Hygroscopy is the phenomenon of attracting and holding
water molecules from the surrounding
environment, which is usually at normal or room temperature. This is achieved through either
absorption or
adsorption with the absorbing or adsorbing substance becoming physically changed somewhat. This could be an increase in volume, boiling point, viscosity, or other physical characteristic or property of the substance, as water molecules can become suspended between the substance's molecules in the process.
Hygroscopic substances include
cellulose fibers (such as cotton and paper),
sugar,
caramel,
honey,
glycerol,
ethanol,
wood,
methanol,
sulfuric acid, many fertilizer chemicals, many
salts (including
sodium chloride), and a wide variety of other substances.
[1]