Mini Chemistry
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Chemical Formulae and Equations

Write formulae, balance equations without changing formulae, add state symbols and form ionic equations by cancelling spectators.

  • SEC G3 Combined Science Chemistry component 2027
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Learning objectives

  • state the symbols of the elements and formulae of the compounds mentioned in the syllabus
  • deduce the formulae of simple compounds from the relative numbers of atoms present and vice versa
  • deduce the formulae of ionic compounds from the charges on the ions present and vice versa
  • interpret chemical equations with state symbols
  • construct chemical equations, with state symbols, including ionic equations.

1. Outcome and prerequisites

By the end, you should be able to write a formula from names or ion charges, balance an equation without changing any substance formula, add state symbols, and cancel spectator ions to form an ionic equation.

The equation is the chemical relationship for every later calculation. If it is wrong, a perfectly executed numerical method still gives the wrong answer.

Keep formulae fixed

A subscript belongs to a substance’s identity. Balance the number of formula units with coefficients; never change a subscript to make the atom count fit.

For an ionic compound, combine ions so that total positive and negative charge is zero. For example, Al³⁺ and O²⁻ form Al₂O₃ because 2(+3) + 3(-2) = 0.

2. Modelled example

Modelled example 1

Establish the full and ionic equations

Core

Problem

Magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen. Write the balanced equation with state symbols, then form the ionic equation.

View solution step by step

  1. Write correct substance formulae

    Method

    Translate each name before balancing.

    Reason

    Mg²⁺ requires two Cl⁻ ions, so magnesium chloride is MgCl₂; hydrogen is diatomic H₂.

    Working

    Mg + HCl → MgCl₂ + H₂

  2. Add states and balance

    Method

    Use coefficients while leaving the four formulae unchanged.

    Reason

    Two HCl units supply the two chlorine atoms and two hydrogen atoms required on the right.

    Working

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

  3. Expose aqueous ions and cancel spectators

    Method

    Split strong aqueous electrolytes into ions, then cancel identical ions on both sides.

    Reason

    Cl⁻(aq) is unchanged, but Mg becomes Mg²⁺ and H⁺ becomes H₂.

    Working

    Mg(s) + 2H + (aq) → Mg²⁺(aq) + H₂(g) Atoms and total charge are balanced.

3. Guided practice

Guided practice 2

Correct an equation one decision at a time

About 6 min

Problem

Calcium reacts with hydrochloric acid to form calcium chloride and hydrogen. Draft the equation before opening the feedback.

Write each step

Hints

Hint 1: charge balance
Start from one Ca²⁺ ion and enough Cl⁻ ions to make total charge zero.
Hint 2: atom balance
Once CaCl₂ is fixed, count chlorine and hydrogen to choose the hydrochloric-acid coefficient.
View solution step by step

  1. Check the product formula

    Method

    Balance the ionic charges before choosing equation coefficients.

    Reason

    One calcium ion has charge 2 +, so two chloride ions are required for a neutral compound.

    Working

    CaCl₂, because two chloride ions balance one Ca²⁺ ion.

  2. Balance without changing that formula

    Method

    Place 2 before hydrochloric acid and then add the stated physical states.

    Reason

    Two HCl units provide the two chlorine atoms in CaCl₂ and the two hydrogen atoms in H₂.

    Working

    Ca(s) + 2HCl(aq) → CaCl₂(aq) + H₂(g)

4. Plausible error contrast

A learner writes 2Al(s) + O₂(g) → Al₂O₂(s) and says it is balanced. The first invalid step is changing aluminium oxide from Al₂O₃ to Al₂O₂. Repair the formula first, then balance:

4Al(s) + 3O₂(g) → 2Al₂O₃(s)
First-error repair

Do not start by adjusting the learner’s coefficients. Once a substance formula is wrong, atom counting describes a different reaction.

5. Changed-context transfer

Mind stretcher 1: Form a precipitation ionic equationExtension

Aqueous silver nitrate and aqueous sodium chloride form solid silver chloride and aqueous sodium nitrate. Write the full equation and then the ionic equation.

Show feedback

First write AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). It is already balanced. Split the aqueous compounds and cancel Na + (aq) and NO₃⁻(aq). The ionic equation is Ag + (aq) + Cl-(aq) → AgCl(s); both atom count and total charge are balanced.

6. Independent evidence

Without answer choices, construct the formula of iron(III) sulfate from Fe³⁺ and SO₄²⁻. Then write and balance the equation for aqueous iron(III) sulfate reacting with aqueous barium chloride to form solid barium sulfate and aqueous iron(III) chloride. Include state symbols and identify the spectator ions. Check every element and total charge before submitting your work.

Continue to Relative Mass and the Mole only when the chemical relationship is secure.