BECE Science 2022 Paper

Concept:Hardness of water is caused by dissolved calcium and magnesium ions.Explanation:These ions react with soap to form an insoluble scum, reducing lathering.Among the given options, $\mathrm{Mg}^{2+}$ is the hardness-causing ion.$\mathrm{Ca}^{2+}$ also causes hardness, but it is not listed in the options.$\mathrm{Na}^{+}$, $\mathrm{CO}_3^{2-}$, and $\mathrm{OH}^{-}$ do not contribute to water hardness.Hence, the ion that causes hardness is $\mathrm{Mg}^{2+}$.Answer:B. $\mathrm{Mg}^{2+}$