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10.1.3Using Resources

Required Practical: Analysis and Purification of Water Samples — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions

5 min read 7 parts Lesson 4 of 8

📚 AQA GCSE Combined Science Trilogy 8464 - Chemistry Hub / Topical Revision
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Part 1 of 5Aim and variables

Aim
  • Aim Analyse samples of water for pH and dissolved solids, then purify one by distillation to obtain pure water.
Independent variable
  • Independent variable The water sample being tested (e.g. A, B, C and D).
Dependent variables
  • Dependent variables The pH of each sample and the mass of solid left after evaporation.
Control variables
  • Control variables Volume of water in each test, the same heating method and drying time, and the same balance.

Part 2 of 5Part 1: pH of each sample

1
Pour a small sample of each water into separate clean containers.
2
Dip a piece of universal indicator paper, or add a few drops of universal indicator solution, into the first sample.
3
Compare the colour with the pH chart and record the pH.
4
Rinse the equipment and repeat for each sample.

Part 3 of 5Part 2: mass of dissolved solids

1
Measure the mass of a clean, dry evaporating basin.
2
Use a measuring cylinder to add a known volume (e.g. 20 cm3) of one sample to the basin.
3
Heat gently over a water bath or Bunsen burner until most of the water has evaporated, then allow the rest to dry off to avoid spitting.
4
Let the basin cool, then weigh it again.
5
Mass of dissolved solid = final mass of basin − original mass of basin. Repeat for each sample using the same volume.

Part 4 of 5Part 3: distillation of salty water

1Heat the flask
  • Heat the flask Salty water is heated gently in a flask connected to a condenser.
2Water evaporates
  • Water evaporates Water boils to steam while the dissolved salt stays behind.
3Steam condenses
  • Steam condenses Cold water around the condenser turns the steam back to liquid.
4Collect distillate
  • Collect distillate Pure water collects in a beaker.
    • Test it: it should leave no residue on evaporation and have a pH close to 7.

Part 5 of 5Safety and sources of error

Hazard or errorPrecaution
Hot apparatus and steamWear eye protection, use tongs and let equipment cool before handling
Spitting of hot solutionHeat gently and finish drying with the heat turned off
Unknown water samplesDo not drink; treat samples as potentially harmful
Solid not fully dryRe-heat and re-weigh until the mass is constant
Unequal volumesMeasure each sample with the same measuring cylinder

Key terms

TermMeaning
Dissolved solidsSubstances such as salts that are dissolved in water and remain as a solid residue when the water evaporates.
DistillationSeparating a liquid from a solution by evaporating it and condensing the vapour.
CondenserApparatus that cools vapour so it turns back into a liquid.
Universal indicatorA mixture of dyes that changes colour across the pH scale.
DistillateThe liquid collected after condensing during distillation.

Avoid these mistakes

1
Using different volumes of each sample, which makes the masses of solid impossible to compare.
2
Weighing the basin while it is still hot or still damp.
3
Heating the basin too strongly so that the solution spits out and solid is lost.
4
Saying that boiling the water kills the dissolved solid. The solid remains in the basin.

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