C3 - Investigate the preparation of soluble salts

• Salts can be prepared from the reactions of acids.

• A pure salt can be prepared from the reaction of an acid with an excess amount of an insoluble solid, eg copper(II) carbonate.

• A pure salt can also be prepared from the reaction of an acid with an exact amount of alkali solution.

Salts are produced in the reactions of acids.

Some salts can be useful, and so chemists sometimes carry out preparations of pure salts.

There are two different approaches to making a pure salt which are shown in the two experiments below.

The most important part of preparing a salt is that it is pure – there can be no leftover reactants that would make the salt impure.

In this experiment you will be making a salt using a metal carbonate.

For example, the salt copper(II) sulfate can be prepared from the reaction of copper(II) carbonate with sulfuric acid:

copper(II) carbonate + sulfuric acid → copper(II) sulfate + water + carbon dioxide

To ensure there is no leftover sulfuric acid in the copper(II) sulfate, an excess of copper(II) carbonate is used to make sure the sulfuric acid is fully neutralised.

The leftover copper(II) oxide solid can be filtered from the solution, leaving the copper(II) sulfate and water behind.

The water can be removed by evaporation, leaving behind pure copper(II) sulfate.

• 100 cm³ beakers (x2), glass rod

• Tripod, gauze, Bunsen burner, heat proof mat

• Filter funnel, filter paper and conical flask

• Evaporating basin

• Copper(II) carbonate (approx 4 g)

• Sulfuric acid (1 mol/dm³ 25 cm³)

Follow the safety advice of your teacher.

1. Place 4 g of copper(II) carbonate in a beaker.

2. Warm 25 cm³ of 1 mol/dm³ sulfuric acid in another beaker, the acid should be no hotter than 60^оC.

3. Add the copper(II) carbonate to the sulfuric acid and stir until there is no further reaction and unreacted copper(II) carbonate is present.

4. Allow to cool and filter the mixture and retain the filtrate.

5. Heat the filtrate in an evaporating basin on a tripod and gauze until the volume is about one half of what it was originally.

6. Allow the basin to cool and the crystals will form.

7. When the crystals have formed, filter if necessary and dry (between two sheets of filter paper or using a desiccator or place in a low temperature oven).

In this experiment it is not as easy to determine when all the acid has been used up as both the acid and alkali are colourless solutions.

Therefore, we have to use an indicatorindicator to determine when neutralisationneutralisation has taken place.

We can either remove the indicator by using decolourising charcoal, or, once we know the volumes required for neutralisation, we can repeat the experiment using the exact volumes without an indicator.

In this experiment you will prepare dry, pure sodium chloride using hydrochloric acid and sodium hydroxide solution:

hydrochloric acid + sodium hydroxide → sodium chloride + water

1. Fill the burette with hydrochloric acid.

2. Using the pipette and pipette filler, place 25cm³ of sodium hydroxide solution into a conical flask.

3. Add 2 drops of phenolphthalein indicator to the conical flask (it should go pink).

4. Carefully add the hydrochloric acid from the burette into the conical flask, swirl gently and rinse any droplets down into the flask with deionised water.

5. Stop adding the hydrochloric acid when the indicator turns colourless and record the volume added by carefully reading the burette.

6. Repeat steps 1-5 with fresh chemicals, but this time without the indicator, adding the recorded volume of hydrochloric acid.

7. Carefully pour the solution into an evaporating basin.

8. Place onto a tripod and gauze and heat gently using a Bunsen burner until about ½ of the solution remains.

9. When the crystals have formed, filter if necessary and dry (between two sheets of filter paper or using a desiccator or place in a low temperature oven).

Alternative method, using charcoal to remove indicator colour.

Follow steps 1-5:

1. After step 5, add 2 spatulas of decolourising charcoal to the conical flask.

2. Heat gently, using tripod, gauze, Bunsen burner and heatproof mat – do not boil.

3. Allow to cool slightly, then filter the mixture into an evaporating basin.

4. Place onto a tripod and gauze and heat gently using a Bunsen burner until about ½ of the solution.

5. When the crystals have formed, filter if necessary and dry (between two sheets of filter paper or using a desiccator or place in a low temperature oven).