Question

Which of the following substances is hygroscopic?
(A)- Cryolite
(B)- Feldspar
(C)- Anhydrous calcium chloride
(D)- Slag

Answer 1

Hint: Hygroscopic substances are the substances which readily absorb water from the surroundings. Some hygroscopic substances may absorb moisture on their surfaces. Ionic anhydrous salts are generally hygroscopic in nature.


Complete step by step solution:
Let us examine all the given options one by one to find out the hygroscopic substance.
Cyrolite
Chemical formula of cryolite is $N{{a}_{3}}Al{{F}_{6}}$. It has a refractive index close to that of water and thus when dissolved in water, it becomes invisible. However, it is not hygroscopic in nature.
Feldspar
Feldspar refers to a group of minerals consisting of aluminosilicates of sodium, calcium or potassium. Feldspar minerals constitute almost half of the earth’s crust. Formula of potassium feldspar is $KAlS{{i}_{3}}{{O}_{8}}$. These minerals do not readily absorb water and thus are not hygroscopic.
Anhydrous calcium chloride
Anhydrous salts do not have any water of crystallization and therefore, such salts can readily absorb water to form hydrated salts. Anhydrous calcium chloride ($CaC{{l}_{2}}$) is hygroscopic in nature. Calcium chloride is generally found in nature as hydrated salt. Anhydrous $CaC{{l}_{2}}$ absorbs moisture from its surroundings and, so it is used as a desiccant to remove moisture for special purposes.
Slag
Slag is a mixture that is left after the extraction of a metal. It usually contains silicon oxides, alumina and other metal oxides. It also contains silica and alumina in large proportions and thus, it is also not hygroscopic.

Anhydrous $CaC{{l}_{2}}$ is hygroscopic and hence, the correct option is (C).

Note: Anhydrous salts are the salts which have their water of hydration removed generally by heating. Hydrated salts have water in their crystal structure which is known as water of crystallization.