4.4.2.3 - Preparing soluble salts

Choosing reactants for a soluble salt

A suitable acid and an insoluble solid can react to make a soluble salt that remains dissolved while unused solid is removed.

The acid determines the negative ion: hydrochloric acid makes chlorides, nitric acid makes nitrates and sulfuric acid makes sulfates. The solid supplies the metal ion. To prepare copper sulfate, choose dilute sulfuric acid and insoluble copper oxide:

Copper carbonate is another possible insoluble reactant. It forms carbon dioxide as well as water:

Both routes produce dissolved copper sulfate. Copper metal is unsuitable for this preparation with dilute sulfuric acid because it is below hydrogen in the reactivity series. The identity of the metal in the product alone is not enough: the selected substance must actually react under the specified conditions.

The general method can use an appropriate insoluble metal, metal oxide, hydroxide or carbonate. Check any supplied solubility and reactivity information before choosing. For example, an insoluble magnesium oxide can make a magnesium salt, but soluble sodium hydroxide cannot be removed by filtering an excess. Acid–alkali mixtures therefore require the measured-volume titration method taught next.

Required practical 1 specifically prepares a pure, dry soluble salt from an insoluble oxide or carbonate. A named method should state the actual acid and solid, rather than leaving both as unspecified reagents.

Using an excess insoluble solid

Adding more insoluble reactant than the acid can use ensures that the finished salt solution contains no unreacted acid.

For copper sulfate, measure dilute sulfuric acid into a beaker. Warm it gently using a Bunsen burner and suitable heat-resistant apparatus, then turn off the burner before adding copper oxide. Warming increases the reaction rate; boiling the acid is unnecessary and increases the risk of splashing and concentrated acid remaining as water evaporates. Wear eye protection and handle hot glassware with appropriate equipment.

Add copper oxide with a spatula in small portions, stirring each time. At first the black solid disappears as it reacts, and blue copper sulfate solution forms. Continue adding until some black solid remains after stirring and sufficient time for reaction. This excess indicates that the available acid has been used up. Simply stopping when the solution turns blue is insufficient: some salt can form while considerable acid remains.

With a carbonate, add small portions to control carbon dioxide bubbling and prevent frothing or loss of liquid. Allow each portion to react before the next. Persistent unreacted solid, together with the stopping of reaction after stirring, is stronger endpoint evidence than a brief pause in bubbling alone.

An exact initial solid mass is not needed to match the acid: excess solid will be removed. Nevertheless, quantities should be small and controlled, and the procedure must allow reaction to finish. Adding a very large amount at once can hinder mixing, spill material and waste reagent without improving purity.

Filtration and crystallisation

Filtration removes the unused insoluble solid, while crystallisation separates dissolved salt from most of the water.

Fold filter paper into a funnel over a clean container and pour the mixture through carefully. Excess copper oxide stays as the residue on the paper. Copper sulfate solution passes through as the filtrate. Dissolved copper sulfate is not trapped by ordinary filter paper; crystals have not yet formed.