5th Standard Science Term-III
AIR
Introduction
Air is present everywhere around us. Though we cannot see it, we can feel it. Air is a mixture of gases like oxygen, nitrogen, carbon dioxide and hydrogen. These gases act as an envelope around the earth form the atmosphere. It is the presence of atmosphere that makes the earth a suitable place for living. In the recent years more number of industries have been established and they release excess of harmful gases like carbon dioxide into the atmosphere. Because of this, air is polluted more than ever before. In this lesson, we are going to study about different layers of atmosphere, air pollution, air borne diseases and the measures to control air pollution.
I. Atmosphere
The earth is surrounded by a layer of gases which is called atmosphere. It is composed mainly of nitrogen (78%) and oxygen (21%). Other gases like carbon dioxide and argon comprise 1% of the atmosphere by volume. The atmosphere is like a blanket that surrounds the earth. It protects the Earth from getting too cold or too hot.
Atmosphere divided into five different layers. The layers from the bottom upwards are: Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.
✧ Troposphere
The troposphere is the lowest layer of the atmosphere. From the sea level it extends upto about 10 km. It is the densest layer and almost 75% of the air in the atmosphere is found in this layer. This layer also has water vapour. We live in the troposphere and most of the weather like clouds, rain, snow are found in this layer. All weather changes also occur in this layer.
✧ Stratosphere
It extends from the top of the troposphere to about 50 km above the ground. Ozone layer found in this layer absorbs harmful ultraviolet rays which can cause damage to our skin and eyes. There is no water vapour in this layer. The temperature in this layer is around -550 C.
✧ Mesosphere
The region above the stratosphere is called mesosphere. It extends upward to a height of about 85 km from the ground. The temperature in this layer decreases with height and it is -100⁰ C. Most of the meteors found in the sky burn in this layer.
✧ Thermosphere
The layer of very rare air above the mesosphere is called thermosphere. It is the fourth layer of the atmosphere.
✧ Exosphere
The outermost layer of the atmosphere is called exosphere. It lies between 400 - 1600 km above the earth. The air here is extremely thin.
II. Importance of Air
Air is important for all the living organisms. Without air no life can exist on the earth. We take in oxygen from the air an d release carbon dioxide. Plants in turn use carbon dioxide present in the air to produce their food. The gases such as oxygen, nitrogen, carbon dioxide and hydrogen present in the air are important to us for many reasons. Let us study about the importance of air in this section.
✧ Water Cycle
Water vapour present in the air is important for the formation of water cycle. Water from the water bodies such as rivers and oceans evaporates and becomes water vapour. This water vapour then forms the clouds. These clouds move to the land and get cooled to give us the rain. This movement of clouds is possible due to air.
✧ Supplies Energy
We breathe oxygen present in the air and it is supplied to the cells in our body. Body cells burn the food molecules with the help of oxygen and produce energy. With this energy we do all kinds of works.
✧ Sound travels through air
We hear many things from the surrounding and people hear what we speak. This is possible due to air because sound needs a medium to travel. Sound travels from the point of generation to the listener through air.
✧ Useful for plants.
Nitrogen present in the air is useful for plants. Nitrogen in the atmosphere is converted into easily absorbable nitrates by plants with the help of some microbes. It is known as nitrogen fixation. These nitrates are useful for the growth of plants. Also, ir is helpful for the pollination of plants. Pollen grains travel from one plant to another plant through air. Thus, cross pollination is achieved with the help of air.
✧ Transport
Movement of ir is called wind. This is helpful for the ships and boats to sail on the water. Airplanes and helicopters travel through air.
✧ Sports
Para-gliding is the recreational and competitive adventure sport of flying. In this sport, pilot sits in a harness suspended below a fabric wing. Hang gliding is also an air sport or recreational activity in which pilot flies a light, non-motorized foot-launched air craft called a hang glider. Para-gliding and hang gliding are possible with the help of air. Other sports like wind surfing, kite surfing and sailing are also possible with the help of air.
✧ Parachutes and Hot air balloons
Parachutes and hot air balloons are used to land from above. In case of emergency, we can use parachutes and come down slowly and safely with the help of air.
✧ Wind energy
Air flows from high pressure region to low pressure region. This flow of air at high speed is called wind. This wind is used to generate electric power with the help of wind mills.
III. Air Pollution
The presence of harmful substances in the air which can have an adverse efffect on living beings and the environment is called air pollution. When harmful substances including gases like carbon dioxide, carbon monoxide, sulphur dioxide etc., and particles like dust and aerosols are released into the earth's atmosphere, air pollution occurs. These substances are released into the atmosphere at a rate which exceeds the natural capacity of the environment to absorb them. Air pollution may cause diseases, allergies and even death to humans. It may also cause harm to other living organisms such as plants and animals. It may damage the natural environment also.
1. Causes of Air Pollution
Air pollution is caused by both natural and manmade activities. Air pollutants are released into the atmosphere through human activities like burning of fuels, releasing industrial wastes, mining etc. Natural events like volcanic eruption also cause air pollution. Let us see about some of the causes of air pollution in detail.
✧ Industries
Many industries have been established to manufacture things. Large amount of carbon monoxide, hydrocarbons, organic compounds, and chemicals are released into the air by these industries. Because of these pollutants quality of air is affected very much.
✧ Burning of fuels
Combustion of fossil fuels like coal and petroleum release sulphur dioxide which is an air pollutant. Major air pollutants are released by the vehicles including bus, cars, trains and airplanes. Improper or incomplete combustion o fuel releases carbon monoxide. Nitrogen oxides which are released by both natural and manmade processes, also cause air pollution.
✧ Agricultural activities
Use of insecticides, pesticides, and fertilizers in agricultural activities has increased in the modern days. They emit harmful chemicals such as ammonia into the air causing air pollution.
✧ Mining
Extraction of minerals from the earth is called mining. Mining processes release dust and chemicals into the air causing massive air pollution. This affects the health conditions of workers and people living in the surrounding areas.
✧ Household activities
Air is polluted through household activities also. While cleaning and painting, we use a lot of chemicals. These chemicals pollute the environment.
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