RP8 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 you can 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 6

what safety precautions need to be considered when purifying water?

Answer 6

• bunsen burner turned off or put on safety flame when not in use, hair tied back
• acids/alkalis used for neutralisation so wear safety goggles
• clear up any glassware that breaks