Water

Water: Complete CBSE Class 6 Science Notes & Guide

Introduction to Water

Water is one of the most essential substances on Earth, forming the foundation of all life. It exists in rivers, ponds, seas, oceans, underground reservoirs, and even in the air we breathe. Water is essential for life all living things (plants and animals) need water to survive. Without water, no life could exist on our planet.

From domestic activities like drinking, cooking, and bathing to large-scale agricultural and industrial applications, water serves countless purposes in our daily lives. Understanding water, its cycle, conservation, and the challenges of pollution is crucial for sustainable living.

Uses of Water in Different Fields

1. Domestic Uses

Water is used in homes for drinking, cooking food, washing utensils, cleaning floors, brushing teeth, bathing, washing clothes, flushing toilets, and watering plants. The consumption of water by individuals varies based on living standards as living standards improve, water requirements typically increase.

2. Agricultural Applications

The largest amount of water is used for irrigation of crops in agriculture due to the high water requirements of food-producing plants. To illustrate the scale:

• Approximately 1,500 liters of water are needed to produce 1 kilogram of wheat grains
• About 4,500 liters of water are required to produce 1 kilogram of rice grains

Water is also essential for growing fiber crops like cotton and jute, and for fish farming in ponds, lakes, and rivers.

3. Industrial Production

Water is used in industries for producing almost all the things we use, including paper, cloth, medicines, chemicals, bread, and biscuits. For example, making the paper used in each page of a typical book consumes approximately 2 glasses of water in the paper mill.

4. Cooling Systems

Water is used to keep things cool, such as in the radiators of vehicles (cars, buses, and trucks) to keep their engines cool.

5. Transportation

Water in rivers and seas is used for transporting passengers and goods from one place to another by boats, sailing boats, motor boats, and ships.

6. Electricity Generation

At hydroelectric power plants, water stored in high dams is allowed to fall from great heights, turning turbines that run generators to produce electricity. Similarly, at thermal power plants, water is boiled to create steam that drives steam turbines connected to electricity generators.

Sources and Distribution of Earth's Freshwater Supplies

Where Do We Get Water From?

The place from where we get water is called a source of water, including rivers, lakes, ponds, wells, and springs. The water we receive through taps in our homes typically comes from rivers, lakes, or tube-wells after being purified and made germ-free.

Primary Water Sources on Land

Rivers get their water from melting snow on mountain peaks (glaciers) and from rains. Lakes, ponds, and wells get their water from rains. These represent the two major sources of freshwater on land: glaciers (snow mountains) and rainfall.

Characteristics of Fresh Water:

• Rain water is the purest form of natural water and is not salty, making it fit for drinking.
• Rivers, lakes, ponds, wells, springs, rain, and glaciers are called fresh water sources as they don't contain much dissolved salts.

Groundwater

Some rainwater seeps through the soil and goes down under the surface of earth, ultimately being stopped by rocks where it collects. This underground water can be accessed by digging wells. Well-water contains some dissolved salts and may contain germs, but water from deep wells is usually fit for drinking.

Ocean Water - The Largest Source

More than two-thirds of the earth is covered with water, with most of this water in oceans and seas. Oceans are the largest source of water on Earth, covering more than two-thirds of the planet's surface.

However, ocean water (or sea water) has large amounts of salts dissolved in it, making it highly saline or salty and not fit for drinking, other domestic purposes, agriculture, or industrial needs.

Important Distinction:

• Fresh water sources: Rivers, lakes, ponds, wells, springs, rain, glaciers (low salt content)
• Saline water sources: Oceans and seas (high salt content)

Ocean water is the most impure form of natural water, and it is very expensive to purify sea water for drinking purposes.

States of Water

Three Physical States

Water can exist in three states: solid, liquid, and gas.

State	Form	Description
Solid	Ice, Snow	Ice is frozen or solidified water; snow is water that falls as white flakes during winter in extremely cold regions.
Liquid	Tap water, River water	Most of the water around us is in the liquid state, such as water flowing from taps.
Gas	Water vapour, Steam	Water vapour is the gas state of water present in air (invisible). Very hot water vapour is called steam.

Changes of State

Water can be changed from one state to another by heating or cooling:

1. Melting: When ice (or snow) is heated, it melts and changes into water
2. Freezing: When water is cooled too much, it freezes and changes into ice (or snow)
3. Evaporation: When water is heated, it evaporates and changes into water vapour
4. Condensation: When water vapour is cooled, it condenses and changes into liquid water

The Water Cycle: How It Works Step by Step

The continuous circulation of water from the earth's surface to atmosphere, and from the atmosphere back to earth, is called the water cycle in nature.

Step-by-Step Process

Stage 1: Evaporation and Transpiration

Heat from the sun evaporates water from rivers, lakes, ponds, oceans, and soil to form water vapour that goes into the air.

Additionally, plants absorb water from soil through their roots and release water as water vapour through small pores in their leaves (stomata) by the process of transpiration.

