Crystallising a soluble salt from solution

A soluble salt can be recovered from its solution as solid crystals. Crystallisation works by removing some water and then cooling the solution, so that some of the dissolved salt comes out of solution as a solid.

Heat the salt solution gently so that some water evaporates. The salt remains in the solution, which becomes more concentrated. Stop heating before all the water has evaporated.

Leave the concentrated solution to cool. Crystals of the salt form as dissolved salt comes out of the solution.

Separate the crystals from the remaining solution by filtration, then dry them, for example between sheets of filter paper.

The result is a pure, dry sample of the soluble salt. Some salt may remain dissolved in the liquid left after the crystals have formed.

Salt preparation using excess insoluble reactants

A soluble salt can be prepared by reacting an acid with an insoluble solid. The solid is added in excess, so that all the acid is used up. The remaining solid can then be filtered off, leaving a salt solution.

The products depend on the solid used:

A suitable metal reacts with the acid to produce a salt and hydrogen.
An insoluble base reacts with the acid to produce a salt and water.
An insoluble carbonate reacts with the acid to produce a salt, water and carbon dioxide.

Preparing the salt solution

Add the insoluble solid to the acid a little at a time, stirring as it reacts. Continue until some solid remains unreacted even after stirring. This indicates that the acid has been used up: there is no acid left to react with the extra solid.

Filter the mixture. The excess solid stays on the filter paper, while the salt solution passes through as the filtrate.

Obtaining salt crystals

Gently evaporate some water from the filtrate to make the salt solution more concentrated. Then leave it to cool. As it cools, salt crystals form from the solution.

Separate the crystals from the remaining liquid by filtration, then dry them, for example between sheets of filter paper.