Chapter 10: Chemistry of the Environment — Part 1

01 Chemical Tests for Water

The presence of water can be identified using anhydrous salts which change colour upon hydration:

• Anhydrous cobalt(II) chloride: Turns from blue to pink.
• Anhydrous copper(II) sulfate: Turns from white to blue.

02 Purity of Water

• Physical Tests: The purity of water is determined by its melting and boiling points. Pure water has a fixed boiling point of 100°C and a melting point of 0°C. Impurities will cause these values to change or occur over a range of temperatures.
• Distilled Water: In practical chemistry, distilled water is preferred over tap water because it contains fewer chemical impurities.

03 Substances in Natural Water

Water from natural sources contains various dissolved substances, some of which are beneficial while others are harmful:

• Beneficial: Dissolved oxygen is essential for aquatic life, and certain metal compounds provide minerals necessary for health.
• Harmful:
  • Metal compounds: Some, like mercury and lead, are toxic.
  • Plastics: These harm aquatic life and break down into microplastics that enter the food chain.
  • Sewage: Contains harmful microbes that cause disease.
  • Nitrates and Phosphates: Often from fertilisers or detergents, these lead to deoxygenation of water and damage aquatic life.

04 Water Treatment

To make water potable (safe to drink), the domestic supply undergoes a series of treatment steps:

1. Sedimentation: Water is held in large tanks to allow large, insoluble particles to sink.
2. Filtration: Removes smaller insoluble particles by passing water through layers of sand and filters.
3. Activated Carbon: Added to adsorb impurities that cause unpleasant tastes and odours.
4. Chlorination: Chlorine gas is injected to kill harmful microbes and bacteria too small to be filtered out.

05 Fertilisers

• Purpose: Ammonium salts and nitrates are used as fertilisers to provide a soluble source of nitrogen for plant growth.
• NPK Fertilisers: These provide three essential elements: nitrogen (N), phosphorus (P), and potassium (K) to improve plant productivity.