Hygroscopic Definition in Chemistry

Hygroscopic Substances Absorb Water

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.

Definition of Hygroscopic

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.

When water vapor is absorbed, the water molecules are taken into the molecules of the substance, often resulting in physical changes, such as increased volume. Color, boiling point, temperature, and viscosity can also change. 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. Silica gel, honey, nylon, and ethanol are also hygroscopic.

Sulfuric acid is hygroscopic, not only when concentrated but also down to a concentration of 10% v/v or even lower.

Germinating seeds also are hygroscopic. After seeds have dried, their outer coating becomes hygroscopic and starts absorbing moisture needed for germination. Some seeds have hygroscopic portions that change the shape of the seed when moisture is absorbed. The seed of Hesperostipa comata twists and untwists, depending on its hydration level, drilling the seed into the soil.

Animals also make use of hygroscopic materials. 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, and then capillary action lets the lizard capture water across its skin.

Hygroscopic Versus Hydroscopic

You might encounter the word "hydroscopic" used in place of "hygroscopic". Although 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, but deliquescent materials absorb moisture to the extent that the substance dissolves in water. Deliquescence may be considered an extreme form of hygroscopy.

A hygroscopic material will become damp and may stick to itself or become caky, while a deliquescent material will liquefy.

Hygroscopy Versus Capillary Action

While capillary action is another mechanism involving the uptake of water, it differs from hygroscopy in that no absorption occurs in capillary action.

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 because of 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.