Chemical Test for Water
This topic covers a specific chemical test to confirm the presence of water using a reagent that changes colour. It is a simple, qualitative test based on a reversible hydration reaction.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Anhydrous copper(II) sulfate is the chemical reagent used to test for water.
- Initially, anhydrous copper(II) sulfate is a white solid powder.
- When water is added, the solid turns a distinct blue colour as it becomes hydrated copper(II) sulfate.
- This colour change from white to blue is the positive result confirming the presence of water.
- The reaction is reversible: heating the blue hydrated copper(II) sulfate will drive off the water and turn it back into a white powder.
Formulae
CuSO4(s) + 5H2O(l) → CuSO4.5H2O(s) This equation shows the hydration reaction. The white anhydrous solid reacts with liquid water to form the blue hydrated solid.
Definitions
- Anhydrous
- A substance that contains no water molecules within its crystal lattice structure.
- Hydrated
- A substance that has water molecules chemically bonded within its crystal lattice structure, often referred to as water of crystallisation.
Worked example
A student is carrying out a distillation to separate a mixture of ethanol and water. To check if the final sample of distillate is pure ethanol, they add a small amount of a white solid which immediately turns blue. What does this result indicate about the distillate?
- 1
Identify the chemical test being described.
A white solid turning blue is the characteristic test for water.
- 2
The white solid used is anhydrous copper(II) sulfate.
- 3
The blue colour indicates the formation of hydrated copper(II) sulfate.
- 4
The formation of the hydrated form confirms that water is present in the sample.
- 5
Therefore, the distillation was not completely effective, and the distillate is not pure ethanol but a mixture containing water.
Answer: The distillate contains water.
Common mistakes
- ×Reversing the colours: A common mistake is to forget whether the test goes from white-to-blue or blue-to-white. Remember, it becomes blue *with* water.
- ×Forgetting the initial state: The test is only valid if you start with *anhydrous* copper(II) sulfate. Blue, hydrated copper(II) sulfate will show no change.
- ×Confusing this test with other copper reactions: Do not mix this up with the test for Cu2+(aq) ions using sodium hydroxide, which forms a blue precipitate.
No-calculator tips
- ✓This is a purely qualitative test. Focus on memorising the exact colours and state change (solid powder) rather than any numbers.
- ✓Mnemonic: Think 'White without water'. The chemical is white when it is dry.
- ✓Visualise the change: Associate the colour blue with water (e.g., oceans, rain). The test substance turns blue when it finds water.