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

Method - Part 1 (Analysing water samples)

  1. For each sample of water you are given, test the pH using either a pH meter or universal indicator and an appropriate colour chart. Record your observations carefully.
  2. For each sample of water, pour 50 cm3 into a clean pre-weighed evaporating basin. 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 pHMass of solid obtained by evaporating 50 cm3 of water sample (g)
A70.00
B81.74
C40.07
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:

Salty water is heated.Salty water is heated

Or you might be using apparatus like this:

Apparatus for distillation

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 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.

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:

HazardPossible harmPossible precaution
Acidic or alkaline water samplesIrritating to eyes or skin Rinse off skin if contact occurs, wear eye protection
Hot apparatus BurnsAllow hot apparatus time to cool before touching
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