Cobalt chloride can be used as a test for water.
Take the mixture/solution that is needed for test for water add COCl3 in it. If the mixture contains water COCl3 will change into COCl3.6H2O (forms pink crystals), if you heat the mixture and water evaporates COCl3.6H2O will again change into COCl3 and turns blue from pink, if this happen mixture contains water.