Which of the following is a deliquescent compound?

Analysis of the Question

The question asks us to identify a deliquescent compound from the given list. To answer this, we must distinguish between the different ways substances interact with atmospheric water vapor.

Concept Explanation

• Deliquescence: This occurs when a substance absorbs so much moisture from the atmosphere that it eventually dissolves in the absorbed water to form a solution. Common examples include Calcium chloride ($CaCl_2$) and Sodium hydroxide ($NaOH$).
• Efflorescence: This occurs when a hydrated salt loses its water of crystallization to the atmosphere upon exposure. An example is washing soda ($Na_2CO_3 \cdot 10H_2O$), which loses water to become a white powder.
• Hygroscopy: This refers to substances that absorb moisture from the atmosphere but do not necessarily dissolve in it (e.g., concentrated $H_2SO_4$ or silica gel).

Step-by-Step Solution

1. Analyze Option A ($NaNO_3$): Sodium nitrate is hygroscopic, but it is not typically classified as deliquescent.
2. Analyze Option B ($CaCl_2$): Calcium chloride is a classic example of a deliquescent substance. It is so effective at absorbing moisture that it is widely used in laboratories as a drying agent (desiccant) in desiccators. It absorbs water until it turns into a liquid solution.
3. Analyze Option C ($CuO$): Copper(II) oxide is an insoluble metal oxide and does not absorb moisture from the air.
4. Analyze Option D ($Na_2CO_3 \cdot 10H_2O$): This is a hydrated salt. As mentioned in the concept section, this substance is efflorescent, meaning it loses water to the atmosphere, not the other way around.

Comparing these, $CaCl_2$ is the only substance listed that exhibits deliquescence.

Conclusion

Exam Tip

To remember the difference:

• Deliquescent = Dissolves in the water it absorbs.
• Efflorescent = Exits (loses) water to the air.