Which of the following are efflorescent substances?

This question has multiple correct options

• A. CuSO₄·5H₂O
• B. Na₂CO₃·10H₂O
• C. K₂SO₄·Al₂(SO₄)₃·24H₂O
• D. (NH₄)₂SO₄

Efflorescent substances are those that lose water of crystallization when exposed to air, often resulting in a powdery residue. Among the options provided, the following are considered efflorescent:

Efflorescent Substances

• CuSO₄·5H₂O - Commonly known as copper(II) sulfate pentahydrate, it readily loses water and turns into a white powder.
• Na₂CO₃·10H₂O - This is sodium carbonate decahydrate, which also loses water when exposed to air.

Non-Efflorescent Options

The other options do not typically exhibit efflorescence:

• K₂SO₄·Al₂(SO₄)₃·24H₂O - This compound is less likely to lose water in the same manner.
• (NH₄)₂SO₄ - Ammonium sulfate does not show efflorescence under normal conditions.

In summary, the efflorescent substances from your list are CuSO₄·5H₂O and Na₂CO₃·10H₂O.