Cambridge O Level Chemistry · Syllabus 5070 · Acids, Bases and Salts
Soluble Salt
What is Soluble Salt?
A salt that dissolves in water, so that after it has been made it is present in the filtrate as a solution and must be recovered by evaporating the solution to the crystallisation point and letting it cool and crystallise.
This definition is part of the Acids, Bases and Salts chapter in Cambridge O Level Chemistry.
Soluble Salt 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.
Questions students ask about Soluble Salt
Why can't you make a soluble salt from an acid and an alkali by filtering?
Both the acid and the alkali are dissolved, and so is the salt formed, so there is no undissolved solid for a filter to remove. The exact volume of alkali that reacts with the acid must instead be found by titration, using an indicator, and then the same volumes are mixed again without indicator so the crystals are not contaminated.
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.