Topic 10.1: Water Revision Notes

1. Overview

Water is the most important solvent in chemistry and is essential for all known forms of life. This topic covers how to chemically identify water, how to ensure its purity for scientific use, and the processes involved in making natural water safe for human consumption.

Key Definitions

• Anhydrous: A substance that contains no water (often used as a test reagent).
• Hydrated: A substance that contains water of crystallisation.
• Distilled Water: Water that has been purified by boiling it into steam and condensing it back into a liquid, leaving impurities behind.
• Sedimentation: The process where large, insoluble particles settle to the bottom of a tank.
• Filtration: The process of passing water through layers of sand and gravel to remove smaller insoluble solids.
• Chlorination: The addition of chlorine to water supplies to kill harmful microbes and bacteria.

Core Content

Chemical Tests for the Presence of Water

These tests confirm the presence of water molecules but do not prove that the water is pure.

1. Anhydrous Cobalt(II) Chloride

   • Test: Add the liquid to blue anhydrous cobalt(II) chloride paper.
   • Observation: The paper changes color from blue to pink.
   • Word Equation: anhydrous cobalt(II) chloride + water → hydrated cobalt(II) chloride
   • Symbol Equation: $\text{CoCl}_2 (\text{s}) + 6\text{H}_2\text{O} (\text{l}) \rightarrow \text{CoCl}_2\cdot6\text{H}_2\text{O} (\text{s})$

2. Anhydrous Copper(II) Sulfate

   • Test: Add the liquid to white anhydrous copper(II) sulfate powder.
   • Observation: The powder changes color from white to blue.
   • Word Equation: anhydrous copper(II) sulfate + water → hydrated copper(II) sulfate
   • Symbol Equation: $\text{CuSO}_4 (\text{s}) + 5\text{H}_2\text{O} (\text{l}) \rightarrow \text{CuSO}_4\cdot5\text{H}_2\text{O} (\text{s})$

Testing for Purity

A chemical test only tells you if water is present. To check for purity, you must measure physical properties:

• Boiling Point: Pure water boils at exactly 100 °C at standard atmospheric pressure.
• Melting Point: Pure ice melts at exactly 0 °C.
• Effect of Impurities: If water is impure, the boiling point will be higher than 100 °C and the melting point will be lower than 0 °C.

Distilled Water vs. Tap Water

In practical chemistry, distilled water is preferred over tap water.

• Tap water contains dissolved chemical impurities (such as calcium or magnesium ions) which can react with reagents and interfere with experimental results.
• Distilled water has been purified, meaning it contains fewer chemical impurities and provides more reliable data in the lab.

Substances in Natural Water Sources

Water from rivers, lakes, and oceans contains various substances. Some are helpful, while others are harmful.

Substance	Status	Effect
Dissolved Oxygen	Beneficial	Essential for the respiration of aquatic life (fish).
Metal Compounds	Beneficial	Some (like Ca²⁺ or Mg²⁺) provide essential minerals for life.
Metal Compounds	Harmful	Some (like lead or mercury) are toxic to humans and animals.
Plastics	Harmful	Can trap or be ingested by aquatic life, causing physical harm.
Sewage	Harmful	Contains harmful microbes (bacteria/viruses) which cause diseases like cholera.
Nitrates & Phosphates	Harmful	From fertilisers/detergents; lead to deoxygenation (eutrophication) of water.

Domestic Water Treatment

To make water safe for drinking (potable), it undergoes several stages:

1. Sedimentation: Water is stored in large tanks where heavy insoluble particles sink to the bottom.
2. Filtration: Water is passed through layers of fine sand and gravel to filter out remaining small insoluble solids.
3. Carbon (Activated Charcoal): Used to remove unpleasant tastes and odours from the water.
4. Chlorination: Chlorine gas is bubbled through the water to kill harmful microbes and bacteria.

Extended Content (Extended Only)

There is no supplement-only content for this specific sub-topic (10.1).

Key Equations

Reaction	Equation	Notes
Cobalt Chloride Test	$\text{CoCl}_2 (\text{s}) + 6\text{H}_2\text{O} (\text{l}) \rightarrow \text{CoCl}_2\cdot6\text{H}_2\text{O} (\text{s})$	Blue to Pink
Copper Sulfate Test	$\text{CuSO}_4 (\text{s}) + 5\text{H}_2\text{O} (\text{l}) \rightarrow \text{CuSO}_4\cdot5\text{H}_2\text{O} (\text{s})$	White to Blue

• Symbols: (s) = solid, (l) = liquid.
• Units: Temperature is measured in degrees Celsius (°C).

Common Mistakes to Avoid

• ❌ Wrong: Saying that cobalt(II) chloride turns blue to pink to prove water is pure.
• ✅ Right: Cobalt(II) chloride only proves the presence of water. Only boiling point or melting point proves purity.
• ❌ Wrong: Forgetting the word "anhydrous" when describing the chemical tests.
• ✅ Right: Always specify anhydrous copper(II) sulfate or anhydrous cobalt(II) chloride.
• ❌ Wrong: Thinking chlorination removes solids from water.
• ✅ Right: Filtration removes solids; chlorination only kills microbes.

Exam Tips

• Command Words: If the question asks you to "State" a test for water, keep it brief: "Anhydrous copper(II) sulfate turns from white to blue."
• Application Questions: You may be given an "unknown" liquid. If it boils at 102 °C, you must conclude it is water (because it is near 100 °C) but it is impure (because it is not exactly 100 °C).
• Contexts: Expect questions about why farmers shouldn't use too much fertiliser near rivers (mention nitrates leading to deoxygenation).
• Typical Values: Be prepared to interpret data where nitrate levels are given in mg/dm³ (e.g., 0.14 or 3.0). Higher values usually indicate higher pollution levels.