Water Cambridge IGCSE Combined Science revision
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In plain words
Water from a tap looks pure, but it isn't: it has all sorts dissolved in it, some useful and some not. Chemistry has simple tests to show that a liquid contains water, and to show whether that water is pure. And before water reaches your tap it has been cleaned in several steps.
Three things to know
- Tests for water: white anhydrous copper(II) sulfate turns blue; blue anhydrous cobalt(II) chloride turns pink. Pure water boils at exactly 100 °C and freezes at exactly 0 °C.
- Treating the water supply: sedimentation and filtration remove solids, carbon removes tastes and smells, and chlorine kills microbes.
- Natural water contains dissolved substances. Some are useful (oxygen for fish, minerals). Some are harmful: sewage and microbes, plastics, toxic metal compounds, and nitrates and phosphates from fertilisers and detergents, which use up the oxygen in rivers.
Tips and tricks
- The chemical tests show water is present. Only the boiling point (or melting point) shows it is pure.
- Give the colour change both ways round: "from white to blue".
It lands in your notebook with its questions as flashcards.
Water: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What colour change shows that water is present when it is added to anhydrous cobalt(II) chloride?
Anhydrous cobalt(II) chloride is blue; with water it turns pink.
Why is chlorine added to the water supply?
It makes the water safe to drink.
What does filtration remove from water?
Dissolved substances pass straight through.
At what temperature does pure water boil?
Impurities change the boiling point.
What harm do nitrates and phosphates from farms do in rivers?
They make water plants and algae overgrow; when these die and rot, the oxygen is used up.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 7 marks. Write your answers on paper, then check them.
Water
Cambridge IGCSE Combined Science 0653 · 7 marks · papermunch.org
Name ______________________________ Date ______________
Describe a chemical test for water, and give the result.[2]
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Add it to anhydrous copper(II) sulfate, which turns from white to blue. (Or anhydrous cobalt(II) chloride: blue to pink.)
Describe how you could show that a sample of water is pure.[1]
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Measure its boiling point: pure water boils at exactly 100 °C.
State why water is filtered and why chlorine is added when it is treated.[2]
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Filtration removes insoluble solids. Chlorine kills microbes (bacteria).
Explain why distilled water, not tap water, is used in practical chemistry.[2]
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It contains fewer dissolved substances (impurities), which could react or interfere with the experiment.
Answers: Water
- 1. Add it to anhydrous copper(II) sulfate, which turns from white to blue. (Or anhydrous cobalt(II) chloride: blue to pink.)
- 2. Measure its boiling point: pure water boils at exactly 100 °C.
- 3. Filtration removes insoluble solids. Chlorine kills microbes (bacteria).
- 4. It contains fewer dissolved substances (impurities), which could react or interfere with the experiment.