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When two compounds react, if their ions are interchanged, the reaction is called a double displacement reaction.
Ions with identical charges can only be interchanged, which means that a cation can be replaced by another cation. This reaction is also called ‘Metathesis reaction’.
Many displacement reactions occur between the ionic compounds that are dissolved in water. A double displacement reaction is shown schematically as follows:
Double displacement reaction
Atoms from two different compounds swap places in a double displacement reaction. The two reactants and products are different compounds. For example:
Double displacement reaction
In a double displacement reaction, one of the products must be either a precipitate or water. In this way, double displacement reaction is further classified as follows:

(i) Precipitation reactions
(ii) Neutralization reactions
Precipitation reactions:
When aqueous solutions of two compounds are mixed, if they react to form an insoluble compound and a soluble compound, it is called a precipitation reaction.
Since one of the products is an insoluble compound, the reaction is called a precipitation reaction.
Mixing the clear aqueous solutions of potassium iodide with lead (II) nitrate is an example of a double displacement reaction.
Precipitation of \(PbI_2\)
In this reaction, potassium and lead displace or replace one another and form a yellow precipitate of lead (II) iodide.
Neutralization reactions:
The reaction between an acid and a base result in the formation of salt and water is called a neutralisation reaction. This is another type of displacement reaction in which both acid and base neutralise each other.
Example: \(1\)

A common neutralisation reaction is the reaction of sodium hydroxide with hydrochloric acid. Here, sodium replaces hydrogen from hydrochloric acid, forming sodium chloride (neutral soluble salt).
\(NaOH_{(aq)} + HCl_{(aq)} → NaCl_{(aq)} + H_2O_{(l)}\)
Neutralisation reaction
Example: \(2\)
The reaction of ammonium hydroxide with nitric acid forms ammonium nitrate and water.