Question

How can I identify double displacement reactions?

Answer 1

As the name suggested that in this reaction double (two) displacement of species like cation and anion takes place and in this reaction precipitate or gas, or water is formed as a result of displacement.

Complete step by step solution:
In double displacement reaction, whether the displacement of two cations observed or the displacement of two anions are observed in between the two compounds which are present as reactant in the chemical reaction.
-In general double displacement reaction is shown as follow:
${{\text{A}}^{\text{ + }}}{{\text{B}}^{\text{ - }}}{\text{ + }}{{\text{C}}^{\text{ + }}}{{\text{D}}^{\text{ - }}} \to {{\text{A}}^{\text{ + }}}{{\text{D}}^{\text{ - }}}{\text{ + }}{{\text{C}}^{\text{ + }}}{{\text{B}}^{\text{ - }}}$
In the above reaction two compounds are present as reactant on the left hand side and in between these two compounds double displacement of ${{\text{A}}^{\text{ + }}}$cation from the 1st compounds and ${{\text{C}}^{\text{ + }}}$ cation from 2nd compounds of the reactant takes place.
-From the definition of double displacement reaction it is clear that in this reaction product is always a gas, or a precipitate, or water.
So one example of double displacement reaction is given below through which we get an idea of identification of this type of reaction:
-When aqueous silver nitrate (${\text{AgN}}{{\text{O}}_{\text{3}}}$) reacts with the aqueous solution of sodium chloride (${\text{NaCl}}$) then formation of silver chloride (${\text{AgCl}}$) precipitate takes place with the formation of aqueous sodium nitrate (${\text{NaN}}{{\text{O}}_{\text{3}}}$), and chemical reaction for this double displacement reaction is shown as follow:
${\text{AgN}}{{\text{O}}_{{\text{3}}\left( {{\text{aq}}} \right)}}{\text{ + NaC}}{{\text{l}}_{\left( {{\text{aq}}} \right)}} \to {\text{AgC}}{{\text{l}}_{\left( {\text{s}} \right)}}{\text{ + NaN}}{{\text{O}}_{{\text{3}}\left( {{\text{aq}}} \right)}}$
And in this reaction double displacement of silver cation (${\text{A}}{{\text{g}}^{\text{ + }}}$) and sodium cation (${\text{N}}{{\text{a}}^{\text{ + }}}$) gets exchanged.

Note: Here some of you may do wrong identification if you only notice the double displacement of ions in the reactant side and will predict that it is a displacement reaction, because it is not necessary that every reaction after displacement will form gas or water or precipitate which are the products of double displacement reaction.