Air and Atmosphere | ICSE Class 6 Chemistry Notes

Air and atmosphere, evidence for the presence of air, its gaseous components and percentage composition, the importance of oxygen and nitrogen, carbon dioxide and its detection, water vapour, moving air, and ways to reduce air pollution.

What are air and the atmosphere?

Definition: The atmosphere is the layer of air surrounding the Earth. Air is a mixture of gases, including nitrogen, oxygen, argon and carbon dioxide.

A mixture contains different substances together. The substances present are its components. Air also contains water vapour, which is water in its gaseous state. Dust and smoke can pollute the air around us.

Air is present everywhere around us. We breathe it, and moving air makes its presence noticeable. Leaves rustling, clothes swaying on a clothes line and pages fluttering when a fan is switched on are familiar evidence of air in motion.

How do air and atmosphere differ?

The word air identifies the mixture itself. The word atmosphere identifies the layer of that mixture surrounding the Earth. These words are closely connected, but neither word means oxygen alone.

Our bodies need oxygen to perform their functions. Most living beings also need oxygen for survival. This makes air essential to life, although different components have different roles.

Air also matters in processes outside living bodies. Oxygen supports a candle burning, water vapour is involved in changes between water's liquid and gaseous states, and moving air can turn a windmill.

Keeping air clean therefore protects a resource used in many ways. Studying air involves identifying what it contains, observing evidence of its presence, and connecting its components with the processes that depend on them.

How can a bottle show that air occupies space?

An apparently empty bottle contains air. Occupying space means taking up room. A bottle and water activity makes this property of air easier to recognise, because the escaping air becomes visible as bubbles in the water.

What is the procedure?

1. Take an empty glass bottle and a tub containing water.
2. Turn the bottle upside down and lower its open mouth into the water.
3. Tilt the bottle while keeping its mouth under water.
4. Watch the air bubbles coming out of the bottle and the water entering it.

The important observation, meaning what is directly noticed, is that bubbles come out as water goes inside. The bottle did not contain liquid at the start, but that does not mean it contained nothing.

The conclusion, meaning the explanation supported by the observation, is that air was present inside the bottle. As this air escapes through the opening, water enters the space it occupied.

Draw a tub of water with an inverted bottle tilted below the water surface. Label the bottle, water and air bubbles escaping from its mouth. Show water entering the bottle.

What does the activity establish?

The activity provides evidence for the presence of air in the bottle. It also connects the movement of the two materials: air comes out and water goes in. Both parts of the observation belong in a description of the result.

It does not identify a particular gas. The escaping bubbles are bubbles of air, a mixture, so they should not be labelled as pure oxygen or pure nitrogen.

Keep the procedure, observation and conclusion separate when recording the activity. Tilting the bottle is an action; seeing bubbles is an observation; explaining that air occupied the bottle is a conclusion. This distinction makes an experimental account clear.

Why is air described as a mixture of gases?

Air contains several gaseous components. Nitrogen and oxygen make up the major part. Argon, carbon dioxide and other gases are also present. Water vapour is another component of the air around us.

A gas does not have a fixed shape and spreads through the available space. Different gases can therefore be present together around us. Calling air a mixture recognises these different components without treating them as a single gas.

Which components should be distinguished?

Limewater is a clear solution of calcium hydroxide in water, used to test for carbon dioxide. Calcium hydroxide is the substance dissolved in this test liquid. The test and its observation are explained below.

Dust consists of small solid particles that can be present in air. Smoke is produced during burning and contains particles and gases. Dust and smoke are treated as pollutants here, not as additional major gases in the composition chart.

A pollutant is a substance that contaminates the environment and can cause harm. Air pollution is the contamination of air by such substances. Smoke contains many harmful gases, so its presence matters even though air also contains useful gases.

Note: Air is not another name for oxygen. Oxygen is one component of air, and different components have different properties and uses.

A complete description of air should therefore separate three ideas: the mixture as a whole, its individual gaseous components, and materials such as dust and smoke that pollute it.

What is the percentage composition of air?

Percentage means the number of parts in a hundred. Its symbol is %. A percentage composition shows how a whole is divided among its components. A diagram with a hundred squares gives a simple way to represent the composition of air.

How should the smaller group be read?

The 1% belongs to argon, carbon dioxide and other gases together. It is not the percentage of carbon dioxide alone. Nor does it mean that each gas in that group separately occupies one square.

The largest share belongs to nitrogen, while oxygen has the next largest share. Oxygen's importance to our bodies does not mean that it makes up most of the air.

