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Cobalt dichloride paper is a paper impregnated with CoCl2 (blue), used to demonstrate the presence of water. When in contact with water, cobalt chloride forms a complex of pink CoCl2 • 6H2O (cobalt dichloride hexahydrate). The reaction is reversible: when the paper dries, it returns to its blue color.
It can be used as a characteristic test of water in a colorless or slightly colored solution (so that the color of the solution does not interfere with this colorimetric test). It is also used to illustrate the presence of hot spots, for example in a microwave oven: the paper is slightly moistened to turn pink, then heated in the oven a few seconds. The hottest places are those that dry the paper the fastest, resulting in blue patterns corresponding to the higher intensity areas of the oven on cobalt paper. edit code de
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