Required practical
Analysis and purification of water
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.
Aims
- To analyse samples of water from different sources in terms of Scale of acidity or alkalinity. A pH (power of hydrogen) value below 7 is acidic, a pH value above 7 is alkaline. and the presence of When something is broken up in a liquid and no longer exists. solids.
- To A separation technique which involves a solution being heated so that the solvent evaporates before being cooled to form a pure liquid. sea water to obtain Water that is safe to drink..
Method - Part 1 (Analysing water samples)
- For each sample of water you are given, test the pH using either a An instrument which can be used to measure the pH of a solution, providing a numerical value which is usually to 1 or 2 decimal places. or A chemical solution that produces many different colour changes corresponding to different pH levels. and an appropriate colour chart. Record your observations carefully.
- For each sample of water, pour 50 cm3 into a clean pre-weighed A round porcelain dish which contains a solution which is heated from below so that the solvent evaporates, leaving the solute in the dish.. Heat gently over a Bunsen burner, tripod and gauze until no liquid remains. Allow to cool, then weigh the evaporating basin again and calculate the mass of solid that remains in the evaporating basin.
Analysis - Part 1 (Analysing water samples)
| Water sample | pH | Mass of solid obtained by evaporating 50 cm3 of water sample (g) |
| A | 7 | 0.00 |
| B | 8 | 1.74 |
| C | 4 | 0.07 |
| Water sample | A |
|---|---|
| pH | 7 |
| Mass of solid obtained by evaporating 50 cm3 of water sample (g) | 0.00 |
| Water sample | B |
|---|---|
| pH | 8 |
| Mass of solid obtained by evaporating 50 cm3 of water sample (g) | 1.74 |
| Water sample | C |
|---|---|
| pH | 4 |
| Mass of solid obtained by evaporating 50 cm3 of water sample (g) | 0.07 |
Question
Which sample was sea water?
Sample B was sea water. It contained the highest mass of dissolved solids.
Question
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.
Method - Part 2 (Distillation)
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.
Analysis - Part 1 (Analysing water samples)
Question
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.
Evaluation
Question
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.
Question
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 A suspension of particles in a liquid formed when a dissolved substance reacts to form an insoluble substance, eg in a precipitation reaction. 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.
Question
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.
Hazards, risks and precautions
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 |
| Hazard | Acidic or alkaline water samples |
|---|---|
| Possible harm | Irritating to eyes or skin |
| Possible precaution | Rinse off skin if contact occurs, wear eye protection |
| Hazard | Hot apparatus |
|---|---|
| Possible harm | Burns |
| Possible precaution | Allow hot apparatus time to cool before touching |