BECE Science 2021 Paper

Concept:Calcium oxide reacts with water in a combination reaction to produce a single product called calcium hydroxide.Explanation:Calcium oxide has the formula $\mathrm{CaO}$.Water has the formula $\mathrm{H}_2\mathrm{O}$.When they react, calcium combines with the hydroxide group $\mathrm{OH}$.Calcium has a valency of $+2$, and each hydroxide group has a valency of $-1$.Therefore, two hydroxide groups are needed to balance the charge.This gives the product formula $\mathrm{Ca}(\mathrm{OH})_2$.The balanced chemical equation is:$$\mathrm{CaO} + \mathrm{H}_2\mathrm{O} \longrightarrow \mathrm{Ca}(\mathrm{OH})_2$$Answer:$\mathrm{Ca}(\mathrm{OH})_2$