Here's why:

* Hygroscopic substances absorb moisture from the air, but they don't necessarily dissolve in it.

* Efflorescent substances lose water of crystallization to the air, becoming powdery.

* Deliquescent substances absorb so much moisture from the air that they dissolve in it, forming a solution.

Potassium hydroxide is very soluble in water and has a strong affinity for moisture. When exposed to air, it readily absorbs water vapor, eventually dissolving into a liquid solution. This is the defining characteristic of a deliquescent substance.