10.1 Water
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1. Describe the chemical test for the presence of water using anhydrous cobalt(II) chloride.
Anhydrous cobalt(II) chloride paper transforms from blue to pink upon coordinating with water molecules.
2. Describe the chemical test for the presence of water using anhydrous copper(II) sulfate.
Anhydrous copper(II) sulfate powder transforms from white to blue as it undergoes hydration.
3. How is the purity of water tested using physical properties?
By verifying its precise thermodynamic transition states: pure water displays a sharp melting point at exactly 0°C and a fixed boiling point at exactly 100°C under a standard atmospheric pressure of 1 atm.
4. Why is distilled water preferred over tap water in practical chemistry?
Because distilled water is chemically purified, containing significantly fewer dissolved ionic salts or chemical contaminants that could distort analytical test results.
5. List the specific substances that may be found in water from natural sources.
Dissolved oxygen (O2), metal compounds, insoluble plastics, untreated sewage, harmful pathogenic microbes, nitrates (from agricultural runoff), and phosphates (from detergents and fertilisers).
6. Which substances in natural water are considered beneficial, and why?
Dissolved oxygen is vital to sustain cellular respiration in aquatic organisms, and specific dissolved mineral compounds supply essential trace ions necessary for healthy growth.
7. What are the potentially harmful effects of metal compounds and plastics in water?
Certain dissolved metal salts (e.g., heavy metal ions like Pb2+ or Hg2+) are highly toxic to living tissues, while microplastics present ingestion and structural toxicity hazards to marine life.
8. What is the danger of sewage in water supplies?
Sewage serves as a vector for pathogenic microbes, which rapidly transmit waterborne infectious diseases (such as cholera).
9. What are the environmental consequences of nitrates and phosphates in water?
They act as plant nutrients that trigger rapid algal blooms. This leads to eutrophication, a process causing severe deoxygenation of the water column and the subsequent collapse of aquatic ecosystems.
10. Describe the two steps in domestic water treatment used to remove solids.
1. Sedimentation: Allowing large, dense particles to sink to the bottom of large tanks.
2. Filtration: Passing the water through beds of fine sand and gravel to trap smaller suspended solids.
2. Filtration: Passing the water through beds of fine sand and gravel to trap smaller suspended solids.
11. What is the purpose of using carbon in water treatment?
Using activated carbon beds absorbs organic contaminants, effectively filtering out unpleasant tastes and volatile odors.
12. Why is chlorination used in the treatment of the domestic water supply?
To act as a powerful chemical disinfectant that kills remaining harmful bacterial and pathogenic microorganisms, rendering the water safe to consume.