IGCSE Chemistry: Acids, Bases and Salts Practice Questions
An acid produces hydrogen ions in aqueous solution and a base neutralises it to form a salt and water. Acids react with metals to give a salt and hydrogen, with bases to give a salt and water, and with carbonates to give a salt, water and carbon dioxide.
Topic 7 of Cambridge IGCSE Chemistry 0620 is examined heavily on Paper 6 as well as the theory papers, because salt preparation is a required practical. Method questions are marked step by step, so the order of operations matters. The questions below cover both routes to a salt.
What you need to know for Acids, Bases and Salts
- Acid and baseAn acid is a proton donor that produces H+ ions in aqueous solution. A base is a proton acceptor. An alkali is a soluble base that produces OH- ions in solution.
- The three acid reactionsAcid + metal gives salt + hydrogen. Acid + base gives salt + water. Acid + carbonate gives salt + water + carbon dioxide.
- Strong and weak acidsA strong acid is fully dissociated into ions in solution, such as hydrochloric acid. A weak acid is only partially dissociated, such as ethanoic acid, so at the same concentration it has a higher pH and reacts more slowly.
- Soluble salt from an insoluble baseAdd excess solid base to warm acid until no more reacts, filter off the excess, then crystallise the filtrate by partial evaporation and slow cooling.
- Soluble salt from a soluble baseUse titration to find the exact volume of alkali that neutralises the acid, then repeat with the same volumes without indicator and crystallise.
- Insoluble salt by precipitationMix solutions of two soluble salts that contain the required ions, filter off the precipitate, wash it with distilled water and dry it.
IGCSE Chemistry Acids, Bases and Salts questions and answers
4 exam-style questions written to the 0620 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Describe how you would prepare pure dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.
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- Warm some dilute sulfuric acid in a beaker. Warming increases the rate of reaction.
- Add copper(II) oxide a little at a time, stirring, until no more dissolves and some solid remains. The excess shows all the acid has reacted.
- Filter the mixture to remove the unreacted copper(II) oxide. The filtrate is copper(II) sulfate solution.
- Heat the filtrate to evaporate about half the water, producing a saturated solution. Test by dipping a glass rod in and checking that crystals form on it.
- Leave the solution to cool slowly so crystals form, then filter them off and dry them between filter papers. Do not evaporate to dryness, as that would drive off the water of crystallisation.
Write balanced equations for the reactions of dilute hydrochloric acid with magnesium, with sodium hydroxide, and with calcium carbonate.
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- Acid plus metal gives salt plus hydrogen: Mg + 2HCl gives MgCl2 + H2.
- Acid plus alkali gives salt plus water: HCl + NaOH gives NaCl + H2O.
- Acid plus carbonate gives salt plus water plus carbon dioxide: CaCO3 + 2HCl gives CaCl2 + H2O + CO2.
- In each case the chloride salt forms because the acid is hydrochloric. Sulfuric acid would give sulfates and nitric acid would give nitrates.
Ethanoic acid and hydrochloric acid solutions have the same concentration, but the hydrochloric acid has a lower pH and reacts faster with magnesium. Explain why.
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- Hydrochloric acid is a strong acid, meaning it is fully dissociated into H+ and Cl- ions in solution.
- Ethanoic acid is a weak acid, meaning only a small proportion of its molecules dissociate into ions at any time.
- At the same overall concentration, the hydrochloric acid therefore contains a much higher concentration of hydrogen ions, which is what determines pH, so its pH is lower.
- A higher concentration of hydrogen ions means more frequent successful collisions with the magnesium, so the reaction is faster.
Describe how you would prepare a pure dry sample of barium sulfate, an insoluble salt, starting from two soluble salts.
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- Choose two soluble salts that between them provide the required ions, for example barium chloride solution and sodium sulfate solution.
- Mix the two solutions. A white precipitate of barium sulfate forms immediately: BaCl2 + Na2SO4 gives BaSO4 + 2NaCl.
- Filter the mixture to collect the precipitate in the filter paper.
- Wash the residue with distilled water to remove the soluble sodium chloride, then leave it to dry in a warm oven or between filter papers.
Common mistakes in this topic
- Confusing strong with concentrated, and weak with dilute.
- Evaporating a salt solution to dryness when crystals are required.
- Forgetting that the acid determines the salt: hydrochloric gives chlorides, sulfuric gives sulfates, nitric gives nitrates.
- Omitting the excess solid step, which is what guarantees all the acid has reacted.
- Writing that an alkali is the same as a base. An alkali is a soluble base.
Exam tips
- Learn the three general acid reactions as word equations first, then substitute the specific reactants.
- Choose the preparation route from the solubility of the salt and of the base: insoluble base gives the excess and filter route, soluble base gives the titration route, insoluble salt gives precipitation.
- Say excess and explain why. It is one of the most reliable single marks in this topic.
- In practical answers, name the piece of apparatus, not just the action. Filter funnel, filter paper, evaporating basin.
- Learn the solubility rules: all nitrates and all sodium, potassium and ammonium salts are soluble.
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Acids, Bases and Salts FAQs
What is the difference between a strong acid and a concentrated acid?
Strong describes how completely an acid dissociates into ions in solution. A strong acid is fully dissociated. Concentrated describes how much acid is dissolved in a given volume. A dilute solution of a strong acid and a concentrated solution of a weak acid are both possible.
How do I prepare a soluble salt?
If the base is insoluble, add it in excess to warm acid, filter off the unreacted solid, then evaporate the filtrate partially and cool slowly to crystallise. If the base is soluble, use a titration to find the exact neutralising volume, repeat without indicator, then crystallise the resulting solution.
Why should a salt solution not be evaporated to dryness?
Heating to dryness drives off the water of crystallisation and can decompose the salt, leaving a powder rather than well formed crystals. Evaporate about half the water to make a saturated solution, then let it cool slowly so that crystals form.
How do I make an insoluble salt?
Use precipitation. Mix solutions of two soluble salts that together supply the ions needed. The insoluble salt forms as a precipitate. Filter it off, wash the residue with distilled water to remove the soluble salt left in solution, then dry it.
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Written to the published Cambridge IGCSE Chemistry (0620) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.