Acids, Bases and Salt Preparations

Method A — acid plus an insoluble base

• Used when the metal involved is insoluble as its base (most transition-metal oxides, carbonates and hydroxides)

• Works for almost any soluble salt of an unreactive or moderately reactive metal — e.g. CuSO₄, ZnCl₂, Fe(NO₃)₃

• Step-by-step:

  • Warm a fixed volume of the dilute acid gently in a beaker (warming speeds up the reaction)
  • Add the insoluble base a little at a time, stirring, until no more dissolves — the base is then in excess, which guarantees that no acid is left over
  • Filter off the unreacted base, collecting the salt solution as the filtrate in a clean container
  • Pour the filtrate into an evaporating basin and heat gently to evaporate most of the water
  • Stop heating when crystals start to form on a cold glass rod dipped into the solution — the solution is now saturated
  • Leave the saturated solution in a warm place to cool and crystallise slowly; decant any remaining liquid and let the crystals dry on filter paper

• Worked example: copper(II) sulfate from copper(II) oxide and dilute sulfuric acid

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

• Reason to add the base in excess: any unreacted acid would become dangerously concentrated during the evaporation step
• Reason for slow crystallisation: large, well-formed crystals only grow when the solvent leaves slowly

Method B — acid plus an alkali (titration method)

• Used when both reactants are soluble — adding excess of one would just contaminate the salt solution, so a titration is used to find the exact end-point first
• Step-by-step:
  • Pipette a fixed volume of the alkali (e.g. 25.0 cm³) into a conical flask with a few drops of indicator
  • Titrate with the acid from a burette to find the exact volume needed for neutralisation (see topic 15 section 3 for full titration technique)
  • Repeat the reaction without the indicator, using the same volumes that were found in the titration — the resulting solution is now a clean salt solution, free of dye
  • Evaporate, cool, decant and dry as in method A
• The indicator-free repeat is essential because the indicator itself would contaminate the salt sample