Editing Hygroscopy (section)

==Overview==
[[File:THC 2003.902.036 Determining Hygroscopicity.tif|thumb|right|Apparatus for the determination of the hygroscopicity of fertilizer, Fixed Nitrogen Research Laboratory, c. 1930]]
Hygroscopic substances include [[cellulose]] fibers (such as cotton and paper), [[sugar]], [[caramel]], [[honey]], [[glycerol]], [[ethanol]], [[wood]], [[methanol]], [[sulfuric acid]], many fertilizer chemicals, many [[Salt (chemistry)|salt]]s and a wide variety of other substances.<ref name="hygroscopiccycle">{{cite web|title=Hygroscopic compounds|url=https://www.hygroscopiccycle.com/hygroscopic-compounds/|website=hygroscopiccycle.com|publisher=IBERGY|access-date=April 7, 2017|archive-date=April 8, 2017|archive-url=https://web.archive.org/web/20170408000150/http://www.hygroscopiccycle.com/hygroscopic-compounds/|url-status=live}}</ref>

If a compound dissolves in water, then it is considered to be [[Hydrophile|hydrophilic]].<ref>{{GoldBookRef |title=hydrophilic |file=H02906 }}</ref>

[[Zinc chloride]] and [[calcium chloride]], as well as [[potassium hydroxide]] and [[sodium hydroxide]] (and many different [[Salt (chemistry)|salt]]s), are so hygroscopic that they readily dissolve in the water they absorb: this property is called [[#Deliquescence|deliquescence]]. Not only is [[sulfuric acid]] hygroscopic in concentrated form but its solutions are hygroscopic down to concentrations of 10% v/v or below.  A hygroscopic material will tend to become damp and cakey when exposed to moist air (such as the salt inside salt shakers during humid weather).

Because of their affinity for [[atmospheric]] [[moisture]], desirable hygroscopic materials might require storage in sealed containers. Some hygroscopic materials, e.g., sea salt and sulfates, occur naturally in the atmosphere and serve as [[Cloud condensation nuclei|cloud seeds]], cloud condensation nuclei (CCNs).  Being hygroscopic, their microscopic particles provide an attractive surface for moisture vapour to condense and form droplets. Modern-day human [[Cloud seeding#History|cloud seeding]] efforts began in 1946.<ref>{{cite journal |last1=Pelley |first1=Janet |title=Does cloud seeding really work? |journal=Chemical & Engineering News |date=May 30, 2016 |volume=94 |issue=22 |url=https://cen.acs.org/articles/94/i22/Does-cloud-seeding-really-work.html?PageSpeed=noscript |access-date=29 January 2023}}</ref>

When added to foods or other materials for the express purpose of maintaining [[moisture content]], hygroscopic materials are known as [[humectant]]s.

Materials and compounds exhibit different hygroscopic properties, and this difference can lead to detrimental effects, such as stress concentration in [[composite material]]s. 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 the coefficient of hygroscopic contraction (CHC)—the difference between the two terms being a difference in sign convention.

Differences in hygroscopy can be observed in plastic-laminated paperback book covers—often, in a suddenly moist environment, the book cover will curl away from the rest of the book. The unlaminated side of the cover absorbs more moisture than the laminated side and increases in area, causing a stress that curls the cover toward the laminated side. This is similar to the function of a thermostat's [[bimetallic strip]]. Inexpensive dial-type [[hygrometer]]s make use of this principle using a coiled strip. Deliquescence is the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution. Deliquescence occurs when the [[vapor pressure|vapour pressure]] of the solution that is formed is less than the partial pressure of water vapour in the air.

While some similar forces are at work here, it is different from [[capillary action|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).