Hygroscopy

Deliquescent materials are sufficiently hygroscopic that they absorb so much water that they become liquid and form an aqueous solution.

Apparatus for the determination of the hygroscopicity of fertilizer, Fixed Nitrogen Research Laboratory, ca. 1930

Materials and compounds exhibit different hygroscopic properties, and this difference can lead to detrimental effects, such as stress concentration in composite materials. The volume of a particular material or compound is affected by ambient moisture and may be considered its coefficient of hygroscopic expansion (CHE) (also referred to as CME, or coefficient of moisture expansion) or coefficient of hygroscopic contraction (CHC)—the difference between the two terms being a difference in sign convention.

While some similar forces are at work here, it is different from capillary attraction, a process where glass or other solid substances attract water, but are not changed in the process (e.g., water molecules do not become suspended between the glass molecules).

The amount of moisture held by hygroscopic materials is usually proportional to the relative humidity. Tables containing this information can be found in many engineering handbooks and is also available from suppliers of various materials and chemicals.

Hygroscopy also plays an important role in the engineering of plastic materials. Some plastics, e. g. nylon, are hygroscopic while others are not.