Cobalt chloride is commonly used as a humidity indicator because it exhibits a unique property known as hygroscopy. Hygroscopy is the ability of a substance to attract and retain water molecules from the surrounding environment, which allows it to act as a moisture-sensitive indicator.

Cobalt chloride is particularly useful in humidity indicators because it undergoes a reversible chemical reaction when exposed to different levels of moisture. Anhydrous cobalt chloride (CoCl2) is a blue crystalline solid that contains no water molecules. However, when it is exposed to moisture, it readily absorbs water molecules and undergoes a chemical reaction to form hydrated cobalt chloride compounds.

The hydrated cobalt chloride compounds can exist in different forms depending on the amount of water absorbed. The most common form is hexahydrate cobalt chloride (CoCl2·6H2O), which has a pink color. When the humidity is low, the hydrated cobalt chloride loses water molecules and returns to its blue anhydrous state.

By utilizing this property, cobalt chloride can be incorporated into a humidity indicator, typically in the form of a paper strip or gel. The indicator material changes color depending on the humidity level in its surrounding environment. When the humidity is high, the cobalt chloride absorbs water, turning pink. Conversely, when the humidity is low, the indicator loses water and returns to its blue color.

This color change of cobalt chloride can be easily observed, providing a visual indication of the humidity level. It finds applications in various areas such as humidity monitoring in packaging, storage of moisture-sensitive materials, and as a component in weather instruments.