Question

Which of the following are efflorescent substances?
This question has multiple correct options
A. \[CuS{O_4}.5{H_2}O\]
B. \[N{a_2}C{O_3}.10{H_2}O\]
C. \[{K_2}S{O_4}.A{l_2}{(S{O_4})_3}.24{H_2}O\]
D. \[{(N{H_4})_2}S{O_4}\]

Answer 1

Hint: In chemistry, efflorescence is the migration of a salt to the surface of a porous material, where it forms a coating. The essential process involves the dissolving of an internally held salt in water, or occasionally in another solvent. The water, with the salt now held in solution, migrates to the surface, then evaporates, leaving a coating of the salt.

Complete answer:
An efflorescent substance is a chemical which has water associated with its molecules, and which, when exposed to air, loses this water through evaporation. An example of an efflorescent substance is the common copper (II) sulfate crystal \[CuS{O_4}.5{H_2}O\], a blue crystalline solid that when exposed to air, slowly loses water of crystallization from its surface to crystallize to form a white layer of anhydrous copper (II) sulfate.

\[N{a_2}C{O_3}.10{H_2}O\] loses 9 water molecules first and then becomes completely anhydrous.

\[{K_2}S{O_4}.A{l_2}{(S{O_4})_3}.24{H_2}O\] is not an efflorescent as its crystals do not lose water of crystallization by efflorescence nor it is oxidised in the air. \[{(N{H_4})_2}S{O_4}\] do not have associated water molecules.

Note: A common example of efflorescent phenomenon is the drying of cement. When cement is poured, it goes through an exothermic chemical reaction, which generates heat, in order to set. Then the set concrete also has to dry out by evaporation. The heat causes various mineral salts which are in solution in the wet concrete to migrate to the surface, where the evaporation of the water leaves a fine layer of crystals on the face of the concrete.