Question
Answers

The deliquescent among the following is:
                   (A)$CaC{l_2}$
                   (B)$FeS{O_4}.7{H_2}O$
                   (C)$CuS{O_4}.5{H_2}O$
                   (D)$BaC{l_2}.2{H_2}O$

Answer Verified Verified
Hint: This substance has the ability to absorb water from surroundings and form a solution by dissolving in it. It is also used as an effective ice melter than rock salt (NaCl).

Complete answer:
-Hygroscopic substances are those ionic compounds that have the ability to becomes hydrates (absorb water) easily by absorbing the water molecules from water vapour in the air.
-The process by which any substance will absorb moisture from the atmosphere until it itself gets dissolved in this absorbed water and forms a solution is known as deliquescence. The materials which have this property are termed to be deliquescent materials.
-Deliquescence will occur only if the vapour pressure of the formed solution is less that the partial pressure of water vapour in air.
-Deliquescent materials are also a type of hygroscopic substance. The rest of the hygroscopic substances are not soluble in water.
-They are usually salts, for example: potassium hydroxide (KOH), sodium hydroxide (NaOH), ammonium chloride ($N{H_4}Cl$), calcium chloride ($CaC{l_2}$), etc.
-Deliquescent substances are used as desiccants because of such high affinity for water. They are also used to remove excess water from phosphoric and sulphuric acids. At the industrial level they are used to remove water from chemical reactions or various kinds of preparations.
-Calcium chloride ($CaC{l_2}$) is highly hygroscopic, attracts moisture from its surroundings very strongly and it releases a large amount of heat in an exothermic reaction on dissolving in water. Thus, making it a good deliquescent material.
It is formed by neutralising hydrochloric acid (HCl) with calcium hydroxide ($Ca{(OH)_2}$) and can be found in various hydrated forms of $CaC{l_2}.x{H_2}O$ where x can be 0, 1, 2, 4 or 6.
The rest options are already hydrated and thus their ability to absorb water is lesser than calcium chloride.
So, the correct answer is “Option A”.

Note: For any substance to be deliquescent it must have the ability to absorb a large amount of water and be sufficiently soluble to dissolve in it. Calcium chloride has both and also is exothermic with water which makes it highly useful in melting ice on sidewalks, roads, etc. It can cause depression in the freezing point and prevent freezing at temperatures even ($ - {52^ \circ }C$) low.