Explain neutralisation and salt formation
Use the ionic neutralisation equation and identify the salt formed from an acid–alkali reaction.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Equilibrium constants apply at a fixed temperature; include only gaseous or aqueous species in the relevant expressions. For acid–base work, distinguish strength from concentration. Written practice is self-checked
Before you begin
- Balance equations, interpret reversible arrows and use concentration in mol dm⁻³.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which ions form water in neutralisation?
What you will learn
- Write the net ionic equation for neutralisation
- Explain that H⁺ and OH⁻ form water
- Identify salt formation from the remaining ions
Neutralisation consumes hydrogen and hydroxide ions
In aqueous neutralisation, hydrogen ions and hydroxide ions form liquid water: H⁺(aq) + OH⁻(aq) → H₂O(l). This is the net ionic equation for acid–alkali neutralisation.
The reaction removes acidic H⁺ and alkaline OH⁻. In a full molecular equation, balance the acid and alkali formulas and include state symbols when known.
The remaining ions form a salt
A salt forms from the cation supplied by the base or alkali and the anion supplied by the acid. For HCl + NaOH → NaCl + H₂O, Na⁺ and Cl⁻ make sodium chloride.
Use charge balance to write the salt formula. Different acids can make different salts with the same alkali because the acid anion changes.
Pause and explain
What is the net ionic equation for acid–alkali neutralisation?
Worked example
Write the molecular and net ionic equations for hydrochloric acid reacting with sodium hydroxide.
Show the worked solution
- The cation and anion left after neutralisation are Na⁺ and Cl⁻.
- Write the salt and water in the molecular equation.
- Remove spectator ions to leave H⁺ + OH⁻ → H₂O.
Answer HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l); net ionic: H⁺(aq) + OH⁻(aq) → H₂O(l).
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the balanced equation for HNO₃ + KOH and name the salt.
Give me a hint
Hydrogen and hydroxide form water; the remaining ions make the salt.
Compare my reasoning
- K⁺ and NO₃⁻ form KNO₃.
- H⁺ and OH⁻ form H₂O.
- Check one K, N and three O atoms on each side.
HNO₃ + KOH → KNO₃ + H₂O; the salt is potassium nitrate.
Look for these in your work
- Salt formula correct
- Water formed
- Equation balanced
Write the net ionic equation for H₂SO₄ neutralised by NaOH and identify the salt formula.
Give me a hint
Sulfuric acid supplies two H⁺ per formula unit; balance the molecular reaction.
Compare my reasoning
- Two NaOH are required per H₂SO₄.
- The full equation is H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.
- The net ionic equation has 2H⁺ + 2OH⁻ → 2H₂O, which simplifies.
Salt: Na₂SO₄. Net ionic: H⁺(aq) + OH⁻(aq) → H₂O(l).
Look for these in your work
- Molecular coefficients correct
- Salt charge balanced
- Net equation simplified
Explain why HCl and HNO₃ with NaOH form different salts even though both neutralisations form water.
Give me a hint
The anion comes from the acid; the cation comes from the alkali.
Compare my reasoning
- HCl leaves Cl⁻ after donating H⁺.
- HNO₃ leaves NO₃⁻ after donating H⁺.
- With Na⁺, the salts are NaCl and NaNO₃; both also form water.
HCl gives Cl⁻ and forms NaCl, while HNO₃ gives NO₃⁻ and forms NaNO₃. Both supply H⁺ that reacts with OH⁻ to form water.
Look for these in your work
- Acid anions distinguished
- Salt formulas correct
- Common water formation stated
Common mistakes
- Leaving water out of the ionic equation
- Calling water the salt
- Writing NaCl₂ without charge balance
- Confusing spectator ions with the ions that make water
What can you explain now?
Which ions form water in neutralisation?
Compare with the explanation
H⁺ and OH⁻
Their combination gives liquid water.
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Make it stick
Which ions form water, and which remaining ions make the salt?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.