

    Working

    The student’s equation is balanced.

  2. Locate the ratio issue

    Method

    Find the common factor of all coefficients.

    Reason

    4, 2 and 4 share factor 2, so the ratio is not simplest.

    Working

    4:2:4÷2 = 2:1:2.

  3. Write the corrected equation

    Working

    2H₂ + O₂ → 2H₂O

Examiner practice 4

Choosing states for a metal–acid reaction

4 marks

Examination question

Write a balanced equation with state symbols for magnesium reacting with dilute hydrochloric acid to form magnesium chloride and hydrogen. [4 marks]

Construct, balance and label

View solution step by step

  1. Write correct formulae

    1 mark

    Method

    Translate each named substance without changing its identity.

    Reason

    Correct formulae are required before coefficients can conserve atoms.

    Working

    Mg + HCl → MgCl₂ + H₂.

  2. Balance coefficients

    1 mark

    Method

    Place coefficient 2 before hydrochloric acid.

    Reason

    This supplies two Cl atoms for MgCl₂ and two H atoms for H₂.

    Working

    Mg + 2HCl → MgCl₂ + H₂.

  3. Label reactants

    1 mark

    Method

    Use solid magnesium and aqueous dilute acid.

    Reason

    Magnesium is a solid metal and “dilute” hydrochloric acid is dissolved in water.

    Working

    Mg(s) and dilute HCl(aq).

  4. Label products

    1 mark

    Method

    Use aqueous magnesium chloride and gaseous hydrogen.

    Reason

    The salt remains in solution and hydrogen is released as a gas.

    Working

    Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Challenge 5

Keep unchanged nitrate groups together

Minimal support

Complex-equation transfer

Balance Pb₃O₄ + HNO₃ → PbO₂ + Pb(NO₃)₂ + H₂O using the smallest whole-number coefficients.

Balance Pb, then nitrate, then H and O

Hints

Hint 1: allocate lead first
Of the three Pb atoms, place one in PbO₂ and two in Pb(NO₃)₂.
Hint 2: preserve nitrate
Once two lead nitrate units are present, four unchanged nitrate groups are needed on the left.
View solution step by step

  1. Balance lead

    Method

    Use one PbO₂ and two Pb(NO₃)₂.

    Reason

    This accounts for all three lead atoms in Pb₃O₄.

    Working

    Pb₃O₄ + HNO₃ → PbO₂ + 2Pb(NO₃)₂ + H₂O.

  2. Balance nitrate and hydrogen

    Method

    Place 4 before HNO₃ and 2 before water.

    Reason

    Four nitrate groups and four hydrogen atoms are then conserved.

    Working

    Pb₃O₄ + 4HNO₃ → PbO₂ + 2Pb(NO₃)₂ + 2H₂O.

  3. Check oxygen and ratio

    Method

    Count oxygen atoms and inspect the coefficients.

    Reason

    Both sides contain 16 O atoms and the coefficient set has no common factor.

    Working

    Final: Pb₃O₄ + 4HNO₃ → PbO₂ + 2Pb(NO₃)₂ + 2H₂O.

Try it yourself

Mind stretcher 1: Correct the reaction productsExtension

Question: A student writes CaCO₃ → CaO + CO for thermal decomposition. Explain what is wrong and give the correct equation.

Show answer

Atom count is wrong: left has 3 oxygen atoms, right has 2 (one in CaO and one in CO).

Correct products are calcium oxide and carbon dioxide:

CaCO₃(s) → CaO(s) + CO₂(g)

Mind stretcher 2: Keep oxygen as a diatomic moleculeExtension

Question: Balance: Al + O₂ → Al₂O₃.

Show answer

Balance oxygen first by making oxygen atoms a multiple of 2 and 3 (LCM 6):

4Al + 3O₂ → 2Al₂O₃

Practise and check

Topic check

See what you know across this topic, then go back to anything you got wrong.

Take the topic check

Syllabus and review details

Last reviewed: