4CH1

Acids, Bases and Salt Preparations

Inorganic Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 8% of your exam marks.

stable
Low
Stable8%

Soluble and insoluble salt preparation methods are consistently tested with multi-step answers.

Practical: making copper(II) sulfate (a soluble salt by the insoluble-base route)

  • Apparatus: 100 cm3 beaker; measuring cylinder; spatula and stirring rod; Bunsen burner, tripod, gauze and heatproof mat; filter funnel, filter paper, conical flask; evaporating basin
  • Materials: dilute sulfuric acid (1.0 mol dm−3); solid copper(II) oxide
  • Method:
    • Measure 40 cm3 of dilute sulfuric acid into the beaker and warm it gently with the Bunsen
    • Add black copper(II) oxide powder a spatula-tip at a time, stirring, until no more dissolves — undissolved black solid in a clear blue solution means the base is in excess
    • Filter the warm mixture; the blue filtrate is impure copper(II) sulfate solution, and the black residue is the unreacted copper(II) oxide
    • Pour the filtrate into an evaporating basin and warm gently until crystallisation just begins on a cold glass rod
    • Set the basin aside in a warm place for several hours to let bright-blue copper(II) sulfate crystals form slowly
    • Decant any remaining mother liquor and dry the crystals between sheets of filter paper
  • Word and symbol equations:

copper(II) oxide + sulfuric acid → copper(II) sulfate + water
CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)

  • Final product appearance: well-formed, deep-blue, regular-shaped crystals of CuSO4·5H2O

Practical: making lead(II) sulfate (insoluble salt by precipitation)

  • Apparatus: 100 cm3 beaker; measuring cylinder; stirring rod; filter funnel, filter paper, conical flask; wash bottle of distilled water; watch glass; oven or warm cupboard for drying
  • Materials: aqueous lead(II) nitrate (0.4 mol dm−3); aqueous potassium sulfate (0.4 mol dm−3)
  • Lead salts are toxic — wear gloves and dispose of waste solutions in the chemical-waste container, not down the sink
  • Method:
    • Measure 30 cm3 of lead(II) nitrate solution into the beaker
    • Add 30 cm3 of potassium sulfate solution and stir briefly — a fine white of lead(II) sulfate appears immediately
    • Filter the mixture, keeping the white residue on the filter paper; the clear filtrate is potassium nitrate solution and is discarded
    • Wash the residue twice with a few cm3 of distilled water from a wash bottle, letting each wash drain through completely
    • Transfer the washed solid to a watch glass and dry it in a warm oven
  • Word and symbol equations:

lead(II) nitrate + potassium sulfate → lead(II) sulfate + potassium nitrate
Pb(NO3)2(aq) + K2SO4(aq) → PbSO4(s) + 2 KNO3(aq)

  • Final product appearance: a fine, dry, white powder of lead(II) sulfate