Bases, alkalis and the neutralisation equation
Distinguish a soluble base from an insoluble one and connect neutralisation with the ions that form water.
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Read ionic charges and identify spectator ions.
- Name salts from acids and positive components.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which is an alkali in the stated examples?
What you will learn
- Describe the syllabus metal-oxide and metal-hydroxide bases, and alkalis as soluble bases.
- Explain acid–alkali neutralisation and the reaction of an alkali with an ammonium salt.
Solubility separates a base from an alkali
In the syllabus examples, bases include metal oxides and hydroxides, while alkalis are soluble bases. Copper(II) oxide is a base but is insoluble and is not an alkali. Sodium hydroxide is soluble and gives an alkaline solution. The category base is wider than alkali; ammonia's proton-accepting behaviour is explained in the Extended lesson rather than being forced into a metal-oxide formula.
Aqueous acids contain H⁺ ions; aqueous alkalis contain OH⁻ ions. The acid–alkali neutralisation equation is H⁺(aq)+OH⁻(aq)→H₂O(l). For HCl+NaOH→NaCl+H₂O, sodium and chloride ions remain in solution as spectator ions. Neutralisation consumes the hydrogen and hydroxide ions; it does not make sodium or chlorine elements.
An ammonium salt gives a different base reaction
A suitable alkali reacts with an ammonium salt to produce ammonia, a salt and water. The illustrative equation NH₄Cl+NaOH→NH₃+NaCl+H₂O shows ammonium chloride with sodium hydroxide. This is distinct from carbonate gas production and from an acid–metal hydrogen reaction. In the supervised qualitative context, warming is used to assist ammonia release.
Ammonia is alkaline when it dissolves in water and can turn damp red litmus blue. A gas observation must include its stated test rather than relying on an unidentified smell. Practical handling and heating belong to teacher supervision; the learning task is to identify products, balance atoms and connect the observation with the proposed gas.
Pause and explain
Which ions form water in acid–alkali neutralisation?
Worked example
Write HCl(aq)+NaOH(aq) neutralisation as a full equation and an ionic equation.
Show the worked solution
- The full equation is HCl(aq)+NaOH(aq)→NaCl(aq)+H₂O(l).
- Na⁺ and Cl⁻ stay unchanged in the ionic accounting and are spectators.
- Removing them leaves H⁺(aq)+OH⁻(aq)→H₂O(l).
Answer HCl+NaOH→NaCl+H₂O; net ionic H⁺+OH⁻→H₂O.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Explain why CuO is a base but not an alkali, then predict its products with HCl.
Give me a hint
Use both acid reaction and water solubility.
Compare my reasoning
- CuO neutralises an acid and is a basic oxide.
- It is insoluble in water, so it is not an alkali.
- CuO+2HCl→CuCl₂+H₂O gives copper(II) chloride and water.
CuO is an insoluble base; with HCl it makes CuCl₂ and water.
Look for these in your work
- Base and alkali are distinguished.
- The neutralisation products are correct.
Balance sulfuric acid with potassium hydroxide, then state the net ionic acid–alkali equation.
Give me a hint
Two hydroxide units are needed for the two acidic hydrogens in the full equation.
Compare my reasoning
- The salt is K₂SO₄.
- H₂SO₄+2KOH→K₂SO₄+2H₂O balances the full equation.
- The net ionic ratio is H⁺+OH⁻→H₂O, with one of each ion per water molecule.
H₂SO₄+2KOH→K₂SO₄+2H₂O; H⁺+OH⁻→H₂O.
Look for these in your work
- Full-equation coefficients conserve atoms.
- The simplest ionic equation excludes spectators.
Predict products when ammonium sulfate reacts with sodium hydroxide in the stated supervised alkaline-gas test. Balance the equation.
Give me a hint
There are two ammonium groups in (NH₄)₂SO₄.
Compare my reasoning
- The products are ammonia, sodium sulfate and water.
- Two ammonium groups require two NaOH and form two NH₃.
- (NH₄)₂SO₄+2NaOH→2NH₃+Na₂SO₄+2H₂O balances atoms; ammonia turns damp red litmus blue.
(NH₄)₂SO₄+2NaOH→2NH₃+Na₂SO₄+2H₂O; damp red litmus turns blue.
Look for these in your work
- Both ammonium groups are included.
- The gas observation matches NH₃.
Common mistakes
- Calling every base an alkali regardless of solubility.
- Keeping spectator ions in a net ionic equation.
- Predicting carbon dioxide from an ammonium-salt reaction.
- Assuming neutralisation always finishes at pH 7 regardless of excess reactant or acid–base type.
What can you explain now?
Name the gas in the stated ammonium chloride–sodium hydroxide reaction and its damp-litmus effect.
Compare with the explanation
Ammonia; damp red litmus turns blue.
Ammonium ions with the alkali give NH₃; the gas is alkaline when dissolved in the moisture of the test paper.
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Make it stick
Explain base versus alkali with two examples, derive the net neutralisation equation and recall the ammonium-salt product pattern.
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.