The invisible gaseous medium around us, mainly constituted by oxygen and nitrogen is known as air. It is through this air that we are able to carry out the process of respiration. Air is transparent and colourless. It occupies space and is present all around us.
Atmosphere: The thin blanket of air surrounding the surface of the Earth is called the atmosphere.
The atmosphere is divided into five distinct layers on the basis of variations in temperature that changes due to increasing altitude. Air gets scant as we move up in the atmosphere. These are as follows:
- Troposphere: This is the first layer to the atmosphere which is nearest to the surface and is responsible for weather conditions. The troposphere itself is said to contain about more than 75% of the atmosphere!
- Stratosphere: This is the layer just above the troposphere which contains the ozone layer and where the aeroplanes fly and is also home to most of the clouds!
- Mesosphere: This is the third and the coldest layer of our atmosphere and extends to a good 80 km above the surface of the Earth.
- Thermosphere: This is the fourth and one of the hottest layers of Earth where temperatures go to a 1500o This is where the space shuttles go to study Earth from space! The air in this layer is very thin and about 99.9% of the atmosphere is said to lie below this particular layer.
- Exosphere: This is the outermost layer of the atmosphere where molecules and atoms escape into space. Beginning at 480 km above the Earth, this layer then extends into the space.
Importance of Air
- We need air for breathing.
- All living beings need air for breathing.
- We need air to burn something.
- Life is possible on earth because of air.
Composition of Air
Air is a mixture of gases, water vapour and dust particles. Some of the gases; present in air; are discussed below.
Nitrogen: Nitrogen is the largest component of air. Nitrogen makes up 79% of air around us. Nitrogen is used by plants to make protein. Plants cannot take nitrogen directly from the air. Some nitrogen fixing bacteria live in soil. They help in nitrogen fixation in soil. Thus, plants are able to take nitrogenous compounds from the soil.
Oxygen: Oxygen is the second largest component of air. Oxygen makes up 21% of the air around us. Oxygen is used by living beings for respiration. After respiration, the living beings produce carbon dioxide.
Carbon dioxide: The remaining 1% of air is composed of carbon dioxide, many other gases, water vapour and dust particles. Carbon dioxide is also important for living beings. Plant need carbon dioxide to make food during photosynthesis. During photosynthesis, plants take in carbon dioxide and release oxygen. Thus, green plants help in maintaining the balance of carbon dioxide and oxygen in atmosphere.
Air in water: Air is also present in water. Aquatic animals breathe the air which remains dissolved in water. When water is boiled, air bubbles can be seen coming up from the bottom of the pan. This simple activity shows that air is present in water.
Activity to show presence of oxygen, carbon dioxide and nitrogen in air:
For this, take a candle, a glass tumbler and a pan which is filled with some water. Keep the candle upright in the pan and light the candle. Now cover the candle with the glass tumbler. It is observed that the candle extinguishes after some time.
This happens because oxygen in the air inside the glass tumbler is utilised in burning the candle. All the oxygen gets converted into carbon dioxide. Carbon dioxide does not support burning and hence candle stops burning.
Once the candle stops burning, some amount of water is sucked inside the tumbler. This happens because the volume of cabron dioxide is less than the volume of oxygen which was displaced.
It is also seen that a lot of air is still inside the tumbler. A major portion of this air is nothing but nitrogen. Nitrogen too does not support burning.
Availability of Oxygen in Water and Soil
It is often asked how animals under the soil and in water are able to breathe. The answer is that both soil and water have air dissolved in them.
- When we heat or boil water, we often notice that bubbles start to form. These bubbles are in fact, an indication that air molecules are present in the water. When water is heated, the air dissolved in it escapes first followed by water itself getting converted into vapour. This is how animals living underwater are able to respire.
Air bubbles can be seen when water is heated
- To see the presence of water in the soil, we take a small lump of it in a beaker and add water to it. We see bubbles coming out of it which as we discussed, is proof of the existence of air molecules in the soil. As water is added, it displaces the water molecules in the soil which we see in the form of bubbles. Animals make use of this air to breathe under the soil. Some animals make holes and burrows in the soil to help make pathways for air to enter the soil. When it rains heavily, earthworms and other animals come out of the soil because these pathways get blocked by the water and they need to come outside to find the air to breathe.
Air particles present in soil
Balance of Oxygen in the Atmosphere
It is common knowledge that humans and animals can’t survive without plants because they produce oxygen via photosynthesis. The balance of oxygen in the environment is thus maintained through the respiratory processes of plants and animals.
1.Question. What is the composition of air?
Answer. Air comprises of water vapor, Oxygen, Nitrogen, Carbon dioxide, dust and smoke.
2.Question. which gas in the atmosphere is essential for respiration ?
3.Question. How will you prove that air support burning?
Answer. Two lighted candles are taken. one candle is covered a fixed in a tumbler containing water. This candle is covered by an inverted glass. After sometime, the covered candle gets extinguished and water rises upto 1/5th is oxygen. The candle extinguished as soon as all the oxygen was used up in burning. Therefore, water occupies that space. This shows that air supports burning.
4.Question. How will you so that air is dissolved in water?
Answer.When a tumbler containing water heated, tiny bubles appear on the inner side. These bubles appear before the water starts boiling. So, these must be air bubles. This experiment shows that water contains air.
5.Question. Why does a lump of cotton wool shrink in water?
Answer.A lump of cotton wool shrink in water because the air inside wool cotton is driven out by water. They layer stic together and hence the lump shrinks.
6.Question. The layer of air around the earth is known as:-
7.Question. The comppnent of used by green plants to make their food, is :
Answer. Carbon doxide.
8.question. List five activities that are possible due to the presenmce of air.
(2) Formation of clouds.
(6) Flying of birds.
9. How do plants and animals help each other in the exchange of gases in the atmosphere?
Plants take carbon dioxide to give out oxygen. This oxygen is used by animals for respiration to expel out carbon dioxide. This carbon dioxide is used by plants to prepare food. This way, plants and animals help each other in the exchange of gases in the atmosphere