Air
Class 6 | Science — Chemistry | Unit 4
Class 6 Unit 4 Science
Unit 4. Air
Learning Objectives
To identify the components and uses of air
To develop skills in performing experiments and arriving at conclusions
To clarify the role of oxygen in the process of burning
To realize the significance of air for the survival of plants and animals on earth
To appreciate the need of air in protecting our atmosphere
Introduction
Air is present everywhere around us. We cannot see air. But we can feel its presence in so many ways. For example, we feel air when the trees rustle, clothes hanging on a clothes-line sway, pages of an open book flutters when the fan is switched on, when kites fly in the sky. We cannot see, touch or taste air but we can feel it. It is the air that makes all these movements possible. Thus, we can understand that air is present all around us.
Air is necessary for us to live. We can live without food for some days, without water for a few hours, but cannot survive without air for more than a few minutes. So, air is very important for all living beings to survive.
When air is moving it is called wind. It is cool and soothing as breeze. When air moves with force, it is called storm. It can even uproot trees and blow off the roof tops. Air is necessary for breathing and also for combustion.
Activity 1 Air is everywhere
Let us take an empty glass bottle. Is it really empty or does it have something inside?
Now, shall we turn the glass bottle upside down? Can you agree that there is still something inside the empty glass bottle?
Let us do the following activity to find what is there inside an empty glass bottle.
Dip the open mouth of the bottle into the trough filled with water as shown in Figure 1. Observe the bottle. Does water enter the bottle?
Now tilt the bottle slightly. Now again dip the open mouth of the bottle as shown in Figure 2. Do you think that water will enter the bottle?
Kindly observe the Figure 2 carefully. You can see bubbles coming out of the bottle. When you perform the experiment, can you hear the bubbly sound? can you now guess what was inside the bottle?
Yes, you are right. It is "air" that was present in the bottle. The bottle was not empty at all. In fact, it was filled completely with air even when you turned it upside down. That is why we notice that water does not enter the bottle when it is pushed in an inverted position, as there was no space for air to escape. When the bottle was tilted, the air was able to come out in the form of bubbles, and water filled up the empty space that the air has occupied. Hence, we can see that air fills all the space inside the bottle.
4.1. Atmosphere
Our earth is surrounded by a huge envelope of air called the atmosphere. Atmosphere extends to more than 800km above the surface of earth and is held in place by the earth's gravity. The atmosphere protects us from many harmful rays coming from the sun.
The air envelope is thicker near the earth's surface and as we go higher the density and the availability of air gradually decreases. This is because, as we go higher, the force of gravity decreases, so it is not able to hold large amount of air.
The atmosphere is made of five different layers – the troposphere, the stratosphere, the mesosphere, the ionosphere and the exosphere.
The troposphere is the layer closest to the earth. It is the layer in which we live. It extends upwards for about 16km above the surface of the earth. Movement of wind takes place in this layer. It also contains water vapour, which is responsible for making clouds. This layer is responsible for the weather we experience on earth. Aircrafts usually fly above this layer to avoid strong winds and bad weather.
The stratosphere lies above the troposphere. This layer has the ozone layer in it. The ozone layer protects all life on earth from the harmful ultraviolet rays of the sun.
Do You Know?
A weathercock shows the direction in which the air is moving at a particular place. You can also make a wind sock to find the direction of the wind. Can you try it yourself?
4.2. Experimental verification of presence of Oxygen, Carbon-dioxide and Nitrogen in Air
Is air a thing or a composite mixture? For long time, that is, until eighteenth century, human thought 'air' as a fundamental constituent of matter. However, an ingenious experiment conducted by Joseph Priestley in 1774 showed that "air is not an elementary substance, but a composition," or mixture of gases. He was also able to identify a colourless and highly reactive gas which was later named 'oxygen' by the great French chemist Antoine Lavoisier.
