CSEC Chemistry: Preparation of Soluble Salts

The method used to prepare a certain salt is dependent on the solubility of the salt in water. So, it is necessary that you first know the solubility rules:

Nitrates- All nitrates are soluble

Chlorides, Bromides and Iodides (Halides)- All halides are soluble, except for the halides of silver and lead.

Sulphates- all sulphates are soluble except for barium sulphate and lead sulphate. Calcium sulphate and silver sulphate are slightly soluble.

Carbonates- all carbonates are insoluble, except for the carbonates of sodium, potassium and ammonium

Hydrogencarbonates- most soluble

All ammonium salts are soluble

All common salts containing group I metals are soluble (sodium, potassium...)

Preparing soluble salts

Soluble salts can be prepared by:

1) Reacting acids and alkalis:

NaOH(aq) +HNO3(aq) → NaNO3(aq) + H2O(l)

or

2) Reacting acids and excess reactive metals, insoluble metal oxides/hydroxides, metal carbonates/hydrogencarbonates:

Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)

Mg(OH)2(s) + 2HNO3(aq) → Mg(NO3)2(aq) + 2H2O(l)

2NaHCO3(s) + H2SO4(aq) → Na2SO4(aq) +2H2O(l) +2CO2(g)

Whenever preparing a salts using an insoluble solid and an acid, the solid is usually added in excess. This is to ensure that all of the acid is used up in the reaction. Under these conditions, the acid would be the limiting reagent.

Once the reaction is completed, the excess solid (unreacted solid) is removed by filtration.

The filtrate, (what gets through the filter paper) can be used to gain the salt using the following procedure:

1. Concentrate the filtrate by gentle evaporation using a water bath and evaporation dish.

2. Cool the concentrate slowly (this is how the salt crystals are formed)

3. Filter the concentrate and collect the crystals.

4. Gently wash and dry the crystals.

Water of Crystallization

During crystallization, water molecules may become a part of the crystal lattice.

This water of crystallization is included as a part of the formula of the salt like this:

CuSO4 • 5H2O - Copper (II) Sulphate pentahydrate

The dot followed by the number of H2O molecules indicates water of crystallization.

All we do when naming these hydrated salts is state the name of the salt followed by -hydrate with a prefix depending on the number of water molecules:

1- mono

2- di

3- tri

4- tetra

5- penta

6- hexa

7- hepta

8- octo

9- nona

10- deca

However, if a hydrated salt (salt with water of crystallization) is evaporated to complete dryness, a powdery, non-crystalline form of the salt is produced. This salt is called an anhydrous salt, as it has no water of crystallization.