Hygroscopy

Hygroscopy is the phenomenon of attracting and holding water molecules from the surrounding environment, which is usually at normal or room temperature. This is achieved through either absorption or adsorption with the adsorbing substance becoming physically changed somewhat. This could be an increase in volume, boiling point, viscosity, or other physical characteristic or property of the substance, as water molecules can become suspended between the substance's molecules in the process. Hygroscopy is the phenomenon of attracting and holding water molecules from the surrounding environment, which is usually at normal or room temperature. This is achieved through either absorption or adsorption with the adsorbing substance becoming physically changed somewhat. This could be an increase in volume, boiling point, viscosity, or other physical characteristic or property of the substance, as water molecules can become suspended between the substance's molecules in the process. The word hygroscopy (/haɪˈɡrɒskəpi/) uses combining forms of hygro- and -scopy. Unlike any other -scopy word, it no longer refers to a viewing or imaging mode. It did begin that way, with the word hygroscope referring in the 1790s to measuring devices for humidity level. These hygroscopes used materials, such as certain animal hairs, that appreciably changed shape and size when they became damp. Such materials were then said to be hygroscopic because they were suitable for making a hygroscope. Eventually, though, the word hygroscope ceased to be used for any such instrument in modern usage. But the word hygroscopic (tending to retain moisture) lived on, and thus also hygroscopy (the ability to do so). Nowadays an instrument for measuring humidity is called a hygrometer (hygro- + -meter). Hygroscopic substances include cellulose fibers (such as cotton and paper), sugar, caramel, honey, glycerol, ethanol, wood, methanol, sulfuric acid, many fertilizer chemicals, many salts (like calcium chloride, bases like sodium hydroxide etc.), and a wide variety of other substances.