Which chemical compound is primarily present in the solution used for whitewashing walls?

Correct Option

The correct option is Calcium hydroxide

Explanation

Whitewashing involves the application of a lime-based solution to walls. The process begins with Quick lime (Cal, which reacts vigorously with water to form Slaked lime (Calcium hydroxide, Ca(OH)₂). This reaction is highly exothermic. The resulting aqueous solution of calcium hydroxide is the substance actually applied to the walls during whitewashing.

Analysis of Chemical Process

• Solution Preparation: When calcium oxide is mixed with water, it forms calcium hydroxide. Therefore, the solution in the bucket used for whitewashing contains Calcium hydroxide.
• Post-Application Reaction: After the calcium hydroxide solution is applied to the walls, it reacts slowly with carbon dioxide (CO₂) present in the air. This reaction forms a thin layer of Calcium carbonate (CaCO₃) over a period of two to three days, which provides the characteristic shiny white finish.

Option Analysis

• Calcium hydroxide Calcium hydroxide: Correct. This is the chemical compound present in the aqueous solution (slaked lime) used for application.
• Calcium oxide Calcium oxide: Incorrect. This is the raw material (quick lime) used to prepare the solution, not the compound present in the solution itself.
• Calcium carbonate Calcium carbonate: Incorrect. This is the solid product formed on the wall after the whitewash dries and reacts with air; it is not the solution used for application.
• Gypsum Gypsum: Incorrect. Gypsum (CaSO₄·2H₂O) is used in the production of Plaster of Paris and cement, not in whitewashing solutions.

Key Takeaway: The solution used for whitewashing is Slaked Lime (Calcium hydroxide). The shiny white finish obtained days later is due to the formation of Calcium carbonate.