Water purification
Analysing and purifying a sample of water and making it
‘safe to drink’.
During this required practical, students are required to test, analyse, and purify a water sample using a simple distillation. The students will measure the pH of the water, comparing it with the pH of pure water and will reveal and remove any dissolved solids by evaporation. They will then distill the water and re-test it to show it is now purified. Their use of laboratory equipment is also assessed.
Equipment
• Safety goggles
• Bunsen burner
• Tripod
• Gauze
• Heatproof mat
• Test tubes
• Test tube rack
• Evaporating basin
• 50ml measuring cylinder
• 100ml conical flask
• Rubber bung with one hole and delivery tube
• Retort stand and clamp
• Crystallising dish
• Full range universal indicator solution
• Full range universal indicator paper
• Water samples
Equipment for the class to share
• Electronic balance (2 decimal place)
• CLEAPSS Hazcards: 47A, 95A
Method
Activity 1: Analysing a water sample
- Students should collect the necessary equipment and set up their Bunsen burner.
- They should use a piece of universal indicator paper to test the pH of the water samples to be investigated and record these in a suitable table.
- They must accurately weigh an evaporating basin, using the electronic balance, and record the value to two decimal places.
- Next, they pour 10mls of the first water sample into the evaporating basin and record the mass.
- They then use their Bunsen burner to carefully heat the evaporating basin until they can start to see the dissolved solids forming.
- Once the majority of the water has been removed, students can turn off their Bunsen burner and must allow the evaporating basin to cool.
- Once cooled, the students can re-weigh the evaporating basin and calculate the mass of the solids that were dissolved in the water. They record the results in their table.
- The procedure is then repeated for the other water samples.
Activity 2: Purifying by distillation
- Students should set up their distillation equipment. 10mls of the first water sample should be added to the conical flask, which is then placed on a tripod and gauze over a Bunsen burner. When the bung is inserted, the other end of the delivery tube should be placed into a test tube, ensuring that there is a gap of 3cm from the bottom of the test tube, which is standing in a beaker of iced water. The test tube should be secured using a retort stand and clamp.
- The water in the conical flask should be gently heated using the Bunsen burner. Once it starts to boil, students should reduce the heat so that the water boils gently.
- Students must watch as the water evaporates in the conical flask and the steam condenses on the insides of the delivery tube. The purified water is collected in the test tube until there is approximately 1cm depth. The students can now turn off their Bunsen burner.
- They will now analyse the water they have distilled by determining its boiling point.
Video produced by revisechemistrywithmrb,
practical GCSE Chemistry tutorials by a specialist Chemistry teacher with over 25 years experience.
Technician tips
• The pH of a water sample could be increased using Hydrochloric Acid.
• Dissolved solids could include Sodium Chloride and Sodium Carbonate.
• To prevent suck-back during the distillation, students should be warned to not let the solution from the conical flask boil into the delivery tube. The end of the delivery tube should be kept at least 3cm away from the bottom of the test tube.
To do this practical you will need
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