Acids, Bases and Salt Preparations

The principle

• An insoluble salt cannot be crystallised out of a salt-plus-water solution — it would just stay dissolved up to its low solubility
• Instead, the salt is made by precipitation: two soluble salts whose ions can pair up to form the wanted insoluble salt are mixed

AB(aq) + CD(aq) → AD(s) ↓ + CB(aq)

• One product is the wanted precipitate (the insoluble salt); the other product is a soluble salt that stays in the surrounding solution and is washed away

• The reaction is sometimes called a double displacement or double decomposition reaction

• Choosing the reactants: pick a soluble salt of each ion that needs to combine — nitrates are always soluble, so a nitrate of the metal is the standard starting material when the salt's other ion is a halide, sulfate or carbonate

• Step-by-step:

  • Measure equal volumes of the two soluble salt solutions of similar concentration into a beaker and stir
  • The wanted salt forms immediately as a fine solid suspended in the liquid
  • Filter, keeping the residue (the precipitate) on the filter paper
  • Wash the residue with distilled water to flush away any soluble salt clinging to the precipitate
  • Leave the washed solid to dry on a watch glass in a warm oven