Cobalt chloride paper tests for the presence of water. If water is present, cobalt chloride will change colors from blue to purple to pink.
When testing for humidity, the experimenter can simply use cobalt chloride paper to detect airborne water particles. Cobalt chloride is an inorganic salt that is bright blue when it is not exposed to humidity. Cobalt chloride paper can be purchased in premade sheets, but it can also be made by dipping paper into a cobalt chloride solution and letting it dry.
As humidity increases, cobalt chloride will shift from blue to pink, with a purplish color serving as the mid-phase hue between these two colors. According to the American Chemistry Council, cobalt chloride has a crystalline structure that changes as the molecules shift to make room for water molecules, leading to the color change that makes this substance ideal for detecting the presence of humidity.
When heated, wet cobalt chloride paper can turn back from pink to blue as the water molecules are burned off and the original crystalline structure of the chemical is restored.
Named for a shade of blue, cobalt chloride in liquid or paper form is toxic when ingested. When conducting experiments, wearing gloves can ensure the substance doesn't transfer to hands.