Is Air Present Everywhere Around Us?

Although we cannot see air, we can feel its presence in many ways. We notice it when the leaves of trees rustle, clothes on a clothes-line sway, or the pages of a book flutter when a fan is switched on. Moving air, or wind, is what allows us to fly kites and is essential for processes like winnowing. During storms, the wind can be powerful enough to uproot trees.

Devices like a firki (pinwheel) and a weather cock are designed to react to moving air. A firki rotates when air moves past it, and a weather cock shows the direction in which the air is moving.

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Air Occupies Space

Air is present everywhere around us, even in things that appear to be empty. An activity with an "empty" bottle demonstrates this property.

The Empty Bottle Experiment

Method: Take an empty, open bottle and push it in an inverted (upside down) position into a bucket of water.

Observation: You will notice that water does not enter the bottle. This is because the bottle is not actually empty; it is completely filled with air. The air inside prevents the water from entering. However, when the bottle is tilted slightly, the air is able to escape in the form of bubbles. As the air comes out, water fills up the empty space that the air had occupied.

Conclusion: This simple activity proves that air occupies space. Air is transparent and colourless, which is why we cannot see it, but it fills any empty container.

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The Atmosphere

Our Earth is surrounded by a thin layer of air. This protective layer extends up to many kilometres above the surface of the Earth and is called the atmosphere.

As we move to higher altitudes, for instance, while climbing high mountains, the air in the atmosphere gets rarer (less dense). This means there is less oxygen available to breathe. This is the reason why mountaineers carry oxygen cylinders with them when they climb high mountains.

What is Air Made Up Of?

Until the eighteenth century, air was thought to be a single substance. However, experiments have proven that air is actually a mixture of many gases. The main components of air are nitrogen, oxygen, carbon dioxide, water vapour, dust particles, and a few other gases.

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Water Vapour

Air contains water in the form of a gas called water vapour. We know this because when air comes in contact with a cool surface, the water vapour in it condenses and forms tiny drops of liquid water. The presence of water vapour in the air is essential for the water cycle in nature.

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Oxygen

Oxygen is a component of air that is essential for burning (combustion) and for respiration.

The Burning Candle Experiment

Method: Two lit candles are placed on a table. One is left in the open, while the other is covered with an inverted glass tumbler.

Observation: The candle covered with the glass tumbler goes out after a short time, while the other candle continues to burn. This happens because the component of air that supports burning is limited inside the tumbler and gets used up. This component is oxygen.

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Nitrogen

In the candle experiment, after the candle goes out, a large amount of air is still present inside the glass tumbler. This indicates the presence of a component in the air that does not support burning. The major part of the air is nitrogen, which makes up about 78% of the air.

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Carbon Dioxide

Carbon dioxide makes up a very small component of the air around us. It is produced during respiration (plants and animals consume oxygen and produce carbon dioxide) and during burning. An excess of carbon dioxide in a closed room, caused by something burning, can lead to a feeling of suffocation.

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Dust and Smoke

Air also contains fine dust particles and smoke. Smoke is produced from the burning of fuel and contains a few gases and fine dust particles, which are often harmful. This is why factories have long chimneys to release smoke high up in the air. Dust particles are always present in the air, and their presence can be seen when a beam of sunlight enters a dark room, illuminating the tiny particles that appear to be dancing in the air.

To prevent harmful dust particles from entering our body, our nose has fine hair and mucus that trap these particles when we breathe.

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Composition of Air

Air is a mixture of gases, water vapour, and dust particles. The composition of gases in the air is as follows:

• Nitrogen and Oxygen: Together, these two gases make up about 99% of the air. (Nitrogen ~78%, Oxygen ~21%).
• Remaining 1%: This small portion is constituted by carbon dioxide, a few other gases (like argon), and water vapour.

How Does Oxygen Become Available in Water and Soil?

Animals and plants that live in water and soil also need oxygen for respiration. They get this oxygen from the air that is dissolved in water and trapped in the soil.

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Air Dissolved in Water

Water contains dissolved air. We can demonstrate this with a simple activity.

The Heating Water Experiment

Method: Take some water in a container and heat it slowly.

Observation: Well before the water starts to boil, you can observe tiny bubbles forming on the inner surface of the container. These bubbles come from the air that was dissolved in the water, which escapes upon heating. As heating continues, the water itself turns into vapour and boils.

Conclusion: Animals living in water, like fish, use this dissolved oxygen for breathing (respiration) through their gills.

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Air Trapped in Soil

Soil also contains air trapped in the spaces between its particles. Organisms living in the soil and the roots of plants use this air for respiration.

The Soil and Water Experiment

Method: Take a lump of dry soil in a beaker and pour some water over it.

Observation: As the water is poured, you will see bubbles coming out from the soil. These bubbles are the air that was trapped in the soil being displaced by the water.

Conclusion: This activity proves that soil contains air. Animals that live in the soil, like earthworms, make burrows and holes, which also create space for air to move in and out. This is why these animals often have to come out to the surface for air when heavy rain fills up all the air spaces in the soil with water.

How is the Oxygen in the Atmosphere Replaced?

A large number of organisms are constantly consuming oxygen from the atmosphere for respiration. So, why doesn't all the oxygen get used up? The oxygen in the atmosphere is continuously being refilled, primarily by plants.

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The Role of Photosynthesis

As we learned in Chapter 7, plants make their own food through a process called photosynthesis. During this process, plants use carbon dioxide and water in the presence of sunlight to produce food. A very important by-product of this process is oxygen, which is released into the atmosphere.

While plants also consume some oxygen for their own respiration, the amount of oxygen they produce during photosynthesis is much more than what they consume. Therefore, we can say that plants produce oxygen.

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The Oxygen-Carbon Dioxide Balance

There is a natural balance of oxygen and carbon dioxide in the atmosphere, which is maintained by the interdependence of plants and animals.

• Animals and Plants (Respiration): Both consume oxygen and release carbon dioxide into the atmosphere.
• Plants (Photosynthesis): Consume carbon dioxide and release oxygen into the atmosphere.

This cycle ensures that the levels of both gases remain relatively stable. It highlights how animals cannot live without plants, as plants are the primary producers of the oxygen that most living things need to survive. This shows the crucial interdependence of plants and animals.

Other Uses of Air

Besides being essential for respiration, air (especially moving air or wind) has many other important uses for life on Earth and for human activities.

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Harnessing Wind Energy

Moving air or wind possesses energy that can be harnessed for various purposes.

Windmills

A windmill is a machine designed to convert the energy of the wind into more useful forms of energy. The wind makes the large blades of the windmill rotate.

Windmills are used to:

• Draw water from tubewells.
• Run flour mills to grind grain.
• Generate electricity.

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Role in Movement and Dispersal

Air plays a crucial role in the movement of many objects and living things.

• Movement and Flight: Air helps in the movements of sailing yachts, gliders, parachutes, and aeroplanes. The presence of air is what makes it possible for birds, bats, and insects to fly.
• Dispersal of Seeds and Pollen: Air helps in the dispersal of seeds and pollen from the flowers of many plants. This allows plants to reproduce and spread to new areas.

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Role in the Water Cycle

As we learned in Chapter 14, air plays a very important role in the water cycle. The movement of air helps in the evaporation of water from water bodies and carries water vapour over long distances, which then condenses to form clouds and falls as rain, thus distributing water across the land.

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