Which of the following are efflorescent substances?

The correct option is D

Both A and B

The correct option is D.

Explanation for correct option:

• ${\mathrm{CuSO}}_{4}5{\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}$ are examples of efflorescent substances.
• Efflorescent substances are substances that lose the water molecules when exposed to air due to evaporation. They form the corresponding salts.
• ${\mathrm{CuSO}}_{4}5{\mathrm{H}}_2\mathrm{O}$ - Copper II sulphate is a blue crystalline solid that when exposed to air loses the water molecules slowly. As a result, it crystallizes to white anhydrous copper sulphate salt.
• ${\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}$ loses 9 water molecules and becomes completely anhydrous.

Explanation for incorrect options:

Option C:

$\mathrm{K}2{\mathrm{SO}}_4.{\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3.24{\mathrm{H}}_2\mathrm{O}$ is a common alum. Its crystals do not lose water molecules by efflorescence.

Final answer: ${\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}$ are efflorescent substances.