The acidity of calcium hydroxide is _______.
(A) 2
(B) 1
(C) 3
(D) 4

Hint: On determining pH of a solution, we can find out whether the substance is acidic or basic in nature. So, the number of \[{H^ + }{\text{ }}or{\text{ }}O{H^ - }\] ions are the factors responsible for calculating the acidity or basicity of any compound.

Complete step-by-step solution:
Calcium hydroxide is commonly known as slaked lime. It is an inorganic compound having formula \[{\text{Ca(}}OH{)_2}\] and is colourless in its hexagonal crystalline form. There exists an ionic bond between the calcium and two hydroxide ions. Direct exposure to concentrated \[{\text{Ca(}}OH{)_2}\] can affect lungs and might even lead to blindness.
It is chemically very reactive. When it is dissolved in water till a saturation point, it produces a solution called limewater (CaO), which acts as a moderate base. Limewater reacts with many acids and forms salts. The saturated solution of calcium hydroxide in water can also react with metal and dissolves them. For example, aluminium. It also reacts with carbon dioxide and forms calcium carbonate \[CaC{O_3}\]. This reaction is commonly known as carbonatation.
\[Ca{(OH)_2} + C{O_2} \to CaC{O_3} + {H_2}O\]
\[CaO + {H_2}O \to Ca{(OH)_2}\]
Calcium hydroxide releases hydroxide ions when dissolved in aqueous solutions. The liberation of \[O{H^ - }\] ions due to electrocyclic dissociation confirms that it is a base. The number of ionizable hydroxide ions that are present in one molecule of base is termed as the acidity of bases. On that basis, they are classified into three different types: monoacidic, diacidic and triacidic.
Therefore, in case of calcium hydroxide, it has two hydroxide ions which dissociate when dissolved in an aqueous medium. This makes it acidic in nature and results in the acidity of the compound.
As there are 2 hydroxide ions present in the solution, the acidity of caustic soda is 2. It is a diacidic base.

Hence, the correct option is (A).

Note: As acidity of a base depends upon the number of free hydroxyl ions released in the solution, similarly we can find out the basicity of an acid which depends upon the number of free hydrogen ions released in the solution when dissolved in water. For example, the basicity of \[{H_3}P{O_4}\] is 3.