What does hygroscopic refer to in terms of substances?

Hygroscopic refers to the property of substances that allows them to absorb moisture from the surrounding environment readily. This characteristic is important in various applications, such as in refrigeration and air conditioning systems, where materials like desiccants are used to prevent moisture accumulation that can lead to corrosion or microbial growth.

In contrast to the other definitions, substances that repel water would be classified as hydrophobic, not hygroscopic. Completely dry substances do not engage in moisture absorption, and the emission of hydrogen gas is unrelated to hygroscopic properties. Therefore, the definition of hygroscopic aligns specifically with the ability to absorb moisture, making it the correct definition in this context.