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 absorbing or adsorbing substance becoming physically changed somewhat. This could be by an increase in volume, boiling point, viscosity or other physical characteristics or properties of the substance, as water molecules can become suspended between the substance's molecules in the process.

Overview

Zinc chloride and calcium chloride, as well as potassium hydroxide and sodium hydroxide (and many different salts), are so hygroscopic that they readily dissolve in the water they absorb: this property is called deliquescence. Not only is sulfuric acid hygroscopic in concentrated form but its solutions are hygroscopic down to concentrations of 10 Vol-% or below. A hygroscopic material will tend to become damp and cakey when exposed to moist air (such as the salt used in salt shakers during humid weather).

Applications

Because of their affinity for atmospheric moisture, hygroscopic materials might be used in various products to keep areas dry. They can be used in dessicant packs in leather products such as shoes and bags to prevent damaging effects from humidity.

Measurement

The amount of moisture held by hygroscopic materials is usually proportional to the relative humidity. Various materials and manufacturing processes rely on this property. For example, many industries must keep the relative humidity inside their plants at specific levels in order to maintain the quality of their products.

See also

• Deliquescence
• Efflorescence
• Humectant
• Desiccant