Priestley took a tub of water and made a float and placed a candle on it. He covered the candle with a glass jar. [As the bottom portion of the jar was filled with water, no air can enter or exit and hence the jar was completely sealed (Figure 1). As you would have guessed the candle flame was extinguished in a very short time. He used a magnifying glass to focus the sun rays to light the candle. Thus, he tried to relight the candle many times without opening the sealed jar (Figure 2). The candle could not be relit. What can we make out of it? It was clear that something in the air was being used for burning and being converted into another substance. Once the substance in the air that was aiding the burning was completely used by the burning flame and converted into another substance, the flame went out. [Later chemist named the substance necessary for burning as oxygen and during the process of burning oxygen is converted mostly into carbon dioxide]
Now as the jar was inside the water, Priestley could gently lift the jar and place a live mouse inside it without allowing outside air to enter the jar (Figure 3). Without oxygen, as you would have guessed, the mouse died (Figure 4). It was clear that oxygen was necessary for the survival of the mouse.
In the next step, he gently lifted the jar and placed a mint plant (Figure 5). (Note: Look at the Figure- 5; you could see that the plant is inserted into the bell jar when the jar is very much inside the water. This is done to ensure that the outside air is not entering into the bell jar.) Plant being a living thing like mouse, perhaps he thought, would die. Instead, the plant survived. After placing the mint plant, he lit up the candle and it continued to burn (Figure 6).
In fourth experiment, he took a jar, burned a candle and converted all oxygen into carbon dioxide. He placed a mint plant and a mouse into this jar. Both the plant and the mouse survived (Figure 7). He found that plants and animals have a synergy. Animals consume oxygen and release carbon-di-oxide and plants take up carbon dioxide and release oxygen.
During 1730 to 1799, Jan Ingenhousz showed that sunlight is essential to plants to carry out photosynthesis and also to purify air that is fouled by breathing animals or by burning candles.
From these experiments it was clear that "air" is a composite mixture of many gases like oxygen and carbon di- oxide.
Proof for release of oxygen in photosynthesis
Activity 2
Take a healthy branch of Hydrilla and place it in a funnel. Invert the funnel in a beaker of water as shown in the figure. Invert a test tube over the stem of the funnel. The stem of the funnel should be kept immersed inside the water. Leave the beaker in sunlight for some time. You will notice some bubbles rising in the test tube. The bubbles contain oxygen released by the plant during photosynthesis. If we show a glowing splinter to the collected air, it burns brightly. This shows that the collected gas is oxygen.
Test for the proportion of oxygen and nitrogen in air
Activity 3
We know that iron undergoes rusting with oxygen and forms iron oxide. This process can be used to estimate the percentage of oxygen in air, which has been removed by the rusting reaction. Take a small portion of iron wool, press it into a 20 ml graduated test tube and wet it with water. Tip away excess of water. Take a 500 milli beaker and fill half of the beaker with water. Invert the test tube and place it in air. Leave the arrangement at least for a week without making any disturbance to the test tube. Observe the changes that had happened in the iron wool and to the level of water inside the test tube. We could see that the water level has increased inside the test tube. The rise in water is because of oxygen in air which has been removed by the rusting reaction. This will be about 20% which is approximately the percentage of oxygen in the air.
More to Know!
Daniel Rutherford, a Scottish chemist, discovered nitrogen. He removed oxygen and converted it into carbon-di-oxide using an inverted bell jar using a burning candle. He passed this air without oxygen through lime water and removed carbon-di-oxide also. Once the carbon-di-oxide was removed in that air, neither a candle burned nor a plant breathed. Hence, he was sure that the remaining air that he had, did not have oxygen and carbon-di-oxide. He was able to produce a gas, which showed the same property of the air without oxygen and carbon-di-oxide. Hence this gas was named 'nitrogen'.
Test for Carbon-di-oxide in air
Pour some lime water in a glass tumbler. Bubble some air using a straw through the limewater. After a few minutes, look at the lime water carefully. The lime water will produce a white precipitate and that the lime water will eventually turn to a milky white solution. This shows the presence of carbon-di-oxide in air.
4.3. Composition of Air
From Priestley's experiment which was followed by Ingenhousz and Rutherford, we came to know that air was not just one substance. We will now describe what air is made up of. This is called composition of air.
The major component of air is nitrogen. Almost four – fifth of air is nitrogen. The second major component of air is oxygen. About one fifth of air is oxygen. In addition to nitrogen and oxygen gases, air also contains small amount of carbon – di – oxide, water vapour and some other gases like argon, helium etc. The air may also contain some dust particles.
