hygroscopic

Hygroscopic describes a substance's tendency to absorb moisture from the surrounding environment through exposure to water vapor in the air. Hygroscopic materials draw water from the atmosphere and may retain it in solid form; some hygroscopic substances become solutions when enough water is absorbed, a process known as deliquescence.

The extent and rate of moisture uptake depend on factors such as relative humidity, temperature, surface area, porosity, and chemical affinity for water. Materials can be broadly categorized as hygroscopic, non-hygroscopic, or deliquescent (the latter specifically forms a liquid solution after absorbing moisture).

Common hygroscopic materials include salts (calcium chloride and, to a lesser extent, sodium chloride), sugars, wood, paper, cotton, and many polymers. Desiccants such as silica gel, clays, and certain porous minerals are designed to absorb moisture from their surroundings without dissolving. Deliquescent substances, such as calcium chloride and magnesium chloride, absorb so much water that they dissolve into an aqueous solution.

Applications of hygroscopic properties are wide. Desiccants control humidity in packaging, storage, and pharmaceutical processes. In the food and agricultural sectors, moisture management helps preserve quality and stability. In materials science and construction, hygroscopic behavior affects durability and dimensional changes. Measuring hygroscopic behavior typically involves equilibrium moisture content at a given relative humidity and related concepts such as water activity and sorption isotherms.