Question

Does colour change occur in a double displacement reaction?

Answer 1

Double displacement reaction:

• Those reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reactions.
• Double displacement reactions typically result in the formation of a product that is a precipitate.

For example: When ferric oxide reacts with hydrogen chloride, it will form ferric chloride as a precipitate (represented by a downward arrow) and water.

Double displacement reaction in which colour change take place.

$\underset{\mathrm{Ferric}\mathrm{oxide}}{{\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)}+\underset{\mathrm{hydrogen}\mathrm{chloride}}{6\mathrm{HCl}\left(\mathrm{l}\right)}\to \underset{\mathrm{ferric}\mathrm{chloride}}{2{\mathrm{FeCl}}_3\left(\mathrm{s}\right)\downarrow }+\underset{\mathrm{water}}{3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

• A double replacement reaction may or may not be resulting in a colour change.
• From the above reaction, the ferric oxide is red-brown in colour and hydrogen chloride is colourless but after the reaction, the precipitate of ferric chloride formed is a black colour compound.

Double displacement reaction in which colour change does not take place.

For example, This is a precipitation reaction: CaSO₄ is the formed precipitate. CaCl₂ and Na₂SO are white in colour and form white solid CaSO precipitate and NaCl crystals.

$$\begin{gathered} {\mathrm{CaCl}}_2\left(\mathrm{aq}\right)+{\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{CaSO}}_4\left(\mathrm{s}\right)\downarrow +2\mathrm{NaCl}\left(\mathrm{aq}\right) \\ \mathrm{calcium}\mathrm{sodium}\mathrm{calcium}\mathrm{sodium} \\ \mathrm{chloride}\mathrm{sulphate}\mathrm{sulphate}\mathrm{chloride} \end{gathered}$$

Therefore no colour change occurred.