Hygroscopicity is the tendency of a solid substance to absorb moisture from the surrounding atmosphere. The process can take on a number of forms. Thus, with a porous solid such as activated carbon, water vapor will be physically adsorbed, both on the external surface and within the pores, to form a condensed layer. The process may initially take place at "active sites" from which spreading then occurs. With other solids, such as silica gel, the interaction at the surface may not be entirely of a physical nature and some loose chemical bonds may be established. Many cellulosic materials, including hair, cotton and wool, are hygroscopic and change their physical dimensions as a result of the take-up of water. Such materials may be used as the active elements in hygrometers (see Humidity Measurement).
The total amount of water, which can be taken up by the hygroscopic material will be a function of the temperature and humidity of the atmosphere in which it is located and will ultimately be determined by the sorption isotherm of the system. In general, the rate of transfer of moisture will fall off progressively as equilibrium is approached, not only because the concentration driving force becomes smaller, but also because the overall diffusional resistance to mass transfer increases as the more easily accessible elements of surface reach their equilibrium state so that, in a porous solid, the vapor must then diffuse into the more remote pores.
A particular example of hygroscopic behavior is deliquescence which is exhibited by many water-soluble solids, including inorganic salts (e.g., calcium chloride). At a given temperature, the vapor pressure of a saturated salt solution will be lower than that of pure water and, if it is less than the partial pressure in the atmosphere, moisture will be tranferred to the surface of the solids, part of which will dissolve to form a saturated solution. When all the solids have dissolved, the process will continue until the partial pressure of the now unsaturated solution equals that in the atmosphere. In some cases, the formation of a hydrate or higher hydrate of the salt may precede the formation of a liquid phase. Liquids (e.g., sulfuric acid) may also be deliquescent.