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Chemistry · Practice book · Senior 2 · Unit 4 of 9

Mineral salts: properties, preparation and identification of ions

Senior 2 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 4 Mineral salts: properties, preparation and identification of ions

Key unit competence: Analyse the properties and preparation methods of inorganic salts and relate them to real-life applications.

Key ideas

  • A salt is a compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion. Name = metal + acid part: chloride (HCl), sulfate (H2SO4), nitrate (HNO3), carbonate (H2CO3).
  • Solubility rules: all sodium, potassium and ammonium salts and all nitrates are soluble; chlorides are soluble except AgCl and PbCl2; sulfates are soluble except BaSO4, PbSO4 (and CaSO4 slightly); carbonates are insoluble except Na, K and NH4+.
  • Solubility usually increases with temperature for solids; a saturated solution holds the maximum solute at that temperature. Solubility = grams of solute per 100 g of water.
  • Preparation of soluble salts: acid + excess insoluble base/metal/carbonate, then filter and crystallise; or titration of acid with alkali (for Na, K, NH4+ salts). Insoluble salts: precipitation by mixing two solutions, then filter, wash and dry.
  • Tests for cations with NaOH(aq): Cu2+ blue precipitate; Fe2+ green; Fe3+ red-brown; Zn2+ white, dissolves in excess; Al3+ white, dissolves in excess; Ca2+ white, insoluble in excess; NH4+ gives ammonia (turns red litmus blue) on warming.
  • Tests for anions: Cl− white ppt with acidified AgNO3; SO42− white ppt with acidified BaCl2; CO32− fizzes with dilute acid giving CO2 (limewater milky); NO3− gives ammonia when warmed with NaOH and aluminium foil.

Common misconceptions

  • "All salts are soluble in water like table salt." No: Many salts such as AgCl, BaSO4 and CaCO3 are insoluble.
  • "Insoluble salts are made by titration." No: Insoluble salts are made by precipitation; titration is used for soluble salts of Na, K and NH4+.
  • "Salt means only sodium chloride." No: Salt is a whole class of compounds; NaCl is just one example.

Worked examples

Example 1. Describe how to prepare dry crystals of zinc sulfate from zinc oxide and dilute sulfuric acid.

Solution

Warm the dilute sulfuric acid and add zinc oxide a little at a time, stirring, until some remains undissolved (excess).

Filter to remove the excess zinc oxide.

Heat the filtrate in an evaporating dish to concentrate it, then leave it to cool so crystals form.

Filter off the crystals and dry them between filter papers.

Example 2. A solution gives a red-brown precipitate with NaOH and a white precipitate with acidified silver nitrate. Identify the ions and write ionic equations.

Solution

Red-brown precipitate: Fe3+: Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s).

White precipitate with silver nitrate: Cl−: Ag+(aq) + Cl−(aq) → AgCl(s).

The salt is iron(III) chloride, FeCl3.

Practice questions

  1. Define a salt.

    Show answer

    A compound formed when the hydrogen ion of an acid is replaced by a metal ion or ammonium ion.

    Common mistake: Saying 'a white crystal'.

  2. Name the salt formed from magnesium and nitric acid.

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    Magnesium nitrate.

    Common mistake: Writing magnesium nitrite.

  3. Give two insoluble salts.

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    AgCl, BaSO4, PbSO4, CaCO3, PbI2 (any two).

    Common mistake: Naming NaCl.

  4. What colour precipitate does Zn2+ form with NaOH, and what happens in excess?

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    White; dissolves in excess to give a colourless solution.

    Common mistake: Saying it stays insoluble.

  5. Describe the test for sulfate ions.

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    Add dilute HCl then barium chloride solution: white precipitate.

    Common mistake: Using silver nitrate.

  6. Name the method to prepare barium sulfate.

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    Precipitation.

    Common mistake: Saying titration.

  7. State the solubility rule for nitrates.

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    All nitrates are soluble.

    Common mistake: Saying only sodium nitrate is soluble.

  8. Why is a saturated solution heated before crystallisation?

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    To evaporate water and make it concentrated, so crystals form on cooling.

    Common mistake: Saying heating makes the salt insoluble.

  9. Write the ionic equation for the formation of blue copper(II) hydroxide.

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    Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s).

    Common mistake: Writing CuOH.

  10. 40 g of salt dissolves in 125 g of water at 30 °C. Find the solubility in g per 100 g water.

    Show answer

    40 × 100 ÷ 125 = 32 g/100 g water.

    Common mistake: Dividing 125 by 40.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Iron(III) ions give what colour precipitate with sodium hydroxide?a. Acidified silver nitrate
    2. Solubility of potassium nitrate is 32 g per 100 g water at 20 °C. How much dissolves in 50 g of water at 20 °C?b. Red-brown
    3. Insoluble salts are prepared by:c. precipitation
    4. Which reagent tests for chloride ions?d. 16 g
    Show answer

    1 → b (Red-brown) · 2 → d (16 g) · 3 → c (precipitation) · 4 → a (Acidified silver nitrate)

    • Fe(OH)3 is red-brown.
    • Half the water dissolves half the mass.
    • Two soluble salts are mixed and the insoluble one precipitates.
    • A white precipitate of AgCl forms.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    zinc sulfate  ·  turns damp red litmus blue  ·  zinc oxide  ·  iron(II) sulfate
    1. The salt formed from zinc and sulfuric acid is .
    2. Ammonium ions are detected by warming with NaOH; the gas produced .
    3. A solid X gives a green precipitate with NaOH and a white precipitate with acidified BaCl2. X is .
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    1: zinc sulfate · 2: turns damp red litmus blue · 3: iron(II) sulfate

    • Sulfuric acid gives sulfate salts.
    • Ammonia is alkaline.
    • Step 1. Green ppt with NaOH: Fe2+.
      Step 2. White ppt with BaCl2: SO42−.
      Step 3. FeSO4.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which carbonate is soluble in water?a. Sodium carbonate
    2. Which salt is insoluble in water?b. Silver chloride
    3. Copper(II) ions give what colour precipitate with sodium hydroxide?c. Titration of KOH with dilute H2SO4, then crystallisation
    4. Which method would you use to prepare potassium sulfate?d. Blue
    Show answer

    1 → a (Sodium carbonate) · 2 → b (Silver chloride) · 3 → d (Blue) · 4 → c (Titration of KOH with dilute H2SO4, then crystallisation)

    • Group 1 carbonates are soluble.
    • AgCl is one of the few insoluble chlorides.
    • Cu(OH)2 is blue.
    • Both reactants are soluble, so titration finds the exact amounts; potassium metal is too dangerous.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    soluble in water  ·  Zn2+ or Al3+  ·  copper(II) chloride  ·  acidic
    1. Solution Y gives a white precipitate with acidified AgNO3 and a blue precipitate with NaOH. Y is .
    2. All nitrates are .
    3. A white precipitate with NaOH that dissolves in excess NaOH indicates .
    Show answer

    1: copper(II) chloride · 2: soluble in water · 3: Zn2+ or Al3+

    • Step 1. White with AgNO3: Cl−.
      Step 2. Blue with NaOH: Cu2+.
    • This is a key solubility rule.
    • Zinc and aluminium hydroxides are amphoteric.

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