Question

When $CO_2$​ is passed through lime water, it turns milky. The milkiness is due to the formation of:
(A) $CaCO_3$​
(B) $Ca(OH)_2$​
(C) $H_2​O$
(D) $CO_2$​

Answer 1

Hint: Lime water is the common name for the solution of calcium hydroxide. So, the chemical formula for lime water is $Ca(OH)_2$. Milkiness is the turning of a clear solution into a turbid solution. So, when a clear solution turns turbid, it appears milky.

Complete step by step answer:
When carbon dioxide $(CO_2)$​ is passed through lime water or a solution of calcium hydroxide, it turns milky due to the formation of calcium carbonate. When carbon dioxide reacts with lime water, it dissolves in the water of the calcium hydroxide solution to form carbonic acid. The reaction can be represented as follows:

$CO_2 + H_2O \rightarrow H_2CO_3$

Now, it is the carbonic acid that reacts with calcium hydroxide. Calcium hydroxide is basic. Carbonic acid reacting with the base neutralizes it and forms the salt calcium carbonate. It is an example of a neutralization reaction which takes place as follows:

$H_2CO_3 + Ca(OH)_2 \rightarrow CaCO_3 + 2H_2O$

The milkiness appears because calcium carbonate does not dissolve in water. It remains insoluble in water due to which turbidity occurs and the solution appears milky.

So, the correct answer is Option A .

Note: The milkiness from the calcium carbonate solution can be removed and a clear solution can be restored by passing carbon dioxide through it.

The overall reaction between carbon dioxide and lime water can be represented through the following chemical equation:

$CO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O$
Neutralization Reaction – When an acid and a base react, they form a neutral compound called salt, with no acidity or basicity, along with water. It is a specific type of double displacement reaction. In general form, it can be represented as follows:

$Acid + Base \rightarrow Salt + Water$