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Term 2 Chapter 2 | 4th Science - Water | 4th Science : Term 2 Unit 2 : Water

Chapter: 4th Science : Term 2 Unit 2 : Water


After learning this lesson, students will be able to • understand the change of states in water • describe the water cycle • explain the importance of water cycle • summarise rainwater harvesting and its uses

Unit 2



Learning Objectives

After learning this lesson, students will be able to

 understand the change of states in water

describe the water cycle

explain the importance of water cycle

summarise rainwater harvesting and its uses



Water is the most abundant and precious resource on the Earth. It is found in oceans, seas, rivers, streams, lakes, ponds and even under the ground. All living things need water to live and to do many other activities. We have already learnt about this in the previous classes. Let us learn more about water in this lesson.


I. Change of States in Water

Let us do

See the given pictures and write down the state of each objects. (Solid, Liquid, Gas )

Water occurs naturally in three forms: ice, water and water vapour.

These forms are also called states of water. Ice is in solid form, water is in liquid form and watervapour is in gaseous form.

These states occur naturally because of temperature changes. Water becomes ice in cold regions, remains as water in oceans/rivers and becomes water vapour due to the heat of the Sun.

More to know: Human body consists of 75% of water.

Let us do

Fill in the boxes given below with correct answers.

(Liquid, Solid, Melting, Freezing, Cooling, Heating, Gas)

Change of States in water


Ice is a solid form of water. At temperature below 0° C water becomes ice. This process is called freezing.


Water in liquid state is available in water bodies such as oceans, river, waterfalls etc.

Write any two water bodies in your area. river,  ocean

GASEOUS STATE (Water vapour)

Water becomes water vapour as the temperature increases due to heat. Heating forms a white mist of extremely small water droplets in the air known as water vapour.

Try to Answer

1. Why water is a good example for all states of matter?

Answer: Because it is the only natural substance that can exist in all three states of matter at the temperatures normally found on Earth.

2. Which of the following is in liquid state? Rain/Ice/Snow

Answer: Rain

Let us do

 Fill water in an ice tray and keep it in the freezer. After few hours, take it out. What happened to the water?

The water changed from liquid state to solid state. It became ice.

Now keep this ice tray on a table for some time. What happens to the ice?

Ice cubes melt and turn into water. Now water changes from solid state to liquid state.

Pour water from the ice tray into a kettle. Ask an elder at your home to heat the kettle and observe the mouth of the kettle when the water starts boiling. What do you see?

We see white smoke (water vapour) coming out from the mouth of the kettle. Water changes from liquid state to gaseous state. It became steam.


II. Water Cycle

In nature, water keeps changing its state. Let us see how this happens. Due to the Sun‛s heat, water in oceans and rivers changes into water vapour and rises up.

Water becoming water vapour on heating is called evaporation. The water vapour, when comes into contact with cool air, becomes water droplets. This is called condensation. The water droplets combine to form clouds. The water droplets continue to combine, and they become big and heavy. These water droplets then fall down as rain. This is called precipitation. In colder places, combined water droplets come down as snow, sleet or hail. This continuous change of water from one state to another in nature is called water cycle.

The continuous cycle through which water is circulated by different processes like evaporation, condensation and precipitation is called water cycle.

Water Cycle

More to know 

Each day, we lose a little more than a cup of water when we exhale.

Think and answer

What happens to the water in a wet cloth?

Answer: The water in a wet cloth gets evaporated. Hence the cloth gets dried up.

Try to Answer

Choose the correct answer.

In the water cycle ________

a. only evaporation is involved

b. only condensation is involved

c. evaporation and condensation are involved

Answer: (c) evaporation and condensation are involved

Importance of water cycle

• To improve the availability of ground water.

• To control the Earth‛s temperature.

• To provide water to plants and animals especially humans.

• To recycle the Earth‛s supply of water constantly.

Try to Answer

1. Unscramble the words and fill in the blanks.

ondencsaonti, vaeionorpat, ecippitatrino

condensation, evaporation, precipitation

2. Is water cycle important for farming? Yes

3. Why is water cycle important to human? To improve the availability of ground water.

4. Name the process which is responsible for the formation of droplets on the surface of a glass of cold drink. condensation

Do you Know?

73% of our brain‛s mass consists of water. The Antartic region has about 70% of the worlds fresh water.


III. Rain Water Harvesting

‘Rainwater harvesting is the process of collecting and storing rainwater for future use. Rainwater can be collected in natural reservoirs or artificial tanks. Roof top harvesting is also a type of rainwater harvesting. The rainwater on the top of the building is collected and passed into the ground through pipes. Water passing through the pipes enters the pit which consist of gravels and Coarse sand. As it passes impurities are filtered and stored as ground water.

More to know: H2O is the chemical formula of water.

Do you Know? 1 inch of rainfall on a 2,000 sq.ft. roof is equal to 4,800 litres of water.

Benefits of rainwater harvesting

• It increases the ground water level

• It can be used for agriculture

• It increases the availability of water for living things

Rain water harvesting is the only way to conserve rain water

More to know

• The rain water harvesting was launched in the year 2001 by the Tamil Nadu government.

• Tamil Nadu is the first Indian state which made rainwater harvesting as mandatory.

Try to Answer

1. List out some other benefits of rain water harvesting.


Reduces flooding and Erosion.

It increases ground water level

It can be used for agriculture

It increases the availability of water for living things

2. Suggest some ideas to save rainwater in your school.


By making rain water storage pits.

By collecting rainwater from roof and allowing it to flow around borewells.

Let us do

Tips to construct a rainwater harvesting pit at school/home.

Construct a pit of any shape or size. It may be generally constructed, 1-2 metre wide and 2-3 metre depth. The pits can be filled with boulders, gravels and Coarse sand.

Put the boulders in the bottom, sand in the top and gravels in the middle.

Let us do

Discuss with your friends.

Rain water harvesting has an important role to protect our environment.

Answer: It increases the ground water level. It reduces flooding and soil erosion and reduces water scarcity.

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