10.4 Required practical: analysing and purifying water
5 exam questions worth 9 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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How can you check that distilled water is pure?
Choose one answer.
Correct Not quite: the answer is It boils at exactly 100 °C
Mark scheme
D
Model answer
D: pure water boils at exactly 100 °C (and leaves no residue).
A 25 cm3 sample of water was evaporated. The mass of dissolved solids left was 0.85 g.
Calculate the concentration of dissolved solids in g/dm3.
Mark scheme
- 25 cm3 = 0.025 dm3 (1)
- 0.85 ÷ 0.025 = 34 g/dm3 (1)
Model answer
Concentration = 0.85 ÷ 0.025 = 34 g/dm3.
Describe how to find the mass of dissolved solids in a water sample.
Mark scheme
- weigh an empty evaporating basin (1)
- add a measured volume of water and evaporate it (1)
- reweigh; subtract the mass of the basin (1)
Model answer
Weigh an empty evaporating basin, add a measured volume of the sample and evaporate the water, then reweigh and subtract the mass of the empty basin.
Explain why the test tube collecting the distilled water is placed in ice-cold water.
Mark scheme
- to cool / condense the steam (1)
Model answer
To cool the steam so that it condenses back into liquid water.
Give one way to test the pH of a water sample and state the expected pH of potable water.
Mark scheme
- universal indicator / pH probe (1)
- about 7 (6.5 to 8.5) (1)
Model answer
Use a pH probe; potable water should be close to pH 7.
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