Water analysis – pH, dissolved solids and distillation
Integrated ScienceMatter & MaterialsAges 11–12
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Sign in to playAnalyse tap water, sea water, rain water, river water and an unknown sample as in the school practical: measure the pH with universal indicator or a pH meter, filter off suspended solids, evaporate a known volume in a weighed basin to calculate the mass of dissolved solids (g/dm³) step by step, then distil the sample and test the distillate: pH ≈ 7, boils at 100 °C, leaves no residue. Set up the thermometer or the cooling water wrongly to see what goes wrong.
Lesson: Analysis and purification of water samples by distillation
What it shows
Natural water is a mixture: it contains dissolved salts and gases, and sometimes insoluble solids. Measuring the pH and the mass of dissolved solids left after evaporating a known volume shows how samples differ. Sea water holds about 35 g/dm³ of dissolved solids, tap water well under 1 g/dm³, and rain water almost none but is slightly acidic from dissolved carbon dioxide. Filtration removes only insoluble particles. Distillation boils the water off and condenses the vapour, leaving the dissolved solids behind, so the distillate is pure: pH 7, boiling point 100 °C and no residue.
How to use
Choose a Sample, the Volume evaporated, pH by and Balance, and tick Filter before evaporating if you wish. Then follow the numbered buttons: measure the pH, filter, weigh the empty basin, evaporate and weigh again, distil, then test the distillate. Auto run does every step. Compare samples in the results table and the bar chart, and try a wrong Setup.
Parameters you can change
- Water sample Tap water, Sea water, Rain water, River water (muddy), Unknown sample (random)
- Volume of sample evaporated 10–100 cm³
- Filter the sample before evaporating
- pH method Universal indicator, pH meter
- Balance resolution 0.01 g, 0.001 g
- Distillation set-up Correct, Wrong: thermometer bulb in the liquid, Wrong: cooling water in at the top
Questions to explore
- Why does filtering lower the mass of residue from river water but not from sea water?
- Why must the thermometer bulb sit level with the side arm rather than in the boiling liquid?
- Which three results show that the distilled water collected is pure?
