During white-washing of walls, slaked lime reacts slowly with carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Which of the following reactions represents this correctly ?
[amp_mcq option1=”$\text{CaO (s) + CO}_2\text{ (g)} \rightarrow \text{CaCO}_3\text{ (s)}$” option2=”$\text{CaO (l) + CO}_2\text{ (g)} \rightarrow \text{CaCO}_3\text{ (s)}$” option3=”$\text{Ca(OH)}_2\text{ (l) + CO}_2\text{ (l)} \rightarrow \text{CaCO}_3\text{ (s)} + \text{H}_2\text{O (l)}$” option4=”$\text{Ca(OH)}_2\text{ (aq) + CO}_2\text{ (g)} \rightarrow \text{CaCO}_3\text{ (s)} + \text{H}_2\text{O (l)}$” correct=”option4″]
This question was previously asked in
UPSC CDS-2 – 2023
White-washing uses slaked lime, which is calcium hydroxide (Ca(OH)2), typically applied as an aqueous suspension or solution. This slaked lime reacts slowly with carbon dioxide (CO2) from the air to form calcium carbonate (CaCO3), a hard, white substance that coats the wall. Water is also produced in this reaction. The correct chemical equation representing this process is Ca(OH)2 (aq) + CO2 (g) $\rightarrow$ CaCO3 (s) + H2O (l).
Slaked lime (Ca(OH)2) is produced by adding water to quicklime (CaO). The reaction of slaked lime with atmospheric CO2 is a slow carbonation process that hardens the white-washed surface. The state symbols are important: slaked lime is in aqueous form (aq), CO2 is a gas (g), calcium carbonate formed is a solid (s), and water is a liquid (l).