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How do we write and balance chemical equations?

Writing chemical formulae from ions, constructing word and balanced symbol equations with state symbols, and writing simple ionic equations for neutralisation and precipitation.

A CCEA GCSE Chemistry answer on chemical formulae and equations, covering how to build formulae from ion charges, write word equations, balance symbol equations with state symbols, and write simple ionic equations for neutralisation and precipitation reactions.

Generated by Claude Opus 4.89 min answer

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  1. What this dot point is asking
  2. Writing formulae from ions
  3. Word and symbol equations
  4. Ionic equations
  5. Worked example
  6. Examples in context
  7. Try this

What this dot point is asking

CCEA wants you to write chemical formulae by balancing ion charges, turn reactions into word equations and then balanced symbol equations with state symbols, and write simple ionic equations for neutralisation and precipitation.

Writing formulae from ions

The quick method ("swap and drop") is to use each ion's charge as the subscript of the other ion, then simplify. So Al3+\text{Al}^{3+} and O2−\text{O}^{2-} give Al2O3\text{Al}_2\text{O}_3. Brackets are used for compound ions present more than once, such as Ca(OH)2\text{Ca(OH)}_2.

Word and symbol equations

A word equation lists reactants and products by name, for example: magnesium + oxygen gives magnesium oxide. A symbol equation replaces the names with formulae and must be balanced: the same number of each type of atom on both sides, because atoms are neither created nor destroyed (conservation of mass).

To balance, only change the big numbers (coefficients) in front of formulae, never the small subscripts inside a formula, because changing a subscript changes the substance.

Ionic equations

Ionic equations are powerful because they capture the essential reaction: every acid-alkali neutralisation is really just hydrogen ions joining hydroxide ions to make water, regardless of which acid and alkali were used.

Worked example

Examples in context

Example 1. Predicting product masses. A balanced equation tells a manufacturer exactly how much product a given mass of reactant should yield, because the coefficients give the reacting ratio. This is the foundation of all the reacting-mass calculations used in industry.

Example 2. Spotting spectator ions. When silver nitrate is added to sodium chloride, the ionic equation shows that only silver and chloride ions react to form the precipitate, while sodium and nitrate are spectators. Writing the ionic equation makes clear what the test is actually detecting.

Try this

Q1. Write the formula of calcium chloride from Ca2+\text{Ca}^{2+} and Cl−\text{Cl}^-. [1 mark]

  • Cue. CaCl2\text{CaCl}_2.

Q2. Write the ionic equation for neutralisation. [1 mark]

  • Cue. H++OH−→H2O\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}.

Exam-style practice questions

Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

CCEA 20193 marksBalance the following equation for the combustion of methane: CH4 + O2 gives CO2 + H2O. Show the balanced equation including state symbols.
Show worked answer →

Markers want a correctly balanced equation with state symbols.

Start by counting atoms. Methane combustion makes carbon dioxide and water:

CH4+O2→CO2+H2O\text{CH}_4 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}

Carbon is balanced (1 each). Hydrogen: 4 on the left, so we need 2 water molecules. Then oxygen: CO2\text{CO}_2 has 2 and 2H2O2\text{H}_2\text{O} has 2, giving 4 oxygen atoms, so we need 2O22\text{O}_2.

The balanced equation with state symbols is:

CH4(g)+2O2(g)→CO2(g)+2H2O(l)\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(l)

Markers reward the balancing numbers 2 in front of oxygen and water, and correct state symbols (gases and liquid water).

CCEA 20213 marksWrite the formula of aluminium oxide, given aluminium forms Al3+ ions and oxygen forms O2- ions, and explain how you worked it out.
Show worked answer →

The marks are for balancing the charges and the resulting formula.

The compound must be neutral overall, so the total positive charge must equal the total negative charge. Aluminium is +3 and oxide is -2.

The lowest whole numbers that balance are two aluminium ions (2 times +3 equals +6) and three oxide ions (3 times -2 equals -6), which cancel.

So the formula is Al2O3\text{Al}_2\text{O}_3.

Markers reward balancing +3 against -2 to give 2 aluminium and 3 oxygen, and the formula Al2O3.

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