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Hygroscopy is the ability of a substance to attract water molecules from the surrounding environment through either absorption or adsorption.

Hygroscopic substances include honey, glycerin, ethanol, methanol, concentrated sulfuric acid, methamphetamine, and concentrated sodium hydroxide (lye). Calcium chloride is so hygroscopic that it eventually dissolves in the water it absorbs: this property is called deliquescence (see below). Because of their affinity for atmospheric moisture, hygroscopic materials may need to be stored in sealed containers. When added to foods or other materials for the express purpose of maintaining moisture content, such substances are known as humectants.

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

A common example where difference in this hygroscopic property can be seen is in a paperback book cover. Often in a relatively 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 increased in area, causing a stress that curls the cover toward the laminated side. This is similar to the function of a bi-metallic strip.

The similar sounding but unrelated word hydroscopic is sometimes used in error for hygroscopic. A hydroscope is an optical device used for making observations deep under water.

The seeds of some grasses have hygroscopic extensions which bend with changes in humidity, enabling them to disperse over the ground. An example is Needle-and-Thread, Hesperostipa comata. Each seed has an awn that twists several turns when the seed is released. Increased moisture causes it to untwist, and upon drying to twist again; the seed is drilled into the ground.

Deliquescent materials are substances (mostly salts) which have a strong affinity for moisture and will absorb relatively large amounts of water from the atmosphere if exposed to it, forming a liquid solution. Deliquescent salts include calcium chloride, magnesium chloride, zinc chloride, Carnallite (KCl*MgCl₂*6H₂O,) and the strong base sodium hydroxide. They are often used as desiccants.

• Hydrophobic and Hydrophilic