Question

Rahul took 25 ml of sea water in a beaker and added one spoon of sodium carbonate to it. Which of the following will he observe?

• A. Bubbles are formed
• B. Precipitate is formed
• C. Solution colour changes to blue
• D. Solution colour changes to red

Answer 1

The correct option is B Precipitate is formed

Sea water is basically hard water. It contains dissolved salts of calcium ions, $\left({\text{Ca}}^{2+}\right)$ or magnesium ions $\left({\text{Mg}}^{2+}\right)$.
Hard water on reaction with sodium carbonate $\left({\text{Na}}_2{\text{CO}}_3\right)$ forms calcium carbonate or magnesium carbonate precipitate. This precipitate can be filtered out and soft water is obtained.
One of the method to remove it is by treating the hard water with washing soda, i.e., $N{a}_{2}C{O}_3$. This forms their respective insoluble carbonates which are removed by filtration.
The reactions involved are:

${\text{Na}}_2{\text{CO}}_3\text{(s)}+{\text{CaSO}}_4\text{(aq)}\to {\text{CaCO}}_3(\downarrow )+{\text{Na}}_2{\text{SO}}_4\text{(aq)}$

${\text{Na}}_2{\text{CO}}_3\text{(s)}+{\text{MgCl}}_2\text{(aq)}\to {\text{MgCO}}_3(\downarrow )+\text{2NaCl(aq)}$