Choosing the method
The way a salt is prepared depends on its solubility and whether it is a sodium, potassium or ammonium salt.
Soluble salts (not Na/K/NH4)
React a dilute acid with an excess of an insoluble solid — a metal, a metal oxide, a base or a carbonate. Then filter off the unreacted solid, and crystallise the filtrate.
Key idea / 要点
CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
Steps: warm acid with excess CuO → filter off excess CuO → evaporate to saturation → cool to crystallise → dry.
Soluble Na/K/NH4 salts
These come only from soluble alkalis, so excess cannot be filtered off. They are made by titration: neutralise a measured volume of acid with alkali using an indicator, note the volume, then repeat without indicator and crystallise.
Insoluble salts
Made by precipitation (double decomposition): mix two soluble salt solutions that supply the required ions; filter, wash and dry the precipitate.
Worked example
Prepare insoluble lead(II) iodide.
Answer: Mix lead(II) nitrate solution with potassium iodide solution: Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq). Filter off the yellow PbI2, wash with distilled water and dry.
Choosing the wrong method wastes reagents or gives an impure product. Adding excess solid only works when that solid is insoluble, so it can be filtered off; for a soluble base such as sodium hydroxide there is no excess to remove, which is why titration is used. For an insoluble product, precipitation is quick because the salt simply drops out of solution and is collected by filtration.
Remember
- Soluble salt (not Na/K/NH4): acid + excess solid, filter, crystallise.
- Na/K/NH4 salt: titration (acid + alkali).
- Insoluble salt: precipitation, then filter, wash and dry.