The square grid contains 100 small squares. Blue represents nitrogen, labelled 78%; red represents oxygen, labelled 21%; and pink represents argon, carbon dioxide and other gases together, labelled 1%.

See Fig. 11.1 in your NCERT textbook

How can the diagram be used?

First identify the whole grid. Then read each coloured region with its label. Finally compare the sizes of the groups. Reading the labels prevents the small pink region from being mistaken for carbon dioxide alone.

The grid describes relative amounts, meaning the amounts compared with the whole. It does not rank usefulness. A component can have an important role even when its share of the mixture is small.

Keep the grouped label intact when copying the chart. Replacing it with the name of a single gas changes the meaning of the information, even if the number beside it remains the same.

Why is oxygen important for living beings?

Oxygen is a gas in the air that our bodies need to perform their functions. The air we breathe in contains oxygen. Most living beings also need oxygen for their survival.

Breathing is the movement of air into and out of the body. When we inhale, air moves in from outside. When we breathe out, air moves from inside the body to the surroundings.

How are breathing and respiration connected?

Breathing is part of respiration, the process that releases usable energy from food in living things. Energy is the capacity to do work or bring about change.

The visible movement of the abdomen in a resting dog, cat, cow or buffalo helps us notice breathing. The abdomen is the belly region of the body. Such movement provides a way of observing breathing in these animals.

Plants also respire. Tiny pores called stomata on the surface of leaves help plants take air in and out. A pore is a small opening. Plants therefore belong in a discussion of air and life, as well as animals.

Why is the exact wording important?

“Most living beings need oxygen” preserves an important limit. It should not be changed into “every living being needs oxygen”. The statement that all living beings respire is different from a claim that every form of respiration needs oxygen.

When discussing air, also distinguish the mixture that enters the body from the component needed for its functions. We breathe air containing oxygen; ordinary surrounding air is not pure oxygen.

These connections explain why air is essential for sustaining life. The atmosphere supplies the air around us, breathing moves air into and out of our bodies, and oxygen in that air supports our body's functions.

What does a covered candle show about oxygen?

Combustion means burning. Oxygen supports combustion, which means it helps a substance continue to burn. A candle covered by an inverted glass provides a simple demonstration of oxygen's role.

How is the demonstration arranged?

1. Place a candle in a shallow container.
2. Put some water in the container and light the candle.
3. Place an inverted glass over the lighted candle.
4. Observe that the candle burns for some time and then goes out.

Inverted means turned upside down. The glass covers the candle and encloses some air around it. Carry out this flame demonstration under adult supervision.

The key observation is that burning continues for some time before the flame is extinguished. The oxygen in the enclosed air is used during burning. Eventually there is too little oxygen to keep the candle burning.

Does all the air disappear?

Nitrogen, the major component of air, remains present above the water level and does not support the burning candle. The flame going out therefore does not mean that every component of the enclosed air has disappeared.

Draw a candle in a shallow container of water and an inverted glass over it. Label the candle, glass, water and enclosed air. Add a second sketch showing the flame extinguished after some time.

Compare this activity with the bottle activity. The bottle activity shows that air is present and occupies space. The candle activity connects a particular component, oxygen, with the continuation of burning.

Describe the result in words rather than assigning an exact burning time. The demonstration gives an observation and an explanation of the role of oxygen; no fixed number of seconds is needed to express that conclusion.

Keep the conclusion focused: oxygen supports burning, while nitrogen does not support this candle burning. This difference is one reason the components of air should be considered separately.

Why is nitrogen important even though it does not support burning?

Nitrogen makes up the largest share of air, shown as 78% in the composition chart. Its role differs from that of oxygen. Nitrogen does not support the burning of a candle, but it is essential for plants.

Plants contain nitrogen in substances such as proteins and enzymes. Proteins are substances needed for the structure and functioning of living things. Enzymes are substances that help chemical reactions in living things take place. A chemical reaction is a change that forms new substances.

How should nitrogen and oxygen be compared?

The comparison separates quantity from function. Quantity describes how much of a component is present. Function describes what it does or why it matters. The largest component need not do the same job as another component.

In the covered candle demonstration, the flame goes out even though nitrogen remains present above the water. The remaining nitrogen cannot take over oxygen's role in supporting the burning candle.

However, that result is not evidence that nitrogen is useless. The activity tests its role in candle burning. Its importance in plant substances is a different part of the study of air.

A clear explanation therefore joins two points without confusing them: nitrogen is the main gaseous component of air, and nitrogen is essential to plants. Its failure to support a candle flame does not cancel either point.

How can carbon dioxide be detected, and why does its amount increase?

