WAEC Chemistry 2011 Paper

Concept:Excess carbon (IV) oxide converts the initially formed calcium carbonate precipitate into soluble calcium hydrogencarbonate.Explanation:When carbon (IV) oxide is passed into calcium hydroxide solution, a white precipitate of calcium carbonate is first formed. The reaction is: $$\mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3 \downarrow + H_2O}$$ If excess $\mathrm{CO_2}$ is bubbled through the solution, this insoluble precipitate reacts further. The calcium carbonate dissolves in the presence of excess carbon (IV) oxide and water to form calcium hydrogencarbonate. The equation for this step is: $$\mathrm{CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2}$$ Therefore, the final product obtained is not $\mathrm{CaCO_3}$, but soluble calcium hydrogencarbonate, $\mathrm{Ca(HCO_3)_2}$.Answer:B. $\mathrm{Ca(HCO_3)_2}$