Question

Which of the following are efflorescent substances?

• A. $Cu{SO}_4.5H_{2}O$
• B. ${Na}_2{CO}_3.10H_{2}O$
• C. $K_2{SO}_4.{Al}_2{\left({SO}_4\right)}_3.24H_{2}O$
• D. ${\left({NH}_4\right)}_2{SO}_4$

Answer 1

Answer: (A), (B)

The correct options are
A $Cu{SO}_4.5H_{2}O$
B ${Na}_2{CO}_3.10H_{2}O$

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 $\left(CuS{O}_4.5H_{2}O\right)$, 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.

$N{a}_{2}C{O}_3.10H_{2}O$ loses 9 water molecules first and then becomes completely anhydrous.

$K_2{SO}_4.{Al}_2{\left({SO}_4\right)}_3.24H_{2}O$ is not an efflorescent as its crystals do not lose water of crystallization by efflorescence nor it is oxidised in the air. ${\left({NH}_4\right)}_2{SO}_4$ do not has associated water molecules.