Water
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
Chemical Tests for the Presence of Water
Two anhydrous salts change colour when water is added to them, so they are used as simple chemical tests for the presence of water:
| Anhydrous salt | Colour change on adding water |
|---|---|
| Cobalt(II) chloride | blue → pink |
| Copper(II) sulfate | white → blue |
Worked Example
- Question: a white solid turns blue when a few drops of liquid X are added to it. Identify the white solid, and state what this shows about liquid X. [2]
- Answer: the white solid is anhydrous copper(II) sulfate [1]; this shows that liquid X contains water [1]
Common Mistakes
- ❌ Mixing up the two colour changes
- ✓ cobalt(II) chloride goes blue → pink; copper(II) sulfate goes white → blue
Distilled Water
Distilled water is used in practical chemistry (rather than tap water) because it contains far fewer dissolved chemical impurities. Tap water's dissolved ions and compounds could react with reagents or interfere with results, so distilled water is used to keep experiments and measurements accurate.
Treating the Domestic Water Supply
Water from natural sources must be treated before it is safe to drink. Three stages are used:
| Stage | What it does |
|---|---|
| Sedimentation and filtration | remove suspended solids - heavier particles settle out (sedimentation), then the water is passed through beds of sand/gravel to remove finer particles (filtration) |
| Use of carbon | removes tastes and odours from the water |
| Chlorination | kills harmful microbes, making the water safe to drink |
Worked Example
- Question: four processes are listed - 1 filtration, 2 sedimentation, 3 distillation, 4 chlorination. Which processes are used in the treatment of the domestic water supply?
- Answer: 1, 2 and 4. Sedimentation and filtration remove suspended solids, and chlorination kills microbes. Distillation is not used - it is far too energy-intensive to apply on the scale of a domestic water supply.
Common Mistakes
- ❌ Including distillation as a stage of domestic water treatment
- ✓ distillation is not part of the domestic treatment process - it is too energy-intensive for the volumes involved; the three real stages are sedimentation/filtration, use of carbon, and chlorination
Interactive revision notes, videos and practice questions load below.