Cobalt chloride (CoCl2) is used in hygrometers because the substance changes colors when humidity levels change in the air, according to the National Weather Service. The chemical is readily available in elementary chemistry sets to demonstrate changes in heat and humidity within the Earth's atmosphere. Cobalt chloride changes from blue to pink on blotter paper.
A simple hygrometer is made by dipping cotton cloth or blotting paper in cobalt chloride and letting the chemical dry when exposed to air. The aqueous solution starts out blue but then changes to pink as it dries. Young scientists can use a hair dryer to speed up the process of the color change during an experiment. Adding water to the paper after it dries to causes the color to change from pink back to blue.
Although not scientifically precise, cobalt chloride can be used to monitor humidity changes throughout a house or room. Scientists observe different changes as the cobalt chloride solution sits next to a bathroom mirror or outside a window.
Cobalt chloride is also used for electroplating and absorbing ammonia gas, according to City Chemical LLC. The chemical is part of gas masks, barometers, solid lubricants, dye mordants, catalysts, flux for magnesium refinement and laboratory reagents. Cobalt chloride can be harmful if swallowed and may cause skin irritation.