Water

1. Chemical tests for the presence of water

• Anhydrous cobalt(II) chloride test:
  • Principle: Anhydrous cobalt(II) chloride is blue; in the presence of water, it becomes hydrated and turns pink.
  • Test: Dip a piece of filter paper in cobalt(II) chloride solution and dry it. Add a drop of the test liquid.
  • Observation: Blue → pink confirms water.
  • Equation (word form):
    • Anhydrous cobalt(II) chloride + water → hydrated cobalt(II) chloride.
  • Equation (chemical symbols):
    • CoCl₂ (blue) + 6H₂O → CoCl₂·6H₂O (pink).
• Anhydrous copper(II) sulfate test:
  • Principle: Anhydrous copper(II) sulfate is white; in the presence of water, it becomes hydrated and turns blue.
  • Test: Add a small amount of anhydrous copper(II) sulfate to the sample.
  • Observation: White → blue confirms water.
  • Equation (word form):
    • Anhydrous copper(II) sulfate + water → hydrated copper(II) sulfate.
  • Equation (chemical symbols):
    • CuSO₄ (white) + 5H₂O → CuSO₄·5H₂O (blue).

2. Testing for purity of water using melting and boiling points

• Pure water:
  • Melting point: 0 °C.
  • Boiling point: 100 °C at 1 atm pressure.
• Procedure:
  • Heat/cool the sample while recording temperature.
  • If melting/boiling occurs exactly at these points → pure water.
  • If temperatures deviate, impurities are present.
• Reason:
  • Impurities lower the melting point and raise the boiling point (freezing point depression and boiling point elevation).

3. Distilled water vs tap water in practical chemistry

• Distilled water:
  • Produced by boiling water and condensing the steam.
  • Contains no dissolved salts, very low levels of other substances.
• Tap water:
  • Contains dissolved salts, minerals, and sometimes chlorine or fluoride.
• Why distilled water is preferred in experiments:
  • Prevents unwanted side reactions.
  • Avoids contamination in chemical tests.

4. Substances found in natural water sources

Natural water (rivers, lakes, reservoirs, groundwater) can contain:
(a) Dissolved oxygen – for aquatic life.
(b) Metal compounds – e.g., calcium and magnesium salts (from rocks).
(c) Plastics – from pollution.
(d) Sewage – waste from households/industries containing organic matter and microbes.
(e) Harmful microbes – bacteria, viruses causing diseases.
(f) Nitrates – from fertilisers (NO₃⁻ ions).
(g) Phosphates – from fertilisers and detergents (PO₄³⁻ ions).

5. Beneficial substances in natural water

• Dissolved oxygen:
  • Supports respiration of fish and other aquatic organisms.
• Essential minerals:
  • Calcium (Ca²⁺) for healthy bones and teeth.
  • Magnesium (Mg²⁺) for enzyme function and chlorophyll in plants.

6. Potentially harmful substances in natural water

• Toxic metal compounds:
  • Lead, mercury, cadmium → harmful to nervous system, kidneys, and other organs.
• Plastics:
  • Microplastics ingested by aquatic life → physical damage and toxic effects.
• Sewage:
  • Contains disease-causing microbes (e.g., cholera, typhoid).
• Nitrates and phosphates:
  • Cause eutrophication:
    • Overgrowth of algae → block sunlight → plants die → decay uses up oxygen → aquatic animals die.

7. Domestic water treatment process

• Step 1 – Sedimentation:
  • Large tanks allow heavy particles (sand, soil) to settle at the bottom.
• Step 2 – Filtration:
  • Water passes through layers of sand and gravel to remove smaller particles.
• Step 3 – Activated carbon treatment:
  • Removes unwanted tastes, odours, and some dissolved organic compounds.
• Step 4 – Chlorination:
  • Chlorine gas is bubbled through to kill harmful bacteria and microbes.
• Outcome:
  • Safe, clean drinking water.