The composition of air in terms of percentage of its various components can be written as follows
The composition of air changes slightly from place to place and also from season to season. For example, Air over industrial cities usually has a higher amount of carbon-di-oxide in it than the air over open spaces. Air in coastal areas may have more water vapour than inland areas. Air also contains more water vapour in rainy season. The amount of dust in the air is more in windy places than other areas.
Test for the presence of dust particles in air
You might have seen the sunlight entering into a dark room through a narrow slit and making shiny dust particles dancing merrily on the path of sunlight. Actually, the air in a room always contains some dust particles, but they are so small that normally they are not visible to us. When a beam of sunlight falls on them, the tiny dust particles become visible.
Activity Calculate dust particles in air
Take a graph sheet. Using marker pens draw a 5x5 cm square on the graph. Apply a thin film of grease on the graph sheet. This sheet will serve as dust collector. Make four or five graph sheets. Discuss in the whole class, as where to place the dust collectors, how long to collect dust particles and place the dust collectors in agreed positions. Ensure that the dust collectors do not get blown away. After the time scheduled for performing this activity is reached, remove the paper and count the number of collected dust particles in the marked area in all the sheets, using a magnifying glass at the dust collector.
Then, calculate the mean number of dust particles in the marked area. Mean equal to total number of dust particles on collector divided by number of squares on collector
The range of the dust can also be calculated as given below Range equal to Maximum value minus minimum value
Collect details from all the areas where we have kept the dust collector sheets. Tabulate the recordings in the table given below
| Location of dust collector | Mean number of dust particles per small square | Range |
|---|
| | |
| | |
| | |
Which area do you think will have the most dust?
Which area do you think will have the least dust?
Test for water vapour in air
Take a few ice cubes in a glass. Keep it on the table for a few minutes. Observe what happens. You could see tiny droplets of water all over the outer surface of the glass. From where do these droplets come? The water vapour present in the air condenses on the cold surface of the glass. This shows that air contains water vapour.
4.4. Burning and Combustion
When we burn a candle, paper, kerosene, coal, wood or cooking gas (LPG), oxygen is needed. The oxygen needed for the burning of candle, paper, kerosene, coal, wood and cooking gas comes from the air around us. Thus, for burning a substance continuously so as to make fire, a continuous supply of fresh air is needed. If we cut off the supply of fresh air to a burning substance, then the burning substance will not get oxygen necessary for burning to continue and hence the substance will stop burning.
In rockets, as they go high in the atmosphere, the availability of oxygen is considerably reduced. Therefore, in rockets along with the fuel, oxygen is also carried for combustion.
The process of burning of a substance in the presence of oxygen and releasing a large amount of light and heat is called burning. If the process does not emit flame then it is called combustion.
Activity 4 Oxygen is necessary for burning
Place two candles on a table. Ensure that both the candles are of same size and height. Mark them as candle 1 and candle 2 using a chalkpiece. Light both the candles. Now, cover candle 2 with glass tumbler as shown in the figure
Observe the happenings at both the candles. What does happen to candle 1?
What does happen to candle 2?
Can you guess why did the covered candle extinguish?
Let us summarize the happenings. The candle 1 continues to burn, unless it is blown – off by strong moving air or any other external force. This is because fresh air is continuously available to the candle for its burning process. Candle 2 glows for a while and then gets put – off. When the burning candle is covered with a glass tumbler, the candle can use the oxygen available in the air inside the glass tumbler. Since only a small amount of air is present inside the glass tumbler – only a small portion of oxygen is available for the candle to continue glowing. When all the oxygen of the air inside the gas jar is used up, then the burning candle gets extinguished.
Now, repeat the candle glowing experiment taking four containers of different sizes. For example, you can take a 250 milli conical flask, a 500 milli bottle, a one-liter jar, a two-liter jar. Cover the burning candle one by one with these containers and find out how long it takes for the candle to extinguish in each case. Record your observations in the following table.
| Serial number | Volume of the Container in (milli liter) | Time taken for candle to extinguish in (second) |
|---|
| 1 | | |
| 2 | | |
| 3 | | |
| 4 | | |
Can you write interpretation based on your observations at the table?
