Cobalt chloride paper
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Cobalt chloride paper is paper soaked in cobalt(II) chloride (CoCl2) solution. It is used to test for the presence of water because it turns from blue to pink upon exposure to water.
The chemical equation for this change is:
CoCl2 (blue) + 6 H2O → [Co(H2O)6]Cl2 (pink)
Pink test paper can be heated to separate the embedded water molecules which get evaporated turning back the paper blue. Hence the reaction above is reversible and the test paper is thus reusable.
The [Co(H2O)6]2+ ion in the pink form has an octahedral structure with the six water molecules attached to the metal centre.
 Anhydrous Cobalt Chloride CoCl2 |
 Hydrated Cobalt Chloride [Co(H2O)6]Cl2 or CoCl2·6H2O |
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