Chapter 10: Water - Chemical Tests, Purity, Natural Sources, and Purification

Chapter 10 Notes

Chemical Tests for Water Presence

1. Test with Anhydrous Cobalt(II) Chloride

• Anhydrous cobalt(II) chloride changes color from blue to pink in the presence of water.
• Cobalt(II) chloride paper, containing anhydrous cobalt(II) chloride, is used for this test.
• Reaction:
  CoCl2(s) + 6H2O(l) \rightarrow CoCl2 "." 6H2O(s)
  (anhydrous cobalt(II) chloride (blue) $\rightarrow$ hydrated cobalt(II) chloride (pink))
• Procedure: Heat the sample in a test-tube and place cobalt(II) chloride paper at the mouth. If water is present, the paper turns from blue to pink.

2. Test with Anhydrous Copper(II) Sulfate

• Anhydrous copper(II) sulfate changes color from white to blue in the presence of water.
• Reaction:
  CuSO4(s) + 5H2O(l) \rightarrow CuSO4 "." 5H2O(s)
  (anhydrous copper(II) sulfate (white) $\rightarrow$ hydrated copper(II) sulfate (blue))
• Procedure: Add a few drops of the sample to anhydrous copper(II) sulfate. If water is present, it turns from white to blue.
• Note: These tests only show the presence of water, not its purity.

Testing Water Purity

• Pure substances have precise melting and boiling points.
• Pure water melts at 0°C and boils at 100°C.
• Impurities:
  • Lower the melting point of ice.
  • Increase the boiling point of water.
  • Cause melting and boiling to occur over a range of temperatures.
• The more impurities present, the more the melting and boiling points deviate from those of pure water.

Use of Distilled Water in Practical Chemistry

• Tap water contains dissolved solids like calcium salts and impurities like chloride and aluminum ions.
• Distilled water consists almost entirely of H2O molecules with close to no impurities.
• Using distilled water ensures more accurate results in practical chemistry.

Water from Natural Sources

Substances in Natural Water Sources

• Water from natural sources (rivers, lakes) contains various substances:
  • Dissolved oxygen
  • Metal compounds
  • Plastics
  • Sewage
  • Harmful microbes
  • Nitrates from fertilizers
  • Phosphates from fertilizers and detergents
• Water is an excellent solvent, transporting minerals and nutrients for plants.
• Bodily fluids contain biologically important solutes dissolved in water.
• Dissolved gases are crucial for respiration of plants, animals, and fish.

Beneficial Substances

1. Dissolved Oxygen:
   • Aquatic life needs dissolved oxygen for breathing.
2. Metal Compounds:
   • Provide essential minerals for plant and animal growth (e.g., aluminum, calcium, and potassium salts).

Potentially Harmful Substances

Figure 15.3 Potentially harmful substances in water from natural sources

1. Metal Compounds:
   • Industrial waste can contain toxic metals like lead and mercury.
   • Pollution from industrial discharge makes water unsuitable for life.
2. Plastics:
   • Plastic waste pollutes water. Non-biodegradable plastics harm aquatic life through choking or poisoning.
3. Sewage:
   • Untreated sewage contains harmful microbes that cause diseases.
4. Nitrates and Phosphates:
   • Fertilizers and detergents contain these, leading to rapid algae growth.
   • Algae respiration depletes dissolved oxygen causing deoxygenation and death of aquatic life.

Purification of Domestic Water Supply

Stages in Water Treatment

Figure 15.4 Main stages in the treatment of water

1. Reservoir: Water is collected and stored.
2. Screening: Water passes through metal grids to remove large solids (leaves, twigs, litter).
3. Coagulants Added: Aluminium sulfate (Al2(SO4)3) and iron (III) chloride (FeCl3) are added. These coagulants cause fine particles of dirt and dissolved impurities to clump together.
4. Sedimentation: Clumped particles increase in size and settle to the bottom of the tank.
5. Filtration: Powdered carbon is added to remove unpleasant tastes and odours. Water seeps through sand and gravel layers, removing remaining fine particles.
6. Chlorination: Chlorine is carefully added to kill microbes, preventing water-borne diseases (typhoid, cholera). This process is called chlorination.
7. Storage Tank: Treated water is stored in a tank for distribution.

• Ozone can be used as a disinfectant to kill bacteria.
• Lime is added to reduce acidity after chlorination.
• Boiling can kill bacteria.