Question

Demonstrate knowledge and understanding of
preparation, separation and purification of salts
as examples of some of the techniques specified
in section 2.2.2 and the reactions specified in
section 8.1

Answer 1

Solubility of the common salts

Solubility of the common salts, CIE IGCSE Chemistry

Preparing soluble salts

Method A: adding acid to a solid metal, base or carbonate.

Preparation of soluble salts, CIE IGCSE Chemistry

Diagram Showing the Filtration of the Mixture to Remove the Excess Base

Method:

Add dilute acid into a beaker and heat using a bunsen burner flame.

Add the insoluble metal, base  or carbonate, a little at a time, to the warm dilute acid and stir until the base is in excess (i.e. until the base stops disappearing and a suspension of the base forms in the acid).

Filter the mixture into an evaporating basin to remove the excess base.

Heat the solution to evaporate water and to make the solution saturated.

Check the solution is saturated by dipping a cold, glass rod into the solution and seeing if crystals form on the end.

Leave the filtrate in a warm place to dry and crystallize.

Decant excess solution and allow crystals to dry.

Preparation of Pure, Hydrated Copper (II) Sulfate Crystals using Method A

Prep of Copper (II) Sulfate Crystals, CIE IGCSE Chemistry

Diagram showing the heating of the mixture to leave a saturated solution

Acid = Dilute Sulfuric Acid                     Insoluble Base = Copper (II) Oxide

Method:

Add dilute sulfuric acid into a beaker and heat using a bunsen burner flame.

Add copper (II) oxide (insoluble base), a little at a time to the warm dilute sulfuric acid and stir until the copper (II) oxide is in excess (stops disappearing).

Filter the mixture into an evaporating basin to remove the excess copper (II) oxide.

Leave the filtrate in a warm place to dry and crystallize.

Decant excess solution.

Blot crystals dry.

Equation Of Reaction:

Copper (II) Oxide    +    Dilute Sulfuric Acid   →       Copper (II) Sulphate    +    Water

       CuO (s)                             H2SO4 (aq)                             CuSO4 (s)                 H2O (l)

Method B: reacting a dilute acid and alkali.

Titration Apparatus, CIE IGCSE Chemistry

Diagram showing the apparatus needed to prepare a salt by titration

Method:

Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (phenolphthalein or methyl orange).

Add the acid into the burette and note the starting volume.

Add the acid very slowly from the burette to the conical flask until the indicator changes to appropriate colour.

Note and record the final volume of acid in burette and calculate the volume of acid added (starting volume of acid – final volume of acid).

Add this same volume of acid into the same volume of alkali without the indicator.

Heat to partially evaporate, leaving a saturated solution.

Leave to crystallise decant excess solution and allow crystals to dry.