AIR

Air is present all around us.

We cannot see, touch or taste air but can only feel it.

The presence of air is felt by the rustling of trees in wind, flying of kites, fluttering of book papers, cool breeze, etc.

Air is very important for the respiration process of all living beings, and so air is essential for survival of all organisms.

Moving air is called wind. Strong winds are very powerful and can even uproot trees.

The direction of wind can be found out by a weathercock or a wind sock.

Air occupies space.

To show this, take an empty bottle, and turn it upside down.

Then, immerse it in a tub of water. We can see that no water enters the bottle.

Now, tilt the bottle to one side. We can see that air bubbles leaving from the bottle as water enters it.

Earlier, there was no space for air to escape. When it got space, air escaped, and water entered the bottle.

This experiment shows that air is present everywhere (even inside the empty bottle) and air occupies space.

Air is present around the earth like a blanket, called atmosphere.

Atmosphere

The huge blanket of air that surrounds the earth is called atmosphere.

Atmosphere extends to more than 800km above the surface of earth and is held in place by the earth’s gravity.

It protects us from the harmful rays of the sun.

Atmosphere is thicker near the surface of earth and becomes thinner with increase in height.

This is because with increase in height, gravity decreases, and lesser air is being held by earth.

There are five layers of atmosphere, namely - the troposphere, the stratosphere, the mesosphere, the ionosphere and the exosphere.

a. The Troposphere

Troposphere is the layer closest to the earth, in which we live.

It extends upwards for about 16km above the surface of the earth.

The movement of wind takes place in this layer.

It also contains water vapour, which is responsible for making clouds.

It is responsible for the climatic and weather changes on earth.

Aircrafts usually fly above this layer to avoid strong winds and bad weather.

b. The Stratosphere

v It is the second layer of the atmosphere.

v It lies above the troposphere, and contains the ozone layer.

v The ozone layer protects all life on earth from the harmful ultraviolet rays of the sun.

c. The mesosphere

v It is the third layer of the atmosphere.

v In mesosphere, temperature decreases with increase in altitude.

v Meteors burn up in the mesosphere.

d. The thermosphere

v The thermosphere is the fourth layer of atmosphere, with low air density.

v Aurora is seen in the thermosphere.

v High temperature is a characteristic feature of thermosphere.

v The thermosphere expands very much with increase in temperature.

e. The exosphere

v The exosphere is the uppermost region of atmosphere

v Satellites and space stations orbit the earth in the exosphere.

Air - a composite mixture

Ø The below experiment shows that air is a mixture of gases

Priestley’s experiments

In 1774, Joseph priestly showed that air was a composite mixture of gases.

He conducted an experiment and verified a colourless, highly reactive gas.

Later, Antoine Lavoisier, the great French chemist called it oxygen.

Priestley took a tub of water and made a float and placed a burning candle on it.

He covered the candle with a glass jar so that no air could enter or exit.

The candle flame was extinguished in a very short time.

v Even though e used a magnifying glass to focus the sun rays to re-light the candle, it could not be relit.

v He came to the conclusion that something present in air, needed for burning was completely used by the flame and converted into another substance.

He carried out his second experiment with a candle and mouse.

v He placed a live mouse and burning candle inside the jar.

v He was able to observe that the mouse died, as the candle consumed oxygen to burn.

v It was clear that oxygen was necessary for the survival of the mouse.

He carried out his third experiment with a candle and a plant.

v He placed a live plant and burning candle inside the jar.

v The plant survived, and the candle continued to burn.

v He concluded that plants released oxygen necessary for burning the candle.

In his fourth experiment, he took a jar, he used a candle, a mouse and a plant.

v He took a jar, burned a candle and converted all oxygen into carbon dioxide.

v He placed a mint plant and a mouse into this jar.

v He observed that both the plant and the mouse survived.

v He concluded that animals consume oxygen and release carbon di-oxide and plants take up carbon dioxide and release oxygen.

Thus, it was clear that “air” was a composite mixture of many gases like oxygen and carbon- dioxide.

During 1730 – 1799, Jan Ingenhousz showed that sunlight is essential for plants to carry out photosynthesis and to purify air that is fouled by breathing animals or by burning candles.

The release of oxygen by plants can be confirmed by the hydrilla experiment.

Air contains about 20% of oxygen.

Daniel Rutherford, a Scottish chemist, discovered nitrogen by the following experiment.

v He repeated Lavoisier’s experiment using a candle, and converted oxygen into carbon di-oxide.

v Then, he passed this air through lime water and removed carbon di-oxide also.

v He was able to produce a gas, which showed the same property of the air without oxygen and carbon di-oxide.

He called this gas nitrogen.

