Cambridge O Level Chemistry · Syllabus 5070 · Acids, Bases and Salts
Precipitation
What is Precipitation?
The formation of an insoluble solid when two aqueous solutions are mixed, because the ions of the insoluble salt cannot remain dissolved together. The solid is separated by filtration as the residue, then washed with distilled water and dried.
This definition is part of the Acids, Bases and Salts chapter in Cambridge O Level Chemistry.
Precipitation in context
Preparing a pure salt follows one route every time: identify the particles present, choose the reacting partner, predict the product, then apply the six solubility rules to decide whether the salt is soluble. A soluble salt made from a dissolved alkali must be prepared by titration, because an excess alkali cannot be filtered off; a soluble salt made from an insoluble solid uses excess solid followed by filtration; and an insoluble salt is made directly by precipitation, filtered, washed and dried. Every soluble-salt route ends the same way — evaporate to the crystallisation point, cool, filter, wash and dry.
Common mistakes with Precipitation
- “All chlorides are soluble — and all sulfates, and all hydroxides.” Repair Chlorides are soluble except lead(II) and silver. Sulfates are soluble except barium, calcium and lead(II). Hydroxides are the other way round: insoluble except sodium, potassium and ammonium. A rule without its exceptions cannot select a precipitation reaction.
Questions students ask about Precipitation
How do you decide which method to use to prepare a given salt?
First check whether the target salt is soluble using the six solubility rules. An insoluble salt is made by precipitation. A soluble salt is made either by titration, if the other reactant is a dissolved alkali, or by adding an insoluble solid — a reactive metal, base or carbonate — in excess and filtering off the excess afterwards.