Carbon dioxide is one of the gases present in air. In the composition chart it belongs to the group containing argon and other gases. That group's combined percentage must not be assigned to carbon dioxide by itself.

What does the limewater test show?

When air is bubbled through limewater, a milky change shows the presence of carbon dioxide. “Milky” describes the cloudy, whitish appearance of the liquid. It is the change in the test liquid that provides the evidence.

1. Take clear limewater as the test liquid.
2. Bubble air through the limewater during a supervised demonstration.
3. Observe the liquid for a change to a milky appearance.
4. Use that change as evidence that the air contains carbon dioxide.

The test concerns presence, meaning that the gas is there. It does not, by itself, provide a percentage for that gas. The milky appearance should not be used to invent a numerical composition.

How do plants use carbon dioxide?

Plants use carbon dioxide and water to make food in the presence of sunlight and chlorophyll, the green pigment in leaves. A pigment is a colouring substance. This food-making process is called photosynthesis, and oxygen is released during it.

Carbon dioxide therefore has a useful role in plant life. Recognising this role is compatible with recognising that burning fuels adds carbon dioxide to the atmosphere. The gas's use by plants and the activities that release it are different parts of the explanation.

How do everyday activities add carbon dioxide?

Fuels are substances used as sources of energy. Burning fuel produces carbon dioxide. Everyday fuel use therefore adds carbon dioxide to the atmosphere.

Fossil fuels include coal, petroleum and natural gas. They formed essentially from the remains of microorganisms and plants buried in the Earth over very long periods. Microorganisms are living things too small to see clearly without magnification.

Petrol and diesel, widely used as vehicle fuels, are obtained from petroleum. Burning fossil fuels produces carbon dioxide and smoke. Heavy dependence on these fuels for transport and domestic uses contributes to air pollution.

The important connection is between an activity and its result: burning fuel adds carbon dioxide to the air. A complete explanation identifies both the activity that produces the gas and the atmosphere into which it is released.

How does water vapour become part of air?

Water vapour is water in its gaseous state. It is invisible and is present in the air around us. Water vapour exists even at room temperature, so water does not have to be boiling for vapour to be present.

How does evaporation add water vapour?

Evaporation is the conversion of water into its vapour state. It takes place continuously, even at room temperature. Water from drying clothes and a mopped floor contributes to the water vapour in surrounding air.

These examples connect a visible change with an invisible material. The wet surface becomes dry, while water enters the air as vapour. The fact that we cannot see the vapour does not mean the water has ceased to exist.

How can water vapour form visible drops?

Condensation is the conversion of water vapour into liquid water. When water vapour in air meets a cold surface, it forms droplets. A droplet is a tiny drop of liquid.

Put cold water and a few ice cubes in a glass tumbler and leave it undisturbed for five minutes. Water droplets appear on its outer surface. The cold surface allows water vapour in the surrounding air to condense.

Distinguish invisible vapour from visible droplets. The drops on the outside of the cold glass are liquid water. They provide evidence of water vapour in the air, but they are not themselves water in the gaseous state.

Evaporation and condensation explain how water can move between a surface and the surrounding air. Both processes help us understand why water vapour belongs in a description of air's components.

How is moving air useful?

Wind is moving air. Sometimes it blows fast, as during a storm, and sometimes it blows gently as a breeze. These differences in movement do not make wind a separate substance from air.

A firki is a paper pinwheel. Air blown on a firki makes it rotate, meaning turn around. Holding it while running also makes it rotate. Its movement gives a simple demonstration of the effect of moving air.

How does a windmill use this movement?

A windmill is a device whose wings are turned by wind. Its working is similar to that of a firki. Windmills can run flour mills, pull water up from wells or generate electricity.

A windmill farm is an area containing many windmills that use wind energy to generate electricity. Here, the movement of air is put to a useful purpose.

These examples show a use of air in motion. They differ from the candle activity, which concerns oxygen's role in burning, and from breathing, which supplies air containing oxygen to the body.

Recognising these differences helps organise the uses of air. Some uses depend on a particular gaseous component. Others depend on the movement of the mixture as a whole. A useful answer should identify which of these ideas applies to the example being explained.

Why should air be kept clean, and what can we do?

Dust and smoke are pollutants of air. Smoke contains many harmful gases. Air remains important for life even when it becomes polluted, which is why keeping it clean is a shared responsibility.

How does fuel use contribute to pollution?

Fossil fuels are used in transport and in homes. Burning them produces smoke and carbon dioxide. Over-dependence on them has resulted in large-scale air pollution. The connection between fuel use and polluted air makes choices about transport and energy important.

