Overview
This chapter explains the importance, properties, and chemical nature of water, its role as a solvent and resource, and issues related to water pollution and conservation.
Importance and Occurrence of Water
- Water is essential for life and covers about 71% of Earth’s surface.
- Water exists in three states: solid (ice), liquid (water), and gas (water vapour).
- It is found in oceans, rivers, lakes, underground sources, glaciers, and the atmosphere.
Physical and Chemical Properties of Water
- Water is colourless, tasteless, and odourless in pure form.
- It has a high boiling point (100°C) and melting point (0°C) at atmospheric pressure.
- Water is a universal solvent, dissolving many ionic and molecular substances.
- The density of water is maximum at 4°C.
Water as a Solvent
- Water dissolves salts, sugars, acids, gases, and many solids.
- Ionic compounds dissociate into ions in water, allowing conductivity.
- Many chemical reactions in living organisms and industry occur in aqueous solutions.
Composition and Structure of Water
- Water is a compound with the formula H₂O, made of two hydrogen atoms and one oxygen atom.
- Water molecules are held together by covalent bonds, and hydrogen bonding exists between molecules.
- Electrolysis of water produces hydrogen and oxygen gases in a 2:1 volume ratio.
Hard and Soft Water
- Hard water does not lather easily with soap due to the presence of calcium and magnesium salts.
- Soft water lathers easily and does not contain these salts.
- Hardness can be removed by boiling (temporary) or by using washing soda/ion exchange (permanent).
Water Pollution and Conservation
- Water is polluted by industrial wastes, sewage, agricultural runoff, and detergents.
- Polluted water can spread diseases and harm aquatic life.
- Conservation involves reducing wastage, treating sewage, recycling, and rainwater harvesting.
Key Terms & Definitions
- Universal solvent — A substance that dissolves many different solutes; water is considered universal.
- Hard water — Water containing dissolved calcium and magnesium salts.
- Electrolysis — Chemical decomposition produced by passing an electric current through a liquid or solution.
- Hydrogen bond — Attraction between molecules caused by hydrogen attached to a highly electronegative atom (like O in H₂O).
Action Items / Next Steps
- Learn and write the chemical equation for electrolysis of water.
- List ways to conserve water at home and in the community.
- Prepare short notes on hard and soft water, including methods of removing hardness.