Water Purification
Q1.
Potable water is water that is safe to drink.
Seawater can be changed into potable water by desalination.
(a)     Name the substance removed from seawater by desalination.
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(1)
(b)     Desalination requires large amounts of energy.
Desalination is only used when there is no other source of potable water.
Give one reason why.
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Water from lakes and rivers can be treated to make it potable.
(c)     The first stage is to filter the water from lakes and rivers.
Why is the water filtered?
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(d)     Chlorine gas is then added to the filtered water.
Why is chlorine gas used to treat water?
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(1)
(e)     Describe a test for chlorine gas.
Give the result of the test if chlorine is present.
Test ______________________________________________________________
Result ____________________________________________________________
(2)
Some students investigated different water samples.
The table shows some of their results.
 
Water	pH	Mass of dissolved solid in g / dm3	
Tap water	6.5	0.5	
Seawater	8.1	35.0	
Pure water	 	 	
(f)      Complete the table above to show the expected results for pure water.
(2)
(g)     What mass of dissolved solid is present in 100 cm 3 of the sample of tap water?
Tick (✔) one box.
 
0.05 g		
0.5 g		
5 g		
50 g		
(1)
(h)     Boiling points can be used to show whether substances are pure.
The diagram shows the apparatus the students used to find the boiling point of tap water.
 
The students made a mistake setting up the apparatus.
What mistake did the students make?
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(1)
(Total 10 marks)