Water purification required practical

Aim: To analyse samples of water for pH and dissolved solids, and to purify a sample of salty water by distillation.

Equipment

• Water samples (e.g. tap water, rainwater, seawater or salty water)
• Universal indicator paper or solution with colour chart, or a pH probe
• Evaporating basin and balance (to 0.01 g)
• Measuring cylinder
• Bunsen burner, tripod, gauze and heatproof mat
• Conical flask with bung and delivery tube (or Liebig condenser)
• Test tube in a beaker of ice-cold water
• Thermometer

Method

1. Test the pH of each water sample with universal indicator or a pH probe and record it.
2. Weigh an empty, dry evaporating basin and record its mass.
3. Measure 10 cm³ of a water sample into the basin and heat it gently until all the water has evaporated.
4. Let the basin cool, then reweigh it; the increase in mass is the mass of dissolved solids.
5. To purify salty water, put about 50 cm³ of it into a conical flask and fit a bung with a delivery tube leading into a test tube standing in ice-cold water.
6. Heat the flask gently so the water boils steadily, and let the steam condense in the cooled test tube.
7. Stop heating before the flask boils dry.
8. Check the distilled water collected: its pH should be 7, it should boil at 100 °C, and evaporating a sample should leave no solid.

Safety

• Wear eye protection, as hot water and solids can spit when evaporating.
• Heat gently and never let the flask boil dry; the apparatus becomes very hot, so let it cool before touching it.
• Make sure the delivery tube is not blocked and does not dip below the water in the collecting tube, to prevent suck-back.
• Do not drink any of the water samples in the laboratory.

Results and calculations

Record the pH of each sample and the mass of the basin before and after evaporation; mass of dissolved solids = final mass − mass of empty basin, and this can be expressed as g/dm³ by dividing by the volume in dm³. For distillation, record the boiling point and appearance of the collected water to show it is pure.

Common mistakes and improvements

• Heating the basin too strongly so solid spits out, giving a low mass of solids; heat gently and use a water bath near the end.
• Weighing the basin while still hot or wet; let it cool and dry fully before weighing.
• Using too small a volume of water, so the mass of solid is too small to measure precisely; use a larger volume or a more precise balance.
• Steam escaping instead of condensing; keep the collecting tube in ice-cold water or use a Liebig condenser, and check the bung fits tightly.

Exam tips

• Know the difference between potable water (safe to drink, low levels of dissolved salts and microbes) and pure water (only H₂O).
• Describe distillation clearly: water evaporates on heating, leaves the dissolved salts behind, and the steam is cooled and condenses back into liquid.
• State how to show distilled water is pure: it boils at exactly 100 °C and leaves no residue on evaporation.
• Link to real life: desalination of seawater by distillation or reverse osmosis needs large amounts of energy, so it is expensive.
• Calculate the mass of dissolved solids from before-and-after masses and convert to g/dm³ when asked.