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5.5–5.6 Salts from Insoluble Bases and Making More Salts

GCSE Chemistry · AQA · Chapter 5: Chemical Changes

A free GCSE Chemistry lesson for AQA (8462) from OBA School: making salts from metal oxides, hydroxides and carbonates, crystallising soluble salts, and making insoluble salts by precipitation. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Write word and balanced symbol equations for acids reacting with metal oxides, hydroxides and carbonates
  • Describe how to make crystals of a soluble salt from an acid and an insoluble base or carbonate
  • Choose an acid and a base to make a named salt, and work out the formula of a salt from the charges on its ions
  • Describe how to make an insoluble salt by precipitation
  • Calculate the mass of salt that can be made and the percentage yield

Key words

  • Base: A substance that neutralises an acid to form a salt and water. Metal oxides and hydroxides are bases.
  • Alkali: A base that dissolves in water, forming OH−(aq) ions, e.g. sodium hydroxide.
  • Neutralisation: The reaction between an acid and a base (or carbonate) that produces a salt.
  • Excess: More than is needed to react completely, so some is left over.
  • Filtrate: The clear solution that passes through filter paper.
  • Crystallisation: Evaporating some solvent and leaving a solution to cool so crystals form.
  • Precipitate: An insoluble solid that forms when two solutions are mixed.

Acids with metal oxides and hydroxides

Metal oxides and metal hydroxides are bases. They neutralise acids to make a salt and water. Many metal oxides, such as copper(II) oxide, are insoluble, but they still react with acids, especially when warmed.

acid + metal oxide → salt + water CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) (black solid → blue solution)

acid + metal hydroxide → salt + water NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) Ca(OH)₂(aq) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)

Key fact: Every alkali is a base, but not every base is an alkali. An alkali is a base that dissolves in water.

Working out the formula of a salt

A salt is an ionic compound: positive metal ions (or ammonium ions, NH₄⁺) with negative ions from the acid. The salt has no overall charge, so the charges on its ions must add up to zero. Transition metals can form ions with different charges; the Roman numeral in the name gives the charge, e.g. iron(III) = Fe³⁺.

Worked example: formula of ammonium sulfate

Ions: NH₄⁺ (charge +1) and SO₄²− (charge −2).
To cancel −2 you need two +1 charges, so the ratio is 2 NH₄⁺ : 1 SO₄²−.
Put brackets round a group ion when you need more than one of it.

Answer: (NH₄)₂SO₄

Exam tip: Quick method: the number of each ion is the size of the other ion’s charge, then cancel if you can. Fe³⁺ and O²− → Fe₂O₃; Ca²⁺ and CO₃²− → Ca₂(CO₃)₂ → CaCO₃.

Acids with metal carbonates

Metal carbonates react with acids to make a salt, water and carbon dioxide. You see fizzing, and the solid disappears.

acid + metal carbonate → salt + water + carbon dioxide ZnCO₃(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l) + CO₂(g) K₂CO₃(aq) + 2HNO₃(aq) → 2KNO₃(aq) + H₂O(l) + CO₂(g)

Most metal carbonates are insoluble in water; the main exceptions are the Group 1 carbonates (such as sodium carbonate) and ammonium carbonate. Farmers use this reaction when they spread powdered limestone (mainly calcium carbonate) on fields that are too acidic.

(Higher tier) For an insoluble carbonate the ionic equation shows the solid reacting with hydrogen ions, e.g. CuCO₃(s) + 2H⁺(aq) → Cu²⁺(aq) + H₂O(l) + CO₂(g). For a dissolved carbonate it is simply CO₃²−(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g).

Key fact: Test for carbon dioxide: bubble the gas through limewater (calcium hydroxide solution). It turns milky (cloudy).

