Question

Which of the following is an example of efflorescent substance?
A. Calcium chloride
B. Glauber’s salt
C. Sodium hydroxide
D. Iron chloride

Answer 1

Hint: We know that efflorescent substances readily lose their water molecules when exposed to heat or air. The chemical formulas for the given compounds are:
Calcium chloride- \[{\mathbf{CaC}}{{\mathbf{l}}_{\mathbf{2}}}\], Glauber’s salt-\[{\mathbf{N}}{{\mathbf{a}}_{\mathbf{2}}}{\mathbf{S}}{{\mathbf{O}}_{\mathbf{4}}}.{\mathbf{10}}{{\mathbf{H}}_{\mathbf{2}}}{\mathbf{0}}\], Sodium hydroxide-\[\;{\mathbf{NaOH}}\] and Iron Chloride-\[{\mathbf{FeC}}{{\mathbf{l}}_{\mathbf{3}}}\].

Complete step by step answer:
There are three properties of compounds that are mostly confused together with:
Deliquescence is the property by which the compound absorbs moisture and gets wet itself. Like common salt \[\left( {NaCl} \right).\]
Hygroscopicity is the property by which the compound absorbs water molecules and forms its hydrate or hydroxide. Compounds like\[\:CaO\] (lime) and \[N{a_2}C{O_3}\](anhydrous) show this property.
Efflorescence is the property of a compound due to which it loses its water molecules when exposed to heat or air. Efflorescences can occur in natural and built environments. When a precipitation occurs directly on the surface, it is termed efflorescence. One of the most common examples of efflorescent substances is Copper sulphate pentahydrate or blue vitriol \[(CuS{O_4}.5{H_2}O).\]
We deal with the efflorescent property of the given compounds here.
Among the given salts or compounds, only Glauber’s salt \[(N{a_2}S{O_4}.10{H_2}0)\]shows this property, as it has \[10\] molecules of water which it can lose when exposed to air or open environment. This salt readily forms anhydrous Sodium Sulphate \[(N{a_2}S{O_4})\]when exposed to air.
So, the correct answer is “Option B”.

Note: The water molecules that are bonded with the molecules of compound or salt, and that can be removed by heating the salt are known as water of crystallization or water of hydration. These molecules are responsible for the efflorescent property of the compound.