What colour change shows that water is present when it is added to anhydrous cobalt(II) chloride?

blue to pink. Anhydrous cobalt(II) chloride is blue; with water it turns pink.

Why is chlorine added to the water supply?

to kill microbes. It makes the water safe to drink.

What does filtration remove from water?

insoluble solids. Dissolved substances pass straight through.

At what temperature does pure water boil?

at exactly 100 °C. Impurities change the boiling point.

What harm do nitrates and phosphates from farms do in rivers?

They lead to the oxygen in the water being used up, killing fish. They make water plants and algae overgrow; when these die and rot, the oxygen is used up.

Water · Cambridge IGCSE Co-ordinated Sciences (Double Award)

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".

5 questions, about 2 minutes.

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?
- white to blue
- blue to pink (the answer)
- pink to blue
- blue to white
Anhydrous cobalt(II) chloride is blue; with water it turns pink.
Why is chlorine added to the water supply?
- to remove solids
- to improve the taste
- to kill microbes (the answer)
- to make it boil sooner
It makes the water safe to drink.
What does filtration remove from water?
- dissolved salts
- insoluble solids (the answer)
- microbes
- chlorine
Dissolved substances pass straight through.
At what temperature does pure water boil?
- above 100 °C
- below 100 °C
- at exactly 100 °C (the answer)
- at any temperature
Impurities change the boiling point.
What harm do nitrates and phosphates from farms do in rivers?
- They make the water acidic.
- They lead to the oxygen in the water being used up, killing fish. (the answer)
- They turn the water blue.
- They make the water boil.
They make water plants and algae overgrow; when these die and rot, the oxygen is used up.

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 Co-ordinated Sciences (Double Award) 0654 · 7 marks · papermunch.org

Name ______________________________ Date ______________

Describe a chemical test for water, and give the result.[2]
Show answerHide answer
Add it to anhydrous copper(II) sulfate, which turns from white to blue. (Or anhydrous cobalt(II) chloride: blue to pink.)
I got this one
Describe how you could show that a sample of water is pure.[1]
Show answerHide answer
Measure its boiling point: pure water boils at exactly 100 °C.
I got this one
State why water is filtered and why chlorine is added when it is treated.[2]
Show answerHide answer
Filtration removes insoluble solids. Chlorine kills microbes (bacteria).
I got this one
Explain why distilled water, not tap water, is used in practical chemistry.[2]
Show answerHide answer
It contains fewer dissolved substances (impurities), which could react or interfere with the experiment.
I got this one

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.