O Level Notes : Chemistry - Salts

Solubility:

All nitrates, and all ammonium, sodium and potassium salts are soluble in water. Most chlorides and sulphates are also soluble, while the majority of carbonates are insoluble.

Learning the solubility of salts is important before learning how the salts are formed. The solubilities in detail are:

Nitrates: All soluble

Chlorides: All soluble, EXCEPT silver chloride AgCl, lead (II) chloride PbCL₂, and mercury (II) chloride HgCl₂.

Sulphates: All soluble, EXCEPT calcium sulphate CaSO₄, barium sulphate BaSO₄ and lead (II) sulphate PbSO₄.

Carbonates: All insoluble, except group I carbonates and ammonium carbonate.

Preparation:

We will learn three methods to prepare salts. Before deciding which method to use, we must know the answer to these two questions.

1. Is the salt to be prepared soluble in water?
2. Are the starting materials soluble in water?

Method 1:

This method is used if the salt to be prepared is soluble in water, while the starting materials are insoluble.

For example, let’s see how to prepare zinc sulphate. We learnt that it is a soluble salt. Let’s think of some starting materials for it now. For the ‘sulphate’ part, we could use Sulphuric acid H₂SO₄. For the ‘zinc’ part, we could use the metal zinc itself.

Zn(s) + H₂SO₄ ZnSO₄ + H₂

In this method, we react the acid with an excess of the substance. Zinc is an insoluble metal. All the acid will be used up as it reacts with the zinc, while the excess zinc can be filtered out.

What we do now is that we take the Sulphuric acid in a beaker and add zinc into the acid while stirring. We keep adding the metal until no more effervescence is observed. Now we have in the beaker zinc sulphate solution and unreacted zinc. We filter the mixture to get rid of the excess metal, and collect the filtrate. This filtrate is only the zinc sulphate solution. Now we heat this filtrate to concentrate the solution, and then leave it to cool so that it can crystallize. These crystals are zinc sulphate crystals. Wash these crystals with distilled water to remove impurities, and dry between sheets of filter paper. These are now pure zinc sulphate crystals.

Method 2:

This method, known as titration, is suitable for preparing salts that are soluble, and their starting materials are also soluble.

For example, let’s see how to make sodium chloride. It is a soluble salt. We can make it with the alkali sodium hydroxide, and the acid hydrochloric acid.

NaOH (aq) + HCl (aq) NaCl (aq) + H₂O

First, we add the sodium hydroxide in a beaker, while the hydrochloric acid is added to the burette, which is clamped to a stand and is placed vertically above the beaker. We add some indicator ‘phenolphthalein’ to the beaker. Phenolphthalein is colorless in acid and neutral solutions while pink in alkali, so it turns pink in the beaker. We open the burette tap to slowly let the acid flow into the burette and watch closely for a color change in the beaker. As soon as it turns colorless, we stop the burette tap and record the reading. In the beaker, we have now sodium chloride salt, as well as the impurity phenolphthalein. As we want only the pure salt, we repeat this whole experiment again, this time without the indicator. We add the same amount of alkali into the beaker, and put in the same volume of the acid that we noted in the first experiment.

Now that we have the salt, we repeat the same processes that were used in the first method. We concentrate the salt by heating it, and then let it cool and dry so that it can crystallize. We filter and collect the crystals, wash them with distilled water, and then dry between sheets of filter paper. We now have pure crystals of sodium chloride.

Method 3:

This method is known as ‘precipitation’. This method is used to prepare insoluble salts. All insoluble salts can be prepared using this method.

For example, let’s learn how to make lead (II) sulphate, PbSO₄.

We will first mix two solutions: one that contains the positive ions, and the other that contains the negative ions of the salt to be prepared. We can use lead nitrate for the lead part, and Sulphuric acid for the sulphate part.

This method, you will learn, is the simplest method. We add the lead (II) nitrate solution to a beaker, and add Sulphuric acid (in excess) and stir until no more precipitate forms. Then, filter to collect the precipitate, which is actually our salt lead (II) sulphate. To remove impurities, wash with distilled water, and then dry between filter papers.

Qualitative Analysis:

The process of identification of cations and anions is called qualitative analysis or salt analysis. We identify which cations (positive ions) are present in a salt by using aqueous sodium hydroxide, and aqueous ammonia. For cation identification, learn the following table.

To identify anions, learn these reactions:

CO₃^2- (carbonate):

Test: add dilute hydrochloric acid. Pass the gas given off into limewater.

Observation: effervescence is observed. Gas given off forms a white ppt with lime water. Carbon dioxide gas is given off.

Cl^- (chloride):

Test: add dilute nitric acid, then add silver nitrate solution.

Observation: a white ppt of silver chloride is formed.

I^- (iodide):

Test: add dilute nitric acid, then add silver nitrate solution

Observation: a yellow ppt of silver iodide is formed.

NO₃^- (nitrate):

Test: add dilute sodium hydroxide. Then add a piece of aluminium foil. Warm the mixture. Test the gas given off with a piece of moist red litmus paper.

Observation: the moist red litmus paper turns blue. Ammonia gas is given off.

SO₄^2- (sulphate):

Test: add dilute nitric acid, then add barium nitrate solution.

Observation: a white ppt of barium sulphate is formed.