Question

Which of the following is not hygroscopic?
A.NaCl
B.${{MgC}}{{{l}}_{{2}}}$
C.${{CaC}}{{{l}}_{{2}}}$
D.LiCl

Answer 1

Hint:Hygroscopy is the property of a substance to absorb the moisture present in the air. Such types of substances are known as hygroscopic substances. For example, insulating materials that are produced from cotton or paper are hygroscopic.

Complete step by step answer:
When hygroscopic salts are left out in the air they tend to absorb moisture present in the air. Pure sodium chloride or common salt does not attract water molecules just from the air.
Hygroscopic compounds tend to absorb moisture from the air due to many reasons. Some of them are:

-Ionic compounds tend to absorb water because of their ability to form stable hydrates.
-Metal cations tend to attract the lone pair of oxygen of the water molecule and form a coordination bond between metal and oxygen atom.
-Similarly, anions like sulfates or nitrates can form strong hydrogen bonds with water molecules.
-Water molecules can diffuse into the hygroscopic material and get trapped inside, especially when the structure of the salt has voids inside them.
-Salts or materials with porous structure also tend to absorb water molecules.
Hence, the correct answer is option A.


Note:
Once the salt absorbs moisture, the water gets to combine with the salt and gets dissolved in it. The resultant salt-water complex gets mixed with more of the salt thus cementing the grains of the salt together into a solid mass.
Hygroscopic salts are used as desiccants. These are used to remove moisture from the surrounding in case an atmosphere free from moisture is required for some purpose.
Some materials also change color after absorbing water which is due to the change in their structure.