A hygroscopic substance readily absorbs water from its surroundings. (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).
An example of a hygroscopic substance is biodiesel, which absorbs water to about 1200 parts per million (PPM). Other examples are honey, ethanol, methanol, glycerin, concentrated sulfuric acid and concentrated sodium hydroxide. Calcium chloride is so hygroscopic that it eventually dissolves in the water it absorbs - this property is called deliquescence. Because of their affinity for atmospheric moisture, hygroscopic materials may need to be stored in sealed containers.
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 or Contraction. The difference (Expansion or Contraction) 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. In effect, what has happened is that the unlaminated side of the cover has absorbed more moisture than the laminated side, and has increased in area, causing a stress that curls the cover toward the laminated side. This is similar to how simple bimetal strips are used as torsion springs in thermometers. A bi-metallic strip consists of two thin layers of metals with different coefficients of thermal expansion bonded together. When the temperature changes, one metal expands more along its length than the other, causing the spring to rotate and create a torque.
Nitrocellulose, unlike gunpowder, is not hygroscopic. Smokeless powder facilitated the development of semi-automatic and fully automatic firearms because the hygroscopic residue of gunpowder would jam the mechanisms of such firearms. Hygroscopic components of bullets generally compensate by being readily ignitable by heat or a spark.
Hygroscopic materials play an important role in buildings; wood is a highly hygroscopic material. Hygroscopic materials have an influence on moisture content in a room. Wood can extract moisture (vapour) from air. The higher the relative humidity, the more vapour wood adsorbs. Many varieties of wood may start to rot if the relative humidity is higher than 80% for long periods.
The seeds of some grasses have hygroscopic extensions which bend with changes in humidity, enabling them to disperse over the ground.
It's possible to split a rock by inserting a piece of wood into a fissure and adding water. As it absorbs water the wood's volume increases and pushes the sections of rock apart, increasing the fissure.