10.4 Required practical: analysing and purifying water

The practical

This is AQA required practical 8: analyse and purify water samples from different sources, including pH, dissolved solids and distillation.

1. Testing the pH

Add a few drops of universal indicator and compare the colour with a chart, or use a pH probe for a more accurate reading. Potable water should be close to pH 7.

2. Mass of dissolved solids

1. Weigh a clean, dry evaporating basin.
2. Measure 50 cm³ of the water sample into it.
3. Heat gently (for example over a water bath) to evaporate the water.
4. Let the basin cool, then reweigh it.
5. Mass of dissolved solids = final mass − mass of empty basin.

Example: 32.61 g − 32.45 g = 0.16 g in 50 cm³, so the concentration is 0.16 ÷ 0.050 = 3.2 g/dm³.

3. Distillation

1. Heat the salty water in a boiling tube or flask.
2. The water evaporates, and the steam passes along a delivery tube.
3. It condenses in a test tube cooled in ice-cold water (or in a condenser).
4. The salt is left behind.

Checking the distilled water is pure

• It should have pH 7.
• Evaporating it should leave no residue.
• It should boil at exactly 100 °C.

Safety

• Wear eye protection. Do not let the salty water boil dry.
• Remove the delivery tube from the cold test tube before you stop heating, to prevent suck-back.