blue to pink, it’s water I think…

Cobalt chloride is another transition metal salt used to test for water. In fact it is the colouring agent added to silica gel for use in dessicators which I mentioned in my January 31st post.

Hydrated cobalt chloride has the formula CoCl2.6H2O and is deep purple in colour.

This is what turns cobalt chloride paper pink when wet!

This is what turns cobalt chloride paper pink when wet!

Cobalt chloride is also toxic. Safety precautions must be employed to avoid contact with skin or inhaling its dust. Always use gloves and a fume cupboard when dealing with cobalt chloride or silica gel impregnated with it.

Cobalt chloride test paper can be used to test for water.
Cobalt chloride paper
Here the blue dihydrate form of cobalt chloride turns pink on addition of water and then back to blue again when heated.

So it’s the opposite of copper (II) sulfate whose hydrated form is blue, CuSO4.5H2O

With cobalt (II) chloride the dehydrated form is blue and the hydrated form is pink, [CoCl2(H2O)4].2H2O

Having covered chemical tests for water in my last two posts, what is the test for hydrogen gas in the school laboratory?

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