Which of the following is a deliquescent compound?

Answer: CaCl₂

Explanation: A deliquescent compound absorbs moisture from the air until it dissolves in the water it has absorbed. Anhydrous calcium chloride (CaCl₂) is strongly deliquescent and is used as a drying agent.

• Na₂CO₃: hygroscopic at most; it does not typically liquefy. Its decahydrate is efflorescent.
• CuO: not hygroscopic or deliquescent.
• Na₂CO₃·10H₂O (washing soda): efflorescent; it loses water to the air rather than absorbing it.