Chemguide: Core Chemistry 14 - 16

Testing for water

This page looks at two common tests for the presence of water - anhydrous copper(II) sulfate and cobalt chloride paper.

Testing using anhydrous copper(II) sulfate

What is anhydrous copper(II) sulfate?

The familiar blue copper(II) sulfate crystals contain molecules of water bound into the crystal structure. We call this water of crystallisation. The copper(II) sulfate is said to be hydrated.

There are 5 molecules of water of crystallisation: CuSO4.5H2O.

If you heat copper(II) sulfate crystals, they lose this water of crystallisation and you get white anhydrous copper(II) sulfate. Anhydrous simply means "without water"

CuSO4.5H2O(s)     CuSO4(s) + 5H2O(l)

That is what is used to test for the presence of water.

Using anhydrous copper(II) sulfate to test for water

If you add water to white anhydrous copper(II) sulfate, it turns blue again as it goes back to the hydrated form.

CuSO4(s) + 5H2O(l)     CuSO4.5H2O(s)

Testing using cobalt chloride paper

Cobalt(II) chloride solution forms pink crystals, CoCl2.6H2O, and if this is heated gently it turns into blue anhydrous cobalt(II) chloride, CoCl2.

The anhydrous form is used as cobalt chloride paper. (The (II) is almost always missed out when you are talking about cobalt chloride paper.)

Filter paper is dipped into cobalt(II) chloride solution and then dried, either by heating it in an oven at less than 100°C, or by leaving it in a desiccator for a longer time.

Note:  A desiccator is a piece of glassware with a close-fitting lid containing some substance which absorbs water under a perforated tray that holds whatever you want to dry.

You can, if you are careful, dry the paper sufficiently just by holding it very close to a Bunsen flame.

The paper turns blue as anhydrous cobalt(II) chloride is formed.

When you add water to it, it turns pink again.

The next short piece of video shows this as well as the use of anhydrous copper(II) sulfate. You may need to turn the volume up - the sound level is a bit low.

Warning:  It is easy to confuse these two colour changes. You need to check your syllabus so that you know which version your examiners are going to ask you about. If it is only one of them, learn that one, and forget about the other one.

If you are doing a UK-based syllabus, you can find links to the Exam Boards' websites where you can download a copy of your syllabus and other useful stuff on the about this part of Chemguide page.

These tests say nothing about it being pure water!

All these tests do is to show that the liquid you have contains water - it doesn't tell you that you have got pure water.

To check its purity, you could check is boiling point at 1 atmosphere pressure. That would have to be 100°C for the water to be pure.

Note:  The pressure is important. At lower or higher atmospheric pressures the boiling point changes. According to Wikipedia, the boiling point of water at an altitude of 8,000 feet (about 2,400 metres) is only 92°C.

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