Best Answer: The standard test for water is to use cobalt (II) chloride paper.
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.