4.5. Importance of air for survival of plants and animals
Respiration in plants
Plants require energy for their growth and hence respiration also occurs in plants. During respiration, plants take in oxygen and release carbon–di–oxide, just as animals do. Gaseous exchange with air in atmosphere takes place in plants with the help of tiny holes called stomata present on their leaves.
Photosynthesis
Plants manufacture food 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. Most plants possess a green pigment called chlorophyll and it is also used-up in the process of photosynthesis. The word equation given below explains the process of photosynthesis.
Plants release oxygen during photosynthesis which is much more than the oxygen consumed by the plants, during respiration.
Respiration in Animals
When we breathe in air, the oxygen present in the air reacts chemically with digested food within the body to produce carbon-di-oxide gas, water vapour and energy.
Light energy – Chlorophyll absorb light from the sun Carbon di ox side - Carbon di ox side enters through the stomata, a natural opening in the leaf Water – water is absorbed through the root and carried through the stem to the rest of the plant water, carbon di ox side and the sun light combine in the leaf to make food Oxygen – Oxygen and water vapour exit the leaf through the stomata water loss from leaves is called transpiration.
This energy is required to carry out many processes in the body such as movement, growth and repair. This process by which oxygen reacts with digested food to form carbon-di-oxide, water vapour and energy is called respiration. The process can be represented by a word equation as given below
Carbon-di-oxide formed during respiration dissolves in the blood and is exhaled out of the body through the lungs. The inhaled and exhaled air thus contain the same substances but in different proportion, except nitrogen which is present in the same amount. Inhaled air contains more oxygen while the exhaled air contains more carbon-di-oxide.
Let us have a look at the following table to compare the composition of air in inhaled and exhaled air.
| Component | Inhaled air | Exhaled air |
|---|
| Nitrogen | 78% | 78% |
| Oxygen | 21% | 16% |
| Carbon-di-oxide | 0.03% | 4% |
| Water vapour | Variable amount | Amount increases in exhaled air |
| Noble gases | 0.95% | 0.95% |
| Dust | Variable amount | none |
| Temperature | Room temperature | Body temperature |
Respiration of plants and animals in water
The water in ponds, lakes, rivers and seas have some amount of dissolved air containing oxygen in it. The plants and animals that live in water use the oxygen dissolved in water for breathing. For example, frogs respire through their skin, fish respire using their gills.
Do You Know?
When carbon-di-oxide is cooled to -57 degree Celsius, it directly becomes a solid, without changing to its liquid state. It is called dry ice and is a good refrigerating agent. Dry ice is used in trucks or freight cars for refrigerating perishable items such as meat and fish while transporting them.
4.6. 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 various kinds of vehicles.
Air plays an important role in maintaining the water cycle in the nature.
Ozone layer, present in the atmosphere, helps in preventing harmful radiations of the sun from reaching the earth's surface.
Under extra – ordinary conditions such as a. a patient having breathing difficulties, b. a mountaineer climbing a high mountain, c. a diver going deep into the sea oxygen gas cylinders are used for breathing purposes.
Blowing air is used to turn the blades of wind mills. The wind mills are used to draw water by running pumps, run flour mills and to generate electricity.
Points to Remember
Air is everywhere around us.
Our earth is surrounded by a huge envelope of air called the atmosphere.
The process of burning of a substance in the presence of oxygen and releasing a large amount of light and heat is called combustion.
Priestley helped us in understanding the presence of oxygen in air that is produced by plants during photosynthesis which can be used by animals for respiration.
Ingenhousz experiment helped us to know the role of sunlight in evolving Oxygen during photosynthesis.
Air contains 78% Nitrogen, 21% Oxygen, 1% of carbon-di-oxide, water vapour, Noble gases, and dust particles.
Composition of air changes slightly from place to place and also from season to season.
Aquatic plants and animals use oxygen dissolved in water for breathing.
The atmosphere is made of five different layers – the troposphere, the stratosphere, the mesosphere, the ionosphere and the exosphere.
Ozone layer, present in the atmosphere helps in preventing harmful radiations hitting the earth directly.
ICT Corner
Air
Through this activity you will be able to understand the atomic level of the process that plants use to convert solar energy into chemical energy.
Step 1: Use the given URL in the browser. 'Illuminating Photosynthesis page will open.