Key Points:

• Evaporation occurs from water bodies and soil
• Transpiration occurs from plant leaves
• Both processes add water vapour to the atmosphere
• Heat energy from the sun drives these processes

Stage 2: Cloud Formation (Condensation)

The air containing water vapour is heated by the sun. Hot air, being lighter, rises high in the sky where it is cold. When air containing water vapour rises to a high altitude, the water vapour gets cooled and condenses to form tiny droplets of water.

These tiny droplets of water formed by condensation remain floating in air and appear to us as clouds.

Stage 3: Precipitation (Rain and Snowfall)

The tiny droplets of water in clouds join together to form bigger drops of water. These drops fall down on earth in the form of rain. In very cold regions, water drops in the sky freeze to form snow (ice), which falls down to earth as snowfall.

Stage 4: Return to Earth

Water formed by the melting of snow and some of the rain water flows into rivers and finally goes into oceans. Rain water also fills lakes and ponds, gets absorbed by soil, and some seeps through the soil to go underground.

In this way, water which was taken from the earth returns to the earth, and hence the water cycle is completed. The water cycle is a continuous process that never ends.

Importance of the Water Cycle

The water cycle is important because:

1. Provides Fresh Water: Sea-water is highly salty and unfit for drinking or plant growth, but rain water (formed from sea-water through evaporation and condensation) is pure water usable by animals and plants.
2. Maintains Water Balance: The water cycle keeps the amount of water on Earth's surface constant by returning water removed during evaporation and transpiration back to Earth during rain and snowfall.

Environmental Impacts of Water Pollution and Solutions

Heavy Rainfall and Flooding

When it rains heavily for a long time, excessive rainfall leads to too much water all around.

Problems Caused by Heavy Rains:

1. Heavy rains in city areas lead to waterlogged roads and streets, disrupting traffic and causing inconvenience.
2. Heavy rains may cause floods when rivers overflow their banks due to excessive water, submerging cities, villages, crop fields, and forests, causing damage to property, human life, domestic animals, standing crops, and wild animals.
3. Animals living in water are carried away by fast-flowing flood waters and often get trapped on land, dying when flood water recedes.
4. Heavy rains kill animals living in soil because they cannot get air to breathe when all soil gets covered with flood water.

Drought Conditions

If it does not rain in a region for a long time (a year or more), it leads to shortage of water causing dryness everywhere, called drought - a long period without rains leading to severe water shortage.

How Drought Develops:

Soil normally has moisture but continuously loses water by evaporation and transpiration. Water lost by soil is usually replenished by rains. If it doesn't rain for a long period, water loss from soil is not made up and the soil becomes dry. Water levels in lakes and ponds drop, and some may dry up completely. Without rains, no water seeps into the ground, causing groundwater levels in wells to fall drastically.

Problems from Drought:

People living in drought-affected areas face:

1. Food Shortage: Soil lacks sufficient moisture to grow crops well, leading to severe food shortages
2. Fodder Shortage: Vegetation like grass doesn't grow well, creating fodder shortages for domestic animals
3. Migration: People and animals migrate to other places in search of food and fodder; some may die of starvation

Methods for Purifying Drinking Water at Home

While the PDF doesn't extensively cover home water purification methods, it emphasizes the need for purification:

Water from rivers, lakes, and ponds contains suspended impurities, soluble impurities, and germs (like bacteria), so it's not fit for drinking as such and must be purified first.

Common Home Purification Methods

Based on scientific principles discussed in the document:

1. Boiling: Heating water to 100°C kills most germs and bacteria
2. Filtration: Removes suspended particles and some microorganisms
3. Sedimentation: Allowing water to stand so heavy particles settle at the bottom
4. Chemical Treatment: Using chlorine or other disinfectants (municipal treatment)
5. Solar Disinfection: Exposing water to sunlight in clear bottles (UV kills germs)

Note: The water drawn from rivers, lakes, and tube-wells is first purified and made germ-free before being supplied to homes through a network of pipes.

Health Effects of Drinking Tap Water vs Bottled Water

While the PDF doesn't directly compare tap and bottled water, it provides important context:

Tap Water Quality

Water we get in taps in our homes comes from rivers, lakes, or tube-wells. This water is first purified and made germ-free before being supplied through a network of pipes.

Advantages:

• Regulated and tested by municipal authorities
• Treated for harmful microorganisms
• Cost-effective
• Environmentally friendly (no plastic waste)

Considerations:

• Quality depends on local water treatment facilities
• May contain trace minerals
• Pipe infrastructure condition affects quality

Natural Water Sources

Rain water is the purest form of natural water, not salty, and fit for drinking. Well-water from deep wells is usually fit for drinking but contains some dissolved salts and may contain germs.

General Water Safety Principles

Based on the document's emphasis on purification:

• All natural water sources should be purified before consumption
• Germ-free water is essential for health
• Dissolved salts in moderate amounts are acceptable
• High salinity (like sea water) makes water unfit for drinking

Water Conservation: How Can We Save Water?

