Which of the following substances is NOT hygroscopic?

Explanation

Hygroscopic substances absorb water from the air around them. Aluminum metal does not readily absorb moisture from the atmosphere under normal conditions.

Sodium chloride, calcium oxide, and copper oxide all attract and absorb water vapor from the air. These compounds can become damp or even dissolve when exposed to humid air.

Pure aluminum metal forms a protective oxide layer that prevents water absorption. This makes aluminum non-hygroscopic, while the other compounds listed actively pull moisture from surrounding air.