10.1 Water

2026 Syllabus Objectives

  1. Core: Describe chemical tests for the presence of water using anhydrous cobalt(II) chloride and anhydrous copper(II) sulfate
  2. Core: Describe how to test for the purity of water using melting point and boiling point
  3. Core: Explain that distilled water is used in practical chemistry rather than tap water because it contains fewer chemical impurities
  4. Core: State that water from natural sources may contain substances, including: (a) dissolved oxygen (b) metal compounds (c) plastics (d) sewage (e) harmful microbes (f) nitrates from fertilisers (g) phosphates from fertilisers and detergents
  5. Core: State that some of these substances are beneficial, including: (a) dissolved oxygen for aquatic life (b) some metal compounds provide essential minerals for life
  6. Core: State that some of these substances are potentially harmful, including: (a) some metal compounds are toxic (b) some plastics harm aquatic life (c) sewage contains harmful microbes which cause disease (d) nitrates and phosphates lead to deoxygenation of water and damage to aquatic life
  7. Core: Describe the treatment of the domestic water supply in terms of: (a) sedimentation and filtration to remove solids (b) use of carbon to remove tastes and odours (c) chlorination to kill microbes

Chemical Tests for the Presence of Water 🔬

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Water is vital to life and plays an important role in many industrial processes. Since many colourless liquids exist (including water, ethanol, and cyclohexane), specific chemical tests are needed to confirm the presence of water.

Test 1: Anhydrous Cobalt(II) Chloride

Anhydrous cobalt(II) chloride is blue in colour. When water is added to it, a chemical reaction occurs forming hydrated cobalt(II) chloride, which is pink.

The reaction can be represented as:

CoCl2+6H2O→CoCl2⋅6H2O\text{CoCl}_2 + 6\text{H}_2\text{O} \rightarrow \text{CoCl}_2 \cdot 6\text{H}_2\text{O}

Observation: Blue cobalt chloride paper turns pink in the presence of water.

This test is usually performed using cobalt chloride paper. The paper is dipped into the liquid being tested, and if water is present, the colour change from blue to pink confirms its presence.

Test 2: Anhydrous Copper(II) Sulfate

Anhydrous copper(II) sulfate is white in colour. When water is added, it forms hydrated copper(II) sulfate, which is blue.

The reaction can be represented as:

CuSO4+5H2O→CuSO4⋅5H2O\text{CuSO}_4 + 5\text{H}_2\text{O} \rightarrow \text{CuSO}_4 \cdot 5\text{H}_2\text{O}

Observation: Solid white anhydrous copper(II) sulfate turns blue when water is added to it.

🔑 Key Point: Both tests are reversible reactions that confirm the presence of water through distinct colour changes.

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