Question

A double displacement reaction is a reaction where ____ reactants exchange their ions to form ____ new compounds in the product.

• A. two, one
• B. two, two
• C. four, two
• D. four, four

Answer 1

The correct option is B two, two

During a double displacement reaction, two reactants exchange their ions (cations and anions) to form two new compounds.
Generalised form of a double displacement reaction is :
$A^{+}{B}^{-}+C^{+}{D}^{-}\to A^{+}{D}^{-}+C^{+}{B}^{-}$Here the positive ions ($A^{+}$ and $C^{+}$) are the cations and the negative ions ($B^{-}$ and $D^{-}$) are the anions. The ions get exchanged because they have more attraction towards oppositely charged ion of other reactant than their own counterpart.
For example,
${\mathrm{BaCl}}_2\left(\mathrm{aq}\right)+{\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\stackrel{\Delta }{\to }{\mathrm{BaSO}}_4\downarrow \left(s\right)+2\mathrm{NaCl}\left(\mathrm{aq}\right)$
In this example, the cations are Ba${}^{2+}$ and Na${}^{+}$ and the anions are Cl${}^{-}$ and SO${}_4$${}^{2-}$. Just swap the anions or the cations and we get our products BaSO${}_4$ and NaCl.
Here a white precipitate of BaSO${}_4$ gets formed.
So such kind of double displacement reaction in which a precipitate is also formed is called precipitation reaction.