Making two sulfates

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

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Copper (II) oxide is insoluble in water (shown on the left), but reacts with sulfuric acid producing a solution of copper (II) sulfate (blue on the right)

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

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Add copper (II) oxide to sulfuric acid whilst heating gently and stirring with a glass rod

Do not boil!

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The copper (II) oxide reacts with the acid producing copper (II) sulfate as a blue solution

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.

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All of the acid has been used up and the excess of insoluble base collects at the bottom of the beaker

The excess copper (II) oxide is removed by filtration.

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Filter off the insoluble black solid

Transfer the filtrate to an evaporating basin.

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The product copper (II) sulfate is dissolved in water as a blue solution

Water is removed by evaporation over a Bunsen burner. Stop heating when crystals start to form at the edge of the solution.

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Solid forming 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).

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The small squares of Universal Indicator paper show the progress of the reaction from right to left, (the tile on the right used up before the tile on the 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.

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Reduce the volume of the reaction mixture by heating over a Bunsen burner.

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Allow crystals to form in a warm, dry place

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?

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