Tests for Water (Edexcel IGCSE Chemistry)

Spec point — describe a test for the presence of water using anhydrous copper(II) sulfate describe a physical test to show whether a sample of water is pure

Test for water

• Water can be identified using a chemical test and/or a physical test

Chemical test for water

• Anhydrous copper(II) sulfate turns from white to blue on the addition of water
• The equation is:

CuSO₄ (s) + 5H₂O (l) → CuSO₄.5H₂O (s)

Copper sulfate turns a light blue colour in the presence of water

Physical test for water

• A physical test to see if a sample of water is pure is to check its boiling point
• A sample of the liquid is placed in a suitable container such as a boiling tube and gently heated
• Using a thermometer, you can check if the boiling point is exactly 100 ^oC
• Any impurities present will usually tend to raise the boiling point and depress the melting point of pure substance

Exam Hint A lot of students are tempted to say you can identify water because it has no taste or smell. While this may be true, it would be extremely hazardous to taste anything in the lab and water is not the only colourless liquid to have no taste or smell!