To conserve water means to save water by using it carefully, wisely, and by preventing its wastage. It is essential to conserve water because the amount of usable water on Earth is limited and there is scarcity of usable water for various purposes.

Why Water Conservation is Critical

The earth has tremendous amounts of water, but most is in oceans. Ocean water is highly saline and cannot be used directly for drinking, irrigation, or industry. Only a small fraction of water available on Earth is fit for use by humans, animals, and plants, available in rivers, lakes, ponds, and as groundwater.

As population increases rapidly, more people use water, and demand increases daily. More water is being used in agriculture for food production and by industries manufacturing various products, leading to water shortages in many parts of the world.

Practical Ways to Conserve Water at Home

Various ways to conserve water or minimize wastage at home include:

1. Turn off taps immediately after use - Get leaking taps repaired immediately
2. Take baths efficiently - Fill water in a bucket rather than bathing directly under flowing tap
3. Wash utensils wisely - Fill water in a basin instead of washing under running tap
4. Reuse water - Use water from washing rice, vegetables, or fruits for watering plants
5. Use toilets efficiently - Don't use a full flush when a half flush is sufficient

Remember: Water is precious. Every drop counts!

Rainwater Harvesting: Increasing Water Availability

The activity of collecting rainwater directly and storing it in big tanks for later use, or making rainwater percolate into ground more efficiently to recharge groundwater, is called rainwater harvesting.

Two Main Techniques

1. Collection and Storage

Collection and storage of rainwater in tanks for future use when there is scarcity of water.

2. Groundwater Recharge

Making rainwater percolate into the ground more efficiently by constructing percolation pits and recharge wells to recharge (or replenish) groundwater.

Why Rainwater Harvesting is Necessary

When too much water is drawn through wells and tube-wells for homes, agriculture, and industry, water levels go down drastically, and some wells may dry up completely, leading to groundwater shortage.

In rural areas, most ground has open soil allowing rainwater to seep naturally. In urban areas, however, most ground is covered with buildings, concrete pavements, and metalled roads, so very little rainwater seeps into ground naturally. Most rainwater in cities flows into dirty drains and goes away.

Rooftop Rainwater Harvesting Methods

Method 1: Direct Storage

Rainwater collecting on the roof of a house is brought down through rainwater pipe and collected in big underground tanks made of concrete. This water may contain soil particles and needs filtering before use. Stored rainwater can be used during water scarcity.

Method 2: Percolation and Recharge

The pipe bringing down rainwater from the roof goes directly into a percolation pit made in the ground. The percolation pit has layers of broken bricks at the bottom, gravel in the middle, and coarse sand at the top, covered by a concrete slab. A narrow bore about 3 meters deep is dug into the ground.

How it works:

• Rainwater flows from roof into percolation pit
• Gets filtered by sand, gravel, and broken bricks
• Filtered water exits through outlet pipe
• Outlet pipe takes water deep into ground where it seeps into soil

Rainwater Harvesting from Open Spaces

Rainwater harvesting from open spaces around buildings in cities is done by constructing percolation pits covered with concrete slabs having holes, connected to a recharge well through a pipe. The recharge well is about 1 meter in diameter and 3 meters deep.

Process:

• Rainwater from open spaces enters percolation pit through holes in concrete slab
• After filtration, water enters recharge well through outlet pipe
• Water gradually seeps into soil from the recharge well

Main Purpose and Benefits

The main purpose of rainwater harvesting is not to hold rainwater on the earth's surface but to make rainwater percolate underground more efficiently to recharge (or replenish) groundwater.

The advantage of rainwater harvesting is that it increases the availability of groundwater and helps in overcoming water shortage.

Formulas and Concepts

Concept	Description	Example/Value
Water for Wheat	Water needed to produce wheat	~1,500 liters per 1 kg of wheat grains
Water for Rice	Water needed to produce rice	~4,500 liters per 1 kg of rice grains
Earth's Water Coverage	Portion of Earth covered by water	More than two-thirds (>66%)
Three States of Water	Physical forms water can take	Solid (ice), Liquid (water), Gas (vapour/steam)
Evaporation	Water → Water vapour (heating)	Occurs at any temperature, fastest at 100°C
Condensation	Water vapour → Liquid water (cooling)	Reverse of evaporation
Freezing	Liquid water → Ice (cooling)	Occurs at 0°C and below
Melting	Ice → Liquid water (heating)	Occurs at 0°C and above
Transpiration	Water loss from plant leaves	Through stomata (leaf pores)
Recharge Well Dimensions	Typical rainwater harvesting well	~1 meter diameter, ~3 meters deep

Conclusion

Water is truly the foundation of life on Earth. Understanding its cycle, sources, uses, and conservation methods is essential for every student and citizen. As we face increasing water scarcity challenges globally, the knowledge of proper water management, rainwater harvesting, and conservation practices becomes more critical than ever.

By implementing simple conservation measures at home and supporting larger initiatives like rainwater harvesting in our communities, each of us can contribute to ensuring water availability for future generations.

Remember: Water is precious. Every drop counts!