2.7.5Inorganic Chemistry
Tests for Water — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 42 of 52
Part 1 of 5Two ways to test for water
Chemical test
- Chemical test A chemical test relies on a new substance being made.
- For water, white anhydrous copper(II) sulfate changes colour when water is present.
- It shows that water is in the sample, but not that the sample is pure.
Physical test
- Physical test A physical test measures a property without changing the substance into something new.
- For water, the property is the boiling point.
- It shows whether the liquid is pure water or a mixture.
Why use both
- Why use both The chemical test answers 'is water present?' and the physical test answers 'is this sample pure water?'.
- They give different information.
Part 2 of 5Anhydrous copper(II) sulfate test
1Start white
- Start white Anhydrous copper(II) sulfate has no water of crystallisation and is a white powder.
2Add the liquid
- Add the liquid Add a few drops of the liquid being tested to the white powder.
3Water is taken in
- Water is taken in The powder combines with water to form hydrated copper(II) sulfate: CuSO4(s) + 5H2O(l) → CuSO4·5H2O(s).
4Turns blue
- Turns blue A white to blue colour change is a positive result.
- The change is reversible, because heating the blue crystals drives the water off and returns the white powder.
Part 3 of 5Testing the boiling point
1
Pour a small sample of the liquid into a boiling tube, no more than about one third full.
2
Add an anti-bumping granule if available and place a thermometer in the liquid with the bulb in the liquid, not touching the glass.
3
Heat gently in a water bath or with a Bunsen burner held at a safe distance, wearing eye protection.
4
Watch the thermometer and note the steady temperature reached while the liquid boils.
5
Compare with 100 °C. Pure water boils at exactly 100 °C at normal atmospheric pressure.
Part 4 of 5Interpreting the evidence
| Observation | Conclusion |
|---|---|
| White solid turns blue | Water is present in the sample |
| No colour change | No water is present |
| Boils at exactly 100 °C | Likely to be pure water |
| Boils above 100 °C | Impurities are dissolved in the water |
| Boils at another fixed temperature | A different liquid, not water |
Part 5 of 5Safety and good practice
Never taste or smell
- Never taste or smell Water has no taste or smell, but many other dangerous liquids are also colourless and odourless.
- Tasting chemicals in a laboratory is never safe, so it is not an acceptable test.
Handle copper sulfate carefully
- Handle copper sulfate carefully Copper(II) sulfate is harmful if swallowed and irritates the eyes, so wear eye protection and wash hands afterwards.
Use a clean, dry start
- Use a clean, dry start Damp glassware turns the white powder blue by itself, so make sure the test tube and spatula are completely dry.
Key terms
| Term | Meaning |
|---|---|
| Anhydrous | Containing no water of crystallisation. |
| Hydrated | Containing water of crystallisation within the crystal structure. |
| Boiling point | The temperature at which a liquid changes to a gas throughout the liquid; fixed for a pure substance. |
| Pure substance | A single element or compound with nothing else mixed in, which has sharp, fixed melting and boiling points. |
| Reversible reaction | A reaction that can go in either direction, such as the hydration and dehydration of copper(II) sulfate. |
Avoid these mistakes
1
Saying water can be identified by its lack of taste or smell. This is unsafe and also true of many other liquids.
2
Claiming that the copper sulfate test shows the water is pure. It only shows that water is present.
3
Describing the colour change the wrong way round. It goes from white to blue when water is added.
4
Stating that impurities lower the boiling point. They usually raise the boiling point and lower the melting point.
5
Forgetting the 100 °C value only applies at normal atmospheric pressure.
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