4th Standard Science Term-II
Water
Learning Objectives
✤ After learning this lesson, students will be able to :
✤ understand the change of states in water.
✤ describe water cycle.
✤ explain the importance of water cycle.
✤ summarise rainwater harvesting and its uses.
Introduction
Water is the most abundant and precious resource on the Earth. It is found in oceans, seas, rivers, streams, lakes, ponds and under the ground. All living things need water to live and to do many other activities. We have already learnt about this in your previous class. Let us learn more about water in this lesson.
Change of States in water
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 water vapour 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 by mass.
SOLID STATE (Ice)
Ice is a solid form of water. At temperature below 00C water becomes ice. This process is called freezing.
Cooling
Water → Ice
below 00C
LIQUID STATE (Water)
Water in liquid state is available in water bodies such as oceans, river, waterfalls etc.
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.
Heating
Water → Water vapour
1000C
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 changing into 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.
More to know
Each day, we lose a little more than a cup of water when we exhale.
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.
Do You Know
73% of our brain's mass consists of water. The Antartic region has about 70% of the worlds fresh water.
III. Rainwater 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 type of rain water 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 consists of gravels and coarse sand. As it passes impurities are filtered and it is stored as ground water.
More to Know
H2 O is the chemical formula of water.
Do You Know
One 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
- Rainwater harvesting was launched in the year 2001 by the Tamil Nadu government.
- Tamil Nadu is the first Indian state which made rainwater harvesting mandatory.
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