Hygroscopic Definition in Chemistry

Substances that absorb water are hygroscopic

The puffball mushroom
The puffball mushroom contains the hygroscopic sugar mannitol. When the mushroom absorbs enough water, it puffs up and releases its spores.

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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.