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the ability of materials to absorb moisture from the air.
Hygroscopicity is a property of wettable (hydrophilic) materials of capillary-porous structure (such as wood and grain), in whose fine capillaries moisture condenses; of water-soluble substances such as table salt, sugar, and concentrated sulfuric acid; and especially of chemical compounds that form, with water, crystalline hydrates. The amount of water absorbed by a porous material (hygroscopic wetness, or Whyg) increases as the moisture content of the air increases, reaching a maximum when the relative humidity of the air is 100 percent. For wood, the maximum Whyg is about 30 percent (in terms of weight), whereas for wheat it is about 35 percent. It is important to know the hygroscopicity of a material in order to determine drying and wetting processes. Hygroscopicity is also considered in long-term storage and transportation of materials, especially by sea. Hygroscopicity explains the dampening and even deliquescence of a number of salts when stored in the open air. Some hygroscopic substances, such as concentrated sulfuric acid, are used to dry the air.