Water Notes
Cambridge IGCSE Chemistry: Revision notes
Key facts
- Anhydrous cobalt(II) chloride turns blue → pink with water; anhydrous copper(II) sulfate turns white → blue.
- Pure water melts at 0 °C and boils at 100 °C; impurities change these.
- Distilled water is made by boiling and condensing.
- Water treatment: sedimentation, filtration, activated carbon, chlorination.
- Fertiliser run-off (nitrates, phosphates) causes excess algal growth.
Testing for water
Anhydrous cobalt(II) chloride and anhydrous copper(II) sulfate both change colour when they become hydrated.
Both tests rely on a colour change as the substance becomes hydrated; the change can be reversed by removing the water.
- 1
Anhydrous solid
cobalt(II) chloride is blue; copper(II) sulfate is white
- 2
Add water
the solid becomes hydrated
- 3
Hydrated solid
cobalt(II) chloride turns pink; copper(II) sulfate turns blue
- 4
Remove the water
the original colour returns
Then back to step 1
| Without water | With water | |
|---|---|---|
| Cobalt(II) chloride | Blue | Pink |
| Copper(II) sulfate | White | Blue |
Without water
- Cobalt(II) chloride:
- Blue
- Copper(II) sulfate:
- White
With water
- Cobalt(II) chloride:
- Pink
- Copper(II) sulfate:
- Blue
Anhydrous copper(II) sulfate turns from white to:
Purity of water
Pure water melts at exactly 0 °C and boils at 100 °C.
Dissolved impurities lower the melting point and raise the boiling point, so any deviation shows that dissolved substances are present. A sample boiling at 102 °C is not pure.
A water sample boils at 102 °C. It is:
Distilled water
It is pure, so it does not interfere with reactions or measurements.
Distilled water has virtually no dissolved or suspended impurities, prevents unwanted side reactions and gives consistent, repeatable results. Tap water contains dissolved minerals and chlorine that can affect reaction rates, products and measurements.
- 1
Boil the water
it changes to steam
- 2
Impurities stay behind
they do not evaporate
- 3
Condense the steam
cool it back to a liquid
- 4
Collect distilled water
pure liquid water
How is distilled water made?
Contaminants
Natural water carries dissolved and suspended substances, some beneficial and some harmful.
Beneficial substances support aquatic ecosystems; harmful ones damage water quality and risk health.
- 1
Fertiliser runs off fields
nitrates and phosphates enter the water
- 2
Excess algal growth
- 3
Dissolved oxygen is used up
oxygen depletion
- 4
Fish and other life are harmed
fish need dissolved oxygen to breathe
| Source | Effect | |
|---|---|---|
| Dissolved oxygen | Atmosphere, aquatic plants | Beneficial: fish need it to breathe |
| Metal compounds | Rock weathering, soil minerals | Mixed: calcium and magnesium essential; lead and mercury toxic |
| Plastics | Human pollution | Harmful: damage organisms, enter food chains |
| Sewage | Human waste, urban run-off | Harmful: disease-causing microbes |
| Nitrates and phosphates | Fertiliser run-off | Harmful: excess algal growth, oxygen depletion |
Source
- Dissolved oxygen:
- Atmosphere, aquatic plants
- Metal compounds:
- Rock weathering, soil minerals
- Plastics:
- Human pollution
- Sewage:
- Human waste, urban run-off
- Nitrates and phosphates:
- Fertiliser run-off
Effect
- Dissolved oxygen:
- Beneficial: fish need it to breathe
- Metal compounds:
- Mixed: calcium and magnesium essential; lead and mercury toxic
- Plastics:
- Harmful: damage organisms, enter food chains
- Sewage:
- Harmful: disease-causing microbes
- Nitrates and phosphates:
- Harmful: excess algal growth, oxygen depletion
Which contaminant is beneficial?
Water treatment
Settling and filtering remove solids, carbon removes taste and odour, and chlorine kills microbes.
Heavy solids settle in tanks (sedimentation), then sand and gravel filters remove remaining solids. Activated carbon, with a large surface area, adsorbs tastes, odours and some chemicals. Chlorine kills bacteria and viruses, protecting against cholera and typhoid, and a little stays in the water as it travels through pipes.
- 1
Sedimentation
heavy solids settle out
- 2
Filtration
sand and gravel remove fine solids
- 3
Activated carbon
removes tastes and odours
- 4
Chlorination
kills harmful microbes
What is chlorine added to water for?
Try an exam question
Describe a chemical test for water and a physical test to show that a sample of water is pure.
[3 marks]
- [1]Add water to anhydrous copper(II) sulfate; it turns from white to blue (or cobalt(II) chloride blue to pink).
- [1]Pure water melts at 0 °C.
- [1]Pure water boils at 100 °C.
That's the notes covered.
Carry on to the next subtopic.