Water

1. Introduction

Water (H2O) is one of the most important substances on Earth. It covers about 71% of the Earth's surface.

Chemical formula: H2O (two hydrogen atoms + one oxygen atom).
Physical states: Solid (ice), Liquid (water), Gas (water vapour).

2. Sources of Water

Natural Sources

Distribution of water on Earth:

• 97% saltwater (oceans and seas).
• 3% freshwater (rivers, lakes, groundwater, glaciers).
• Of the 3% freshwater, about 2% is locked in glaciers and ice caps.

Exam Focus (2 marks): 'Why is only a small percentage of Earth's water available for human use?'

Most of Earth's water (97%) is saline and undrinkable. Of the remaining 3% freshwater, a large portion is frozen in glaciers. Only a tiny fraction is available in rivers, lakes, and as groundwater.

3. The Water Cycle

The continuous movement of water between the Earth's surface and the atmosphere is called the water cycle (hydrological cycle).

Stages of the Water Cycle

1. Evaporation: Water from oceans, rivers, and lakes turns into water vapour due to the Sun's heat.
2. Transpiration: Water vapour is released from plant leaves.
3. Condensation: Water vapour cools and forms tiny water droplets (clouds).
4. Precipitation: Water falls as rain, snow, or hail.
5. Collection: Water collects in rivers, lakes, oceans, and seeps into the ground.

Worked Example: Trace the journey of a water molecule from the ocean to a river.

The water molecule evaporates from the ocean -> rises and condenses to form a cloud -> falls as rain over land -> flows through streams into a river.

4. Importance of Water

For Living Organisms

• Drinking: All organisms need water to survive.
• Digestion: Water helps break down food.
• Transport: Blood (mostly water) carries nutrients.
• Temperature regulation: Sweating cools the body.
• Photosynthesis: Plants need water to make food.

For Daily Life

• Cooking, cleaning, bathing, washing.
• Agriculture and irrigation.
• Industries (cooling, processing, cleaning).
• Generating electricity (hydel power).
• Recreation (swimming, boating).

Common Mistake: Thinking water is only important for drinking. Water is essential for virtually every life process and human activity.

5. Water Purification

Methods of Purification

Worked Example: Which method would you use to purify water at home during a flood?

Boiling is the most practical method at home. It kills bacteria and viruses that may have contaminated the water during the flood.

Common Mistake: Thinking filtered water is always safe to drink. Filtration only removes insoluble impurities. Boiling or chlorination is needed to kill germs.

6. Water Conservation

Why Conserve Water?

• Freshwater is limited.
• Population is growing, increasing demand.
• Many regions face water scarcity.

Methods of Water Conservation

1. Rainwater harvesting: Collecting and storing rainwater for later use.
2. Drip irrigation: Watering plants directly at the roots to reduce wastage.
3. Fixing leaks: Repairing dripping taps.
4. Turning off taps: When not in use (while brushing).
5. Reusing water: Using vegetable washing water for plants.
6. Building check dams: To collect and store rainwater.

Exam Focus (3 marks): 'Suggest three ways to conserve water at home.'

(1) Turn off the tap while brushing teeth. (2) Fix leaking taps immediately. (3) Use water from washing vegetables to water plants.

7. Self-Test

1. List three natural sources of freshwater.
2. Draw a simple diagram of the water cycle and label the stages.
3. Why is water important for: (a) plants (b) animals?
4. Differentiate between boiling and chlorination as methods of water purification.
5. What is rainwater harvesting? Write its one benefit.
6. Name the process by which water changes from: (a) liquid to vapour (b) vapour to liquid.
7. Why is distillation more effective than filtration?

8. Answers to Self-Test

1. Rivers, lakes, groundwater, ponds, rainwater.
2. Diagram should show: evaporation from ocean, condensation forming clouds, precipitation as rain, collection in rivers/lakes.
3. (a) Plants need water for photosynthesis and transport of nutrients. (b) Animals need water for digestion, circulation, and temperature regulation.
4. Boiling uses heat to kill germs; chlorination uses chemicals (chlorine) to disinfect. Both kill germs but boiling is more thorough for home use.
5. Collecting and storing rainwater for later use. Benefit: reduces dependence on groundwater.
6. (a) Evaporation (b) Condensation.
7. Distillation removes all impurities (including dissolved ones) by evaporation and condensation, while filtration only removes insoluble impurities.