Test for Carbon di-oxide in air

v Take some limewater in a glass tumbler.

v Bubble some air using a straw.

v After a few minutes, the limewater turns milky white in colour.

v This shows the presence of carbon-di-oxide in air.

Composition of Air

The major component of air is nitrogen.

Nitrogen constitutes 78% of air.

The second major component of air is oxygen.

It constitutes 21% of air.

In addition to these, air also contains a small amount of carbon di- oxide (0.04%), other gases like argon, water vapour, and dust particles.

The increase in amount of carbon di-oxide in air leads to global warming.

The composition of air changes slightly from place to place and also depends on climate. For eg:

v Air over industrial cities usually has a higher amount of carbon-di-oxide in it than the air over open spaces.

v Air in coastal areas may have more water vapour than inland areas.

v Air also contains more water vapour in rainy season.

v The amount of dust in the air is more in windy places than other areas.

Test for the presence of dust particles in air

Tiny dust particles are present everywhere in the air.

When a beam of sunlight falls on them, the tiny dust particles become visible.

v Take a graph paper and apply grease on it.

v Paste it on different places in your area.

v After some days, take the paper and count the number of dust particles. We get a similar observation like in the figure.

The mean number of dust particles per unit area can be calculated by,

Mean =

The range of dust particles = Maximum value – minimum value

Often, indoor air is more polluted than outdoor air.

Test for water vapour in air

v Take a glass of ice cubes and place it in a table for some time.

v After some time, tiny water droplets are seen all over the outer surface of the glass.

v The water vapour present in the air condenses on the cold surface of the glass.

This experiment shows that air contains water vapour.

Burning and Combustion

Oxygen is necessary for the process of burning or combustion.

This is confirmed from Priestley’s experiment.

Thus, for burning a substance continuously so as to make fire, a continuous supply of fresh air is needed.

In rockets, along with the fuel, oxygen is also carried for combustion, as the availability of oxygen decreases with increase in altitude.

Burning	Combustion
Ø In burning, heat and light is produced. Ø In burning, less amount of heat is produced. Ø It takes place in hydrocarbons. Ø For eg: Burning of wood.	Ø In combustion, no flame is produced. Ø More amount of heat is produced Ø It takes place in solid fuels. Ø For eg: Combustion of wax.

Importance of air for survival of plants and animals

We have seen from Priestly’s experiments that air is a mixture of gases and is vital for the survival of all living organisms.

Air is used by plants for

The exchange of gases in plants take place through tiny pores in the leaves called stomata.

1. Photosynthesis in plants

Plants prepare their food in the presence of sunlight by a process called photosynthesis.

During photosynthesis, Carbon-di-oxide from the air and water from the soil react in the presence of sunlight to produce food.

Plants possess a green pigment called chlorophyll used for the process of photosynthesis.

Plants store this prepared food in the form of carbohydrates.

Plants release oxygen during photosynthesis which is much more than the oxygen consumed by the plants, during respiration.

2. Respiration in plants

Plants require energy for their growth and respiration occurs in plants for fulfilment of energy requirements.

During respiration, plants take in oxygen and release carbon di–oxide, just as animals do.

Usually, plants intake carbon di- oxide during daytime for photosynthesis and intake oxygen during night for respiration.

3. Respiration in Animals

This process by which oxygen reacts with digested food to form carbon di-oxide, water vapour and energy is called respiration.

When we breathe in air, the oxygen present in the air reacts chemically with digested food within the body to produce carbon di-oxide, water vapour and energy.

This energy is required to carry out many processes in the body such as movement, growth and repair.

The carbon di-oxide formed during respiration dissolves in the blood and is exhaled out of the body through the lungs.

The inhaled air contains more oxygen while the exhaled air contains more carbon di-oxide.

4. Respiration of plants and animals in water

The water bodies like lakes, rivers, seas, etc contains some dissolved oxygen in it.

The aquatic plants and animals use the dissolved oxygen in water for breathing.

For example: frogs respire through their skin, fish respire using their gills.

Uses of Air

Air is used by plants and animals for breathing.

Air is used for burning fuels like wood, coal, kerosene, LPG etc.

Compressed air is used to fill tyres of vehicles.

Air plays an important role in maintaining the water cycle in nature.

Ozone layer, present in the atmosphere, prevents harmful radiations of the sun from reaching the earth’s surface.

Oxygen cylinders are used for breathing support for patients with breathing difficulties, like patients in trauma, surgery, etc.

Oxygen cylinders are used by mountaineers, while climbing a mountain.

Deep sea divers also use oxygen cylinders for breathing.

Wind is used as a source of renewable energy, by windmills (wind mills are used to draw water, run flour mills and to generate electricity).