It is important in this core practical to use apparatus and substances carefully and safely, and to observe chemical changes.
Eye protection must be worn at all times.
|Water sample||pH||Mass of solid obtained by evaporating 50 cm3 of water sample (g)|
Which sample was sea water?
Sample B was sea water. It contained the highest mass of dissolved solids.
Which sample was obtained from a region affected by acid rain?
Sample C was acidic, so it could have been obtained from a region affected by acid rain.
1. Set up your distillation apparatus as demonstrated by your teacher. You may be able to use a conventional condenser like this:
Or you might be using apparatus like this:
2. Heat your sea water solution until it boils gently.
3. After a period of time you should see distilled water being produced.
Why is it not safe to drink the distilled water sample?
It might be contaminated because the apparatus might not have been cleaned properly before the practical.
Why was it important to test heat 50 cm3 of each water sample to dryness in the first part of the practical?
So the mass of the dissolved solids could be compared in a valid test.
How could you test to see if the distilled water contained sodium ions and chloride ions?
Sodium ions would give a yellow colour in a flame test. Chloride ions would give a white precipitate with silver nitrate solution after a small amount of nitric acid was added. For a reminder about these chemical tests, see this guide on chemical tests.
It is essential that the end of the delivery tube in the method described by the second diagram above is above the level of the filtrate. This is for safety reasons, when you stop heating the sea water mixture. Explain what would happen if the heat was removed and the end of the delivery tube was below the surface of the distillate.
As the gas inside the boiling tube and delivery tube cools, it will contract, sucking the distillate up the delivery tube. When this cool water reaches the hot glass, it may cause the glass to break.
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
|Hazard||Possible harm||Possible precaution|
|Acidic or alkaline water samples||Irritating to eyes or skin||Rinse off skin if contact occurs, wear eye protection|
|Hot apparatus||Burns||Allow hot apparatus time to cool before touching|