Science, Tech, Math › Science Hygroscopic Definition in Chemistry Substances that absorb water are hygroscopic Share Flipboard Email Print The puffball mushroom contains the hygroscopic sugar mannitol. When the mushroom absorbs enough water, it puffs up and releases its spores. 3283197d_273/Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated December 08, 2019 Water is an important solvent, so it's unsurprising that there is a term specifically related to water absorption. A hygroscopic substance is able to absorb or adsorb water from its surroundings. Typically, this occurs at or near ordinary room temperature. Most hygroscopic materials are salts, but many other materials display the property. How it Works When water vapor is absorbed, the water molecules are taken into the molecules of the hygroscopic substance, often resulting in physical changes, such as increased volume. Color, boiling point, temperature, and viscosity can also change. In contrast, when water vapor is adsorbed, the water molecules remain on the surface of the material. Examples of Hygroscopic Materials Zinc chloride, sodium chloride, and sodium hydroxide crystals are hygroscopic, as are silica gel, honey, nylon, and ethanol.Sulfuric acid is hygroscopic, not only when concentrated but also when reduced down to a concentration of 10% v/v or even lower.Germinating seeds are hygroscopic. After seeds have dried, their outer coating becomes hygroscopic and begins absorbing the moisture required for germination. Some seeds have hygroscopic portions that cause the shape of the seed to change when moisture is absorbed. The seed of Hesperostipa comata twists and untwists, depending on its hydration level, drilling the seed into the soil.Animals can also have characteristic hygroscopic properties. For example, a species of lizard commonly called the thorny dragon has hygroscopic grooves between its spines. Water (dew) condenses on the spines at night and collects in the grooves. The lizard is then able to distribute water across its skin by means of capillary action. Hygroscopic vs. Hydroscopic You might encounter the word "hydroscopic" used in place of "hygroscopic," however, while hydro- is a prefix meaning water, the word "hydroscopic" is a misspelling and is incorrect. A hydroscope is an instrument used to take deep-sea measurements. A device called a hygroscope in the 1790s was an instrument used to measure humidity levels. The modern name for such a device is a hygrometer. Hygroscopy and Deliquescence Hygroscopic and deliquescent materials are both able to absorb moisture from the air. However, hygroscopy and deliquescence don't mean precisely the same thing: Hygroscopic materials absorb moisture, while deliquescent materials absorb moisture to the extent that the substance dissolves in water. A hygroscopic material will become damp and may stick to itself or become caky, while a deliquescent material will liquefy. Deliquescence may be considered an extreme form of hygroscopy. Hygroscopy vs. Capillary Action While capillary action is another mechanism involving the uptake of water, it differs from hygroscopy in that no absorption occurs in the process. Storing Hygroscopic Materials Hygroscopic chemicals require special care. Typically, they are stored in airtight containers. They may also be maintained under kerosene, oil, or within a dry atmosphere. Uses of Hygroscopic Materials Hygroscopic substances are used to keep products dry or to remove water from an area. They are commonly used in desiccators. Hygroscopic materials may be added to products due to their ability to attract and hold moisture. These substances are referred to as humectants. Examples of humectants used in food, cosmetics, and drugs include salt, honey, ethanol, and sugar. The Bottom Line Hygroscopic and deliquescent materials and humectants are all able to absorb moisture from the air. Generally, deliquescent materials are used as desiccants. They dissolve in the water they absorb to yield a liquid solution. Most other hygroscopic materials—which don't dissolve—are called humectants.