How can you test for the presence of water in a substance?

You can test for the presence of water in a substance using anhydrous copper(II) sulfate or cobalt(II) chloride.

Anhydrous copper(II) sulfate is a white powder that turns blue when it comes into contact with water. This is due to the formation of hydrated copper(II) sulfate. To perform this test, you would add a small amount of the substance to be tested to some anhydrous copper(II) sulfate. If the copper(II) sulfate turns blue, then water is present in the substance.

Similarly, anhydrous cobalt(II) chloride is a blue powder that turns pink when it comes into contact with water, forming hydrated cobalt(II) chloride. The test is performed in the same way as with copper(II) sulfate. If the cobalt(II) chloride turns pink, then water is present.

These tests are qualitative, meaning they can tell you whether water is present, but not how much. They are also not specific to water; other substances that can form hydrates will also cause the colour change. Therefore, it's important to use these tests as part of a wider suite of tests to confirm the presence of water.

Another method to test for water is by using a simple heating test. If the substance is heated and steam is produced, this indicates the presence of water. However, this method should be used with caution as other volatile substances may also produce steam when heated.

In a laboratory setting, a more accurate method to test for water is by using Karl Fischer titration. This method can quantitatively determine the amount of water in a sample. However, this method requires specialised equipment and chemicals, and is therefore not typically used in a school laboratory setting.

Remember, safety is paramount when conducting any chemical tests. Always wear appropriate protective equipment and work in a well-ventilated area.