Salt Preparation Direct Combination and Double Decomposition

Some soluble salts are prepared through the reaction of a metal with a non-metal.

Anhydrous aluminium chloride and anhydrous iron (III) chloride are examples of such salts.

These salts can be prepared by directly combining a metal and a non-metal. So the method is called direct combination or synthesis.

Let’s look at the preparation of anhydrous iron chloride by direct combination.

First, iron powder is taken in a combustion tube.

Then, dry chlorine gas is passed through the combustion tube. As a result, the air inside the apparatus is expelled.

A reaction occurs between iron and chlorine to give iron chloride.

Since the reaction is exothermic, the iron turns red hot.

The iron chloride volatilizes and then condenses in the receiver as a solid.

Since it is deliquescent (easily becomes a liquid), it is kept dry using fused calcium chloride.

This is what anhydrous iron chloride looks like. When hydrated, it would acquire a yellow colour.

Moving on, let’s study the preparation of salts by double decomposition.

A double decomposition reaction involves the exchange or switching of the ions between two reactants.

Insoluble salts can be prepared by mixing two different salts to give an insoluble solid precipitate.

This is why double decomposition method is also called precipitation.

Let’s look at the preparation of lead chloride and calcium carbonate by double decomposition (precipitation).

Lead chloride can be prepared through the reaction of dilute hydrochloric acid with lead nitrate.

Alternately, it can also be prepared through the reaction of sodium chloride with lead nitrate.

In this method, the solution of sodium chloride is first added to a solution of lead nitrate.

The resulting solution is filtered to obtain a white residue of lead chloride.

The lead chloride is washed repeatedly with water. It is then taken in a china dish along with some water and heated.

Eventually, lead chloride dissolves. The solution is then cooled.

As a result, needle-shaped crystals of pure lead chloride are obtained.

Now, let’s see how calcium carbonate is prepared by double decomposition.

First, a solution of sodium carbonate is added to a hot solution of calcium chloride.

As a result, a white precipitate of calcium carbonate is formed. This is repeatedly washed with cold water and then dried.

A white amorphous powder of calcium carbonate is thus obtained.

Revision

Some soluble salts like anhydrous aluminium chloride and anhydrous iron (III) chloride are prepared through the reaction of a metal with a non-metal.

The method is called direct combination or synthesis.

To prepare iron chloride, iron powder is taken in a combustion tube and dry chlorine gas is passed through the tube.

A reaction occurs between iron and chlorine to give iron chloride.

Insoluble salts like lead chloride and calcium carbonate can be prepared by mixing two different salts.

This method is called double decomposition or precipitation.

Lead chloride can be prepared through the reaction of dilute hydrochloric acid or sodium chloride with lead nitrate.

Calcium carbonate is prepared through the reaction of sodium carbonate and calcium chloride.

The End