BECE Science 2021 Paper

Concept:Permanent hardness is caused by dissolved chlorides and sulphates of calcium and magnesium, and it can be removed using sodium carbonate ($\mathrm{Na}_2\mathrm{CO}_3$).Explanation:Sodium carbonate, also known as washing soda or soda ash, reacts with these dissolved salts to form insoluble carbonates.For example: $\mathrm{CaCl}_2 + \mathrm{Na}_2\mathrm{CO}_3 \rightarrow \mathrm{CaCO}_3 \downarrow + 2\mathrm{NaCl}$.The insoluble carbonates, such as $\mathrm{CaCO}_3$ and $\mathrm{MgCO}_3$, can be removed by filtration, leaving the water soft.Thus, sodium carbonate ($\mathrm{Na}_2\mathrm{CO}_3$) is the chemical used to remove permanent hardness.Answer:Option B: $\mathrm{Na}_2\mathrm{CO}_3$