Question

When a crystal of $NaOH$ is exposed to air, a liquid layer is developed because:
(A) crystal of $NaOH$ melts
(B) it loses water to the surroundings
(C) it absorbs moisture and $CO_2$​ from the surroundings
(D) it undergoes sublimation

Answer 1

Hint: Different substances show different behavior on being exposed to air (atmosphere), based on which they are classified as hygroscopic, deliquescent, or efflorescent substances. The chemical name for $NaOH$ is sodium hydroxide, and it is deliquescent.

Complete step by step answer:
To understand the behavior of a substance on being exposed to air, let us first recall the definition of air or what air is. Air is a mixture of various gases such as $78\%$ of nitrogen, $21\%$ of oxygen, and $1\%$ of other gases such as carbon dioxide, water vapor, etc.
When a crystal of $NaOH$ is exposed to air, it can develop a liquid layer only when it absorbs the moisture or water vapor $H_2O$ present in it, and also gets dissolved in it. This property is called deliquescence.
Along with moisture, $NaOH$ also absorbs carbon dioxide from the air and forms sodium carbonate. It forms a solution of sodium carbonate due to the absorption of carbon dioxide from the atmosphere and its deliquescent nature. The reaction that $NaOH$ goes through on being exposed to air can be represented through the following equation:

$2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O$

Therefore, due to the absorption of moisture and $CO_2$​ from the surroundings, a liquid layer is developed on the surface of $NaOH$ crystal on being exposed to air.

So, the correct answer is Option C .

Note: Hygroscopic substance:
A substance which absorbs moisture from the atmosphere, but does not itself dissolve in it, is called a hygroscopic substance. For example - $NaCl, NaNO_3$, etc.

Deliquescent substance:
Deliquescence is the advanced form of hygroscopy. A substance that absorbs moisture from the atmosphere, and also gets dissolved in it, is called a deliquescent substance. For example - $NaOH\ ,\ CaCl_2$, etc.

Efflorescent substance:
An efflorescent substance is a substance that loses its moisture and becomes dry on being exposed to the atmosphere. Such substances are mostly compounds containing water of crystallisation, like, $Na_2SO_4.10H_2O\ ,\ Na_2CO_3.10H_2O$, etc.