Making a soluble salt from an insoluble base

  • Add the solid in excess to warm dilute acid and stir. When some solid stays undissolved, all the acid has reacted, so no acid is left in the product. With a carbonate, fizzing stops when the acid is used up.
  • Filter to remove the unreacted solid. The filtrate is a solution of the salt in water.
  • Crystallise: heat the filtrate gently (a water bath stops spitting) until crystals start to form at the edge, then leave it to cool. Pat the crystals dry with filter paper.

You will plan and evaluate this method in detail in the required practical (5.9).

Making soluble salts from alkalis, and ammonium salts

If the base is an alkali (soluble), you cannot add “excess” and filter it off. Instead, do a titration with an indicator to find the exact volumes that react. Then mix the same volumes again without the indicator and crystallise the salt.

In every acid + alkali reaction, the H⁺(aq) ions from the acid join with the OH−(aq) ions from the alkali: H⁺(aq) + OH−(aq) → H₂O(l). The other ions stay in solution as the salt.

Ammonia solution is a weak alkali, because only some ammonia molecules react with water: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH−(aq). It neutralises acids to make ammonium salts, which are used as fertilisers: NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq).

  • soluble; base is insoluble (e.g. CuSO₄); Choose acid + excess insoluble oxide, hydroxide or carbonate; Method add excess, filter, crystallise
  • soluble; base is an alkali (e.g. KNO₃); Choose acid + alkali (or soluble carbonate); Method titrate, repeat without indicator, crystallise
  • insoluble (e.g. BaSO₄); Choose two soluble salt solutions; Method precipitate, filter, wash, dry

Insoluble salts by precipitation

An insoluble salt is made by mixing two solutions that each contain one of its ions. The salt forms as a solid precipitate.

To get a pure, dry sample: filter the mixture, wash the solid on the filter paper with distilled water to remove the soluble salt, then dry it in a warm oven. The ionic equation shows only the ions that form the solid: Ba²⁺(aq) + SO₄²−(aq) → BaSO₄(s).

Worked example: mass of salt and percentage yield

MgO + H₂SO₄ → MgSO₄ + H₂O. A student reacts 2.0 g of MgO with excess acid. (Ar: Mg = 24, O = 16, S = 32)
moles of MgO = 2.0 ÷ 40 = 0.050 mol, so 0.050 mol MgSO₄ can form (1 : 1)
Mr of MgSO₄ = 24 + 32 + (4 × 16) = 120, so maximum mass = 0.050 × 120 = 6.0 g
She collects 4.8 g of dry salt. Percentage yield = (4.8 ÷ 6.0) × 100 = 80%

Answer: maximum 6.0 g; yield 80%

Common mistakes

  • Forgetting carbon dioxide as a product when an acid reacts with a carbonate.
  • Saying the excess solid is added “to make more salt”. It is added so that all the acid reacts and none is left in the product.
  • Heating the solution until it is completely dry. This can spoil the crystals; heat until crystals start to form, then leave it to cool.
  • Calling every base an alkali. Copper(II) oxide is a base but it is insoluble, so it is not an alkali.

Revision questions

Try these, then check your answers below each one.

Question 1. What are the products when a metal carbonate reacts with a dilute acid? (Choose from: a salt, water and carbon dioxide, a salt and hydrogen, a salt and water only, a salt, hydrogen and carbon dioxide.)

Answer: a salt, water and carbon dioxide

Working: acid + metal carbonate → salt + water + carbon dioxide, e.g. CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂.

Question 2. Name the salt made when copper(II) oxide reacts with dilute sulfuric acid.

Answer: copper sulfate

Working: CuO + H₂SO₄ → CuSO₄ + H₂O. The salt is copper(II) sulfate.

Question 3. When making a salt from an insoluble base, why is the base added in excess? (Choose from: to make sure all the acid reacts, to make more salt than the acid allows, to speed up crystallisation, to make the base dissolve in water.)

Answer: to make sure all the acid reacts

Working: Adding excess base makes sure all the acid is used up. The extra solid is then removed by filtering.

Related lessons

Continue in the full course

This lesson is section 5.5–5.6 (AGCCH0505) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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