Petrol and diesel are widely used for vehicles. Natural gas is also used for cooking, electricity generation and vehicles. In vehicles it can be used as Compressed Natural Gas, abbreviated CNG, meaning natural gas compressed for storage and use.

CNG is a cleaner fuel than petrol or diesel. “Cleaner” is a comparison. It should not be changed into a claim that using the fuel causes no pollution.

Which actions can reduce dependence on fossil fuels?

• Walk or cycle to nearby places. These choices help conserve fossil fuels used for transport.
• Use public transport. This is another way to contribute to conserving fossil fuels.
• Use alternatives that cause less pollution. Electric vehicles, for example, do not release smoke.

The statement about electric vehicles concerns smoke released by the vehicle. Keep that meaning clear rather than expanding it into a claim about every stage of producing or using electricity.

Windmills provide another connection with energy choices because moving air can be used to generate electricity. Air is therefore both a resource to protect and a source of useful movement.

Understanding pollution begins with recognising its sources. Connecting burning fuel with smoke and carbon dioxide helps explain why conserving fossil fuels and using less polluting alternatives matter for the air shared by living things.

Glossary

• Air — A mixture of gases surrounding us, including nitrogen, oxygen, argon, carbon dioxide and water vapour.
• Atmosphere — The layer of air that surrounds the Earth and provides air for living things.
• Component — One of the different substances present as part of a mixture.
• Percentage — The number of parts in a hundred, written using the symbol %.
• Oxygen — A gas in air that supports burning and is needed by our bodies.
• Nitrogen — The largest gaseous component of air, essential for plants and present in proteins and enzymes.
• Carbon dioxide — A gas present in air that makes limewater milky and is produced by burning fuel.
• Combustion — Burning, illustrated by a lighted candle using oxygen from the air around it.
• Limewater — A clear solution of calcium hydroxide in water, used to test for carbon dioxide.
• Water vapour — The invisible gaseous state of water, present in the air around us.
• Evaporation — The process in which liquid water changes into its gaseous state, water vapour.
• Condensation — The process in which water vapour changes into liquid water, forming droplets on cold surfaces.
• Wind — Moving air, which can turn a paper pinwheel or the wings of a windmill.
• Pollutant — A substance that contaminates the environment and can cause harm to living things.
• Fossil fuels — Fuels such as coal, petroleum and natural gas formed from buried remains over very long periods.

Common errors and misconceptions

• Misconception: An empty bottle contains nothing. Correct: It contains air; tilting it under water lets air escape as bubbles while water enters.
• Misconception: Air and oxygen mean the same thing. Correct: Air is a mixture, and oxygen is one of its components.
• Misconception: Carbon dioxide makes up 1% of air. Correct: The 1% shown in the composition chart belongs to argon, carbon dioxide and other gases together.
• Misconception: Every living being needs oxygen. Correct: Most living beings need oxygen for survival. All living beings respire, but these statements do not mean the same thing.
• Misconception: The covered candle goes out because all the air disappears. Correct: Oxygen becomes insufficient to support burning; nitrogen remains and does not support the candle flame.
• Misconception: Nitrogen is useless because it does not support burning. Correct: Nitrogen is essential for plants and is present in proteins and enzymes.
• Misconception: Water vapour is the visible liquid on a cold glass. Correct: Water vapour is invisible; the visible drops are liquid water formed by condensation.
• Misconception: A cleaner fuel causes no pollution. Correct: Cleaner is a comparison. CNG is described as cleaner than petrol or diesel.

Exam-style questions with model answers

Q1. Define the atmosphere and explain how oxygen is related to air. [2 marks]

1. The atmosphere is the layer of air that surrounds the Earth.
2. Oxygen is one gaseous component of air, which is a mixture of different gases.

Q2. A composition chart shows nitrogen 78%, oxygen 21%, and argon, carbon dioxide and other gases together 1%. Identify the largest component, explain the grouped 1%, and describe the same chart using 100 squares. [3 marks]

1. Nitrogen is the largest component shown because its share is 78%, greater than the other shares in the chart.
2. The 1% belongs to argon, carbon dioxide and other gases together. It is not the percentage of carbon dioxide alone.
3. In a grid of 100 squares, 78 represent nitrogen, 21 represent oxygen, and one represents the combined group of other gases.

Q3. An empty bottle is inverted in a tub of water and then tilted. Bubbles escape and water enters the bottle. Explain the observation, what was originally inside, and the conclusion about space. [3 marks]

1. The bubbles are air escaping through the bottle's mouth when the inverted bottle is tilted under water.
2. The bottle originally contained air even though it was described as empty. That word meant it did not contain liquid or another visible filling.
3. Water enters the space from which air escapes. This provides evidence that the air inside the bottle occupied space.

