Calcium oxide reacts vigorously with water to produce slaked lime whose solution is used for white-washing walls. This slaked lime reacts with component (A) in air to form a thin layer of component (B) to give a shiny finish. What are the components (A) and (B) ?

The initial reaction is CaO(s) + H₂O(l) → Ca(OH)₂(aq). This slaked lime solution is used for white-washing. The slaked lime then reacts with atmospheric carbon dioxide: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). Component (A) from the air is carbon dioxide (CO₂), and the component (B) formed as a thin layer on the wall is calcium carbonate (CaCO₃). This layer of calcium carbonate gives the walls a smooth, shiny finish after a few days as the reaction completes and the water evaporates.