At GCSE level Chemistry making salts is divided into three categories:
1. by reacting an acid with an excess of an insoluble base
2. by titration
3. by making a precipitate
Making the salt copper (II) sulfate from copper (II) oxide falls into the first category, since copper (II) oxide is a basic oxide which is insoluble in water, but it reacts with sulfuric acid in a typical acid/base reaction.
acid + base -> salt + water
Making ammonium sulfate from aqueous ammonia requires a titration technique, since both the acid (sulfuric acid in this case) and the base (ammonia) are soluble in water. We call bases that are soluble in water alkalis, so we can re-write the general equation here as:
acid + alkali -> salt + water
Making copper (II) sulfate
Do not boil!
Stop adding copper (II) oxide when no more reacts and the black solid collects at the bottom of the beaker. At this stage all of the acid has reacted.
The excess copper (II) oxide is removed by filtration.
Transfer the filtrate to an evaporating basin.
Water is removed by evaporation over a Bunsen burner. Stop heating when crystals start to form at the edge of the solution.
Place the evaporating basin on a windowsill or in a drying cabinet to allow crystals to form.
Making ammonium sulfate
In an approximation to carrying out a titration, small volumes of ammonia can be added to sulfuric acid in a beaker using a dropping pipette. After each addition of ammonia a sample of the reacting mixture is spotted onto a small piece of Universal Indicator paper using a glass rod. Stop adding ammonia when the Universal Indicator turns green or green/blue.
In the picture below the beaker in the centre shows the reaction mixture, the small squares of Universal Indicator paper on the tiles have been spotted after adding aliquots of ammonia (from right to left).
Stop adding ammonia when the indicator turns green. The last four squares of paper on the bottom left (of the tile of the left) were not used.
The third method of making a salt, by precipitation, is not illustrated above.
Which sulfate is formed as a white precipitate and used as a test for the presence of the sulfate ion in GCSE Chemistry?