An example of a hygroscopic substance is

Analysis

The question asks for an example of a hygroscopic substance among the given options. To answer this, we must distinguish between hygroscopic, deliquescent, and efflorescent substances.

Concept Explanation

• Hygroscopic substances: These are substances that absorb moisture (water vapor) from the atmosphere but do not dissolve in the absorbed water to form a solution. They remain as solids. Examples include concentrated sulfuric acid ($H_2SO_4$), quicklime ($CaO$), and copper(II) oxide ($CuO$).
• Deliquescent substances: These are substances that absorb so much moisture from the atmosphere that they eventually dissolve in the absorbed water to form a saturated solution. Examples include sodium hydroxide ($NaOH$), calcium chloride ($CaCl_2$), and magnesium chloride ($MgCl_2$).

Step-by-Step Solution

1. Evaluate Option A ($CuO$): Copper(II) oxide is a solid that absorbs moisture from the air without dissolving. It is a classic example of a hygroscopic substance.
2. Evaluate Option B ($MgCl_2$): Magnesium chloride is highly soluble in water and absorbs moisture until it turns into a liquid solution. It is deliquescent.
3. Evaluate Option C ($CaCl_2$): Calcium chloride is a very common drying agent used in laboratories, but because it absorbs enough water to form a liquid solution, it is classified as deliquescent.
4. Evaluate Option D ($NaOH$): Sodium hydroxide pellets absorb moisture from the air and eventually turn into a liquid solution. It is also deliquescent.

Conclusion

While $CaCl_2$ and $NaOH$ are often used as drying agents, they are technically deliquescent because they form solutions. $CuO$ is the substance listed that absorbs moisture without becoming a solution.

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Exam Tip

To remember the difference:

• Hygroscopic: "H" for "Hold" (it holds the water but stays solid).
• Deliquescent: "D" for "Dissolve" (it absorbs water and dissolves into a liquid).