Required practical 8 - Water purification

Question 1

How can you test the pH of a sample of water?

Answer 1

Use universal indicator paper or a digital pH probe.

Question 2

What would you expect the pH of a sample of pure water to be? What colour would this be on universal indicator paper?

Answer 2

pH 7 - green.

Question 3

How could you find the mass of salt dissolved in a sample of water?

Answer 3

• Weigh an empty evaporating basin.
• Fill with a sample of the water and heat with a bunsen burner until all the water evaporates.
• Reweigh any salts left in the evaporating basin and calculate the mass of solids that were dissolved in the sample.

Question 4

State how can you purify a sample of water

Answer 4

Distillation.

Question 5

Describe how a sample of water can be purified by distillation

Answer 5

• First test the pH. Add acid/alkali as necessary to obtain pH 7.
• Place the water sample in a conical flask and connect the flask to distillation apparatus (including a condenser).
• As the water in the flask is heated, it evaporates and enters the condenser as steam.
• Cold water around the condenser cools the steam, which condenses to form water before it is collected in a beaker.

Question 6

How could you test that the water collected from a distillation experiment is pure water?

Answer 6

Check the pH is still pH 7.

Determine the boiling point and check it matches that of water (100°C).

Question 7

What safety precautions need to be considered when purifying water?

Answer 7

• Bunsen burner turned off or put on safety flame when not in use.
• Hair tied back.
• Acid/alkalis used for neutralisation so wear safety goggles.
• Clear up any glassware that breaks.