Q4. A lighted candle stands in a shallow container of water. An inverted glass is placed over it. It burns for some time and then goes out. Describe the setup, observation, role of oxygen, reason for extinction and role of the remaining nitrogen. [5 marks]

1. The setup has a lighted candle in a shallow container of water, with an inverted glass enclosing air around the flame.
2. The observation is that the candle continues burning for some time after it is covered and then the flame goes out.
3. Oxygen in the enclosed air supports the burning of the candle and is used during this process.
4. The flame goes out when too little oxygen remains to support the candle's continued burning inside the glass.
5. Nitrogen remains present above the water level, but it does not support the burning candle. The result does not mean all air has disappeared.

Q5. Explain why nitrogen is important to plants and why its presence does not keep a covered candle burning. [2 marks]

1. Nitrogen is essential for plants and is present in substances such as proteins and enzymes.
2. Nitrogen does not support a burning candle, so it cannot replace oxygen's role in keeping the flame alight.

Q6. Air bubbled through clear limewater makes it milky. Separately, burning fossil fuels releases smoke and carbon dioxide. Identify the gas detected, explain the test observation, connect fuel burning with atmospheric carbon dioxide, and state one way to conserve transport fuel. [4 marks]

1. The gas detected in the air is carbon dioxide, one of the gaseous components of the air around us.
2. The evidence is the change from clear limewater to a milky appearance when air is bubbled through it.
3. Burning fossil fuels releases carbon dioxide into the atmosphere, so everyday use of these fuels adds this gas to the air.
4. Walking or cycling to nearby places helps conserve the fossil fuels that would otherwise be used for transport.

Q7. A glass contains cold water and ice cubes. After five minutes, liquid droplets appear on its outside. Name the water present in air, identify the process, and explain the droplets' origin. [3 marks]

1. The air contains water vapour, which is the invisible gaseous state of water and is present even at room temperature.
2. The process is condensation, meaning the conversion of water vapour into liquid water rather than the reverse change.
3. Water vapour in the surrounding air comes into contact with the cold outer surface of the glass and forms the observed liquid droplets.

Q8. Explain what wind is and how it turns a firki. Then describe three uses of windmills: running flour mills, pulling water from wells and generating electricity. [5 marks]

1. Wind is air in motion. It can blow fast during a storm or move gently as a breeze.
2. A firki is a paper pinwheel. Blowing air on it makes it rotate, showing the effect of moving air.
3. Wind rotates the wings of a windmill, and this movement can be used to run a flour mill.
4. A windmill can also use the movement produced by wind to pull water up from a well.
5. Windmills can generate electricity. A windmill farm contains many windmills that use the energy of wind for this purpose.

Key takeaways

• Air is a mixture of gases, while the atmosphere is the layer of air surrounding the Earth.
• The composition chart shows nitrogen at 78%, oxygen at 21%, and argon, carbon dioxide and other gases together at 1%.
• Bubbles escaping from a tilted bottle under water show that an apparently empty bottle contains air.
• Our bodies need oxygen, and most living beings need it for survival; oxygen also supports a candle burning.
• Nitrogen does not support candle burning, but it is essential for plants and occurs in proteins and enzymes.
• Carbon dioxide turns limewater milky, and everyday burning of fuels adds carbon dioxide to the atmosphere.
• Evaporation adds water vapour to air, while condensation changes water vapour into visible liquid water droplets.
• Moving air can turn windmills; walking, cycling and public transport help conserve fossil fuels and support cleaner air.

Test yourself

What is the atmosphere?

The atmosphere is the layer of air that surrounds the Earth.

Why should the bubbles from an inverted bottle be labelled air?

The bottle contains air, a mixture of gases. The activity does not separate or identify a pure gas.

Which group occupies 1% in the composition chart?

Argon, carbon dioxide and other gases together occupy the 1% group shown in the chart.

What does nitrogen's failure to support candle burning tell us about its importance to plants?

It does not remove nitrogen's importance to plants. Plants contain nitrogen in substances such as proteins and enzymes.

What observation detects carbon dioxide with limewater?

Limewater changes from clear to milky when carbon dioxide is passed through it.

How do evaporation and condensation differ?

Evaporation changes liquid water into water vapour. Condensation changes water vapour into liquid water.

What is a windmill farm?

It is an area with many windmills that use wind energy to generate electricity.

Why does cleaner not mean pollution-free when describing CNG?

Cleaner compares CNG with petrol or diesel. It does not claim that the fuel causes no pollution.