Hygroscopic means capable of attracting and holding water from environment, either through absorption or adsorption. Typically, this process occurs near ambient or room temperature. Hygroscopy is the ability to attract and hold water. Both terms originate from a 1790s device called a hygroscope, which measured humidity by measuring the change in a hygroscopic material, such as hair.
Examples of Hygroscopic Substances
Many salts, fibers, and porous materials are hygroscopic. Examples of hygroscopic substances include:
- Table salt (sodium chloride)
- Sodium hydroxide
- Potassium hydroxide
- Sulfuric acid
- Brown sugar
- Ethanol
- Methanol
- Hair
- Honey
- Molasses
- Caramel
- Glycerol
- Many fertilizers
- Paper
- Wool
- Cotton
- Nylon
- Poly(methyl methacrylate) (PMMA and Plexiglas)
- Silica gel
How Hygroscopy Works
Hygroscopic substances are hydrophilic (“water-loving”). Chemically, they are polar or support hydrogen bonding. Some hygroscopic substances (like salt and alcohol) dissolve in water, while others do not (like nylon and silica gel).
Aside from polarity, there are three main processes, which may work together:
- Absorption: Absorption is when a substance enters the body of a material. For example, cotton absorbs water.
- Adsorption: Adsorption is when molecules adhere or stick to surface. For example, water adsorbs onto Plexiglas.
- Capillary action: Capillary action draws water through pores and narrow spaces due to the adhesive and cohesive properties of water. Silica gel beads are hygroscopic because silica attracts water, while tiny pores and irregularities collect it via capillary action.
Difference Between Hygroscopic and Deliquescent
A deliquescent substance attracts water from its environment and dissolves into aqueous solution. This process is called deliquescence. Deliquescence is a form of hygroscopy.
Calcium chloride (CaCl2) is an example of a deliquescent substance. You can find this chemical in commercial products, such as Damp Rid. The salt absorbs so much moisture from the air that it eventually dissolves in it.
Uses of Hygroscopic Materials
Hygroscopic materials have many uses, both commercially and in nature.
- Some grass seeds have hygroscopic surfaces that bend and change shape as humidity changes. These changes allow the seeds to twist or drill into the ground.
- Desert lizards called thorny dragons have hygroscopic grooves between their spines that help the animals capture and condense dew. The lizard uses capillary action to draw moisture through skin into its body.
- Packaging often contains hydroscopic materials called desiccants that absorb or adsorb moisture to protect fabrics, electronics, leather, dry foods, and other goods from moisture damage.
- Mixing a bit of rice into a salt shaker helps keep the salt dry because the rice is more hygroscopic than the salt.
- Hygroscopic substances find use in skin moisturizers.
- Baked goods often contain hygroscopic ingredients to keep them moist. In this context, the substance is called a humectant. For example, brown sugar or molasses makes cakes moist and cookies soft.
Difference Between Hygroscopic and Hydroscopic
Both “hygroscopic” and “hydroscopic” are real words relating to water, but they have very different definitions. The word “hydroscopic” refers to measurements made with a hydroscope.
A hydroscope is an instrument that either detects subsurface water or else makes observations below the surface of water. To make matters even more confusing, a hygrometer is an instrument that measures relative humidity, while a hydrometer is a device that measures relative density of liquids relative to the density of water. A hydrometeor is a condensed water particle in the atmosphere large enough to cause precipitation.
References
- IUPAC (1997). Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Blackwell Scientific Publications, Oxford. doi:10.1351/goldbook
- Wells, Mickey; et al. (1997). “Potassium carbonate as a desiccant in effervescent tablets”. International Journal of Pharmaceutics. 152 (2): 227–235. doi:10.1016/S0378-5173(97)00093-8
- Worthington, David (2003). Dictionary of Environmental Health. London: Spon Press. ISBN 0415267242.