Cobalt chloride paper. Hands demonstrating the colour change from blue to pink when cobalt chloride (CoCl2) paper is dipped in water. The change occurs due to a hydration reaction. The original blue form is cobalt chloride dihydrate, with two attached water molecules. When dipped, the compound incorporates four more water molecules to become the pink hexahydrate. The reaction can be reversed by heating. Cobalt chloride paper can be used to indicate humidity.