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How To Write A Chemical Equation

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

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • interpret chemical equations with state symbols
  • construct chemical equations, with state symbols, including ionic equations.

Balancing equations is not a “math trick”. It is conservation of atoms. If your equation changes the number of atoms, it is wrong.

1. Definition

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).

2. Key Ideas

  • 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 formulas.
  • Balanced equation means atoms match, not “charges match” (charges matter for ionic equations; different lesson).
  • Use the smallest whole-number coefficients (simplest ratio).
  • Add state symbols when required: (s), (l), (g), (aq).

3. Detailed Explanations

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, Cl₂, Br₂, I₂.
  • Add correct state symbols when asked: (s), (l), (g), (aq).

A. Parts of a Chemical Equation

  • 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, e.g. H₂O(l).

B. How to Write an Equation (Reliable Method)

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

C. Balancing Rules (Non‑Negotiable)

  • 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.

D. State Symbols (When to Use Them)

State symbols describe the physical state:

  • (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?.

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 atom counts, then add state symbols2Fe(s) + 3Cl₂(g) → 2FeCl₃(s)
Write correct formulas first, then balance with coefficients only and add state symbols to the final equation.

4. Common Mistakes

  • 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.

5. Exam Tips

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.

6. Worked Examples

Modelled example 1

Balancing by Inspection (Iron(III) chloride)

Core

Problem

Balance the formation of iron(III) chloride from iron and chlorine gas, including state symbols.
Study the worked solution

  1. Write correct formulas

    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 physical states.

    Reason

    This conserves two iron atoms and six chlorine atoms.

    Working

    2Fe(s) + 3Cl₂(g) → 2FeCl₃(s)

Guided practice 2

Combustion (Methane)

About 6 min

Problem

Balance methane combustion and add state symbols: 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. Add state symbols

    Working

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

Common misconception 3

Simplest Ratio Check

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

State Symbols (Aqueous vs Solid)

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 formulas

    1 mark

    Method

    Translate each named substance without changing its identity.

    Reason

    Correct formulas 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

Complex Balancing (Keep Polyatomic Ions 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.

7. Mind Stretchers

Mind stretcher 1: Correct the Wrong “Balanced” EquationExtension

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 Diatomic Elements CorrectExtension

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

8. Quiz

Quiz time

Ready to check your understanding? Try the interactive quiz, then review any questions you missed.

Go to quiz