Choosing a method
The way you prepare a salt depends on whether the salt is soluble or insoluble in water, and whether it is a salt of sodium, potassium or ammonium.
Key idea
Solubility rules: all nitrates and all sodium, potassium and ammonium salts are soluble. Most chlorides are soluble (except AgCl, PbCl2). Most sulfates are soluble (except BaSO4, PbSO4).
Three routes
- Soluble salt (not Na/K/NH4): add EXCESS insoluble metal oxide, hydroxide, carbonate or metal to the acid; filter off the excess solid; then crystallise the filtrate.
- Soluble salt of Na/K/NH4: use titration – there is no excess to filter, so react exact volumes of acid and alkali (found with an indicator), then repeat without indicator and crystallise.
- Insoluble salt: use precipitation – mix two soluble solutions, filter the precipitate, wash with distilled water and dry.
Worked example
Prepare copper(II) sulfate crystals.
Copper is below hydrogen, so use its insoluble oxide with excess:
CuO + H2SO4 → CuSO4 + H2O
Warm the acid, add CuO until no more dissolves, filter off excess CuO, then heat the blue filtrate to the crystallisation point and leave to form crystals.
Remember
- Using EXCESS solid ensures all the acid is used up.
- To make an insoluble salt, pick two solutions that between them provide the correct ions.