Which of the following substance is used for white washing the walls?

Slaked lime

The correct answer is option 2. Slaked lime.

A solution of slaked lime (Calcium hydroxide) is used for whitewashing walls.

Calcium hydroxide reacts slowly with the carbon dioxide in the air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of whitewashing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO₃.

Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O(l)