Step 2: Three buttons given on the top of the activity window to explore. Click the 'The Cycle' button, in this window you can open the curtain and water the plant by click on the curtain and the watering pot.
Step 3: Explore the atomic level process of the photosynthesis by clicking the 'Atomic Shuffle' button.
Step 4: Click 'Replay" to view the process again and 'Next' to view the next level of the process.
Illuminating Photosynthesis URL: http://www.bbc.co.uk/schools/scienceclips/ages/10_11/rev_irrev_ changes_fs.shtml
*Pictures are indicative only
Evaluation
I Choose the appropriate answer
1. _____ is the percentage of nitrogen in air.
a. 78% b. 21% c. 0.03% d. 1%
2. Gas exchange takes place in plants using _____.
a. Stomata b. Chlorophyll c. Leaves d. Flowers
3. The constituent of air that supports combustion is _____.
a. Nitrogen b. carbon-di-oxide c. Oxygen d. water vapour
4. Nitrogen is used in the food packaging industry because it _____
a. provides colour to the food b. provides oxygen to the food c. adds proteins and minerals to the food d. keeps the food fresh
5. _____ and _____ are the two gases, which when taken together, make up about 99 percentage of air.
i. Nitrogen ii. carbon-di-oxide iii. Noble gases iv. Oxygen
a. i and ii b. i and iii c. ii and iv d. i and iv II Fill in the blanks
1. is the active component of air.
2. The gas given out during photosynthesis is .
3. gas is given to the patients having breathing problems.
4. can be seen moving in a beam of sunlight in a dark room.
5. gas turns lime water milky.
III True or False. If False, give the correct statement 1. Inhaled air contains a large amount of carbon-dioxide. 2. Planting trees help in decreasing global warming. 3. The composition of air is always exactly the same. 4. Whales come up to the water surface to breathe in oxygen. 5. The balance of oxygen in atmosphere is maintained through photosynthesis in animals and respiration in plants. IV Match the following
| 1. Moving Air | |
| 2. Layer in which we live | |
| 3. Stratosphere | |
| 4. Oxygen | |
| 5. carbon-di-oxide | |
Options: Photosynthesis, Troposphere, Wind, Ozone layer, Combustion
V Arrange the following statements in correct sequence Plants manufacture food by a process called photosynthesis. Plants require energy for their growth. Plants take in oxygen and release carbon-di-oxide just as animals. Plants take carbon-dioxide from the atmosphere, use chlorophyll in the presence of sunlight and prepare food. Such oxygen is available to animals and human beings for breathing. During this process, oxygen is released by plants. VI Analogy
1. Photosynthesis is :: Respiration is Oxygen
2. 78% of air is :: Does not support combustion :: is Supports combustion
VII Observe the given figure carefully and answer the questions
1. What will happen if we remove plants from the aquarium?
2. What will happen if we remove the fish from the aquarium and keep it (with green plants) in a dark place?
VIII Give very short answer
1. What is atmosphere? Name the five layers of atmosphere.
2. How do the roots of land plants get oxygen for breathing?
3. What should be done if the clothes of a person catch fire accidentally? Why?
4. What will happen if you breathe through mouth?
IX Give short answer
1. Biscuits kept open on a plate during monsoon days lose its' crispness. Why?
2. Why do traffic assistants wear a mask on duty?
X Answer in detail
1. How do plants and animals maintain the balance of oxygen and carbon-dioxide in air?
2. Why is atmosphere essential for life on earth?
XI Question based on Higher Order Thinking Skills
1. Can you guess why fire extinguishers throw a stream of carbon-di-oxide while putting - off fire?
📝 Summary
- Air is present everywhere around us and is essential for all living beings to survive - we cannot live without air for more than a few minutes.
- Air occupies space as demonstrated by the experiment where an inverted bottle doesn't allow water to enter until tilted, showing air was filling the space.
- The atmosphere is a huge envelope of air that surrounds Earth, extending to more than 800km above the surface and is held in place by Earth's gravity.
- The atmosphere consists of five different layers - the troposphere, stratosphere, mesosphere, ionosphere and exosphere, with the troposphere being closest to Earth where we live.
- Air is necessary for breathing, combustion (burning), and protects us from harmful rays coming from the sun while supporting the survival of plants and animals on Earth.