Selina Concise Solutions for ICSE Class 7 Geography Chapter 3 Composition and Structure of the Atmosphere

Discuss

Question: Discuss the importance of the different layers of the atmosphere ?
Answer:
The earth is the only planet in the universe that has life. One of the reasons why life exists on this planet is an encompassing blanket of gases called the atmosphere. It is like a blanket of air that protects the earth from the harmful rays of the sun. It also controls the temperature and, most important of all, it contains oxygen, which is essential for every form of life. This almost invisible blanket is kept intact around the planet by another invisible entity called gravity.
The atmosphere acts as a protective shield that filters out dangerous solar radiation while keeping the planet's temperature stable. It provides the essential oxygen we breathe and supports the water cycle, making life possible on Earth.
Teacher's Tip: Think of the atmosphere as Earth's cozy, protective blanket that keeps us warm and safe.
Exam Tip: When discussing importance, always mention both the "protective" function and the "life-supporting" gases like oxygen.

THINK AND ANSWER

Question: What would happen to the earth if the atmosphere disappeared ?
Answer:
If you would like to see what the earth would look like, take a look at the moon, with all its craters. Another thing that would happen if the earth had no atmosphere is the oceans would boil away, leaving no water here on earth because there is not atmosphere to help complete the water cycle.
Without the atmosphere, the Earth would experience extreme temperature shifts and be vulnerable to constant meteor strikes. The lack of air pressure and protection would cause all liquid water to evaporate, turning the planet into a barren desert.
Teacher's Tip: Use the Moon as a visual reference to remember that no atmosphere means no protection and no life.
Exam Tip: Be sure to highlight the collapse of the water cycle as a primary consequence of losing the atmosphere.

VALUES AND LIFE SKILLS

Question: We often leave the lights switched on and fans running when we are not in the room. We switch on the air-conditioning even though we don’t really need it. We insist on being dropped to school in the car every day. Thus, we waste valuable energy which is generally produced by burning coal and petroleum. As you know, the burning of these fuels release gases that pollute the atmosphere. Make a list of the similar things that we do without realizing that we are destroying the environment. Think of the ways that we can behave differently.
Answer:
The list of the similar things that we do without realizing that we are destroying our environment and the ways that we can behave differently and save are :
1. Leaving The Lights On : You’ve probably heard this a million times before but turning the light off when you leave the room, even if you’re only going for a few minutes, really does make a difference to the environment, since it saves a finite source of energy that can’t be replaced.
2. Boiling The Kettle : Many people guess the amount of water they need when they boil the kettle, and they end up boiling too much. Boiling a kettle actually uses a lot of power- enough to light a whole household- which also costs the householder money. There are energy efficient kettles available that can help to reduce energy usage, and kettles that measure how much water is needed for a single mug or a pot of tea.
3. Eating Farmed Meat : Maybe you’re not ready to take a step in the veggie or vegan direction, but if you’re eating farmed meat, you’re supporting an incredibly environmentally damaging industry. Many acres of rainforest are cleared to grow soy crops and grains to feed cattle, and overgrazing of cattle is a major reason for global soil depletion and source of climate pollution.
4. Commuting : Whether you’re flying away on business trips or commuting to work on a daily basis, the way we travel is one of the biggest environmental polluters in our lives. You may not be able to give up these habits, but taking steps to reduce them will impact the environment, local pollution levels, and your health.
5. Wasting Paper : We live in a world of mass paper usage, where recycling can easily ease an otherwise guilty conscience and prompt even the most devoted re-users to waste paper. We may all be guilty of throwing away pieces of paper that could otherwise be reused at least once, if only for note-taking purposes and to-do lists, but it’s easy to forget that recycling still uses energy, which can be reduced if you decide to reuse.
6. Leaving The Tap Running : Whether you’re washing the dishes or brushing your teeth, leaving the tap running while you do so wastes a vast amount of water. It may seem like a small water saving, but soaking dishes before washing them and turning off the faucet while you brush your teeth really does reduce the amount of water you use.
7. Using Plastic Bags : It’s hard to make the connection between the single plastic bag you get at the grocery store once a week, and the Great Pacific Garbage Patch. However, there is a strong likelihood that the plastic bags you bring home end will end up in the ocean, or worse. Consider buy a cloth bag instead and make a habit of keeping it in your pocket or bag.
Our daily habits have a direct impact on the health of our planet through energy consumption and waste production. By making small changes like switching off appliances and reducing plastic use, we can significantly lower our carbon footprint.
Teacher's Tip: Use the "Reduce, Reuse, Recycle" rule to evaluate your daily actions and save the environment.
Exam Tip: In long-form answers about environment, listing specific points with clear headings helps in scoring better marks.

EXERCISES

A. Fill in the blanks.

Question 1: 78% per cent of the atmosphere is made of nitrogen.
Answer: 78%
Nitrogen is the most abundant gas in our atmosphere and is vital for plant growth. It acts as a diluter for oxygen, making our air safe to breathe and preventing rapid fires.
Teacher's Tip: Remember the number 78; it is almost 4/5ths of the entire atmosphere.
Exam Tip: Be careful not to confuse the percentage of Nitrogen (78%) with Oxygen (21%).

Question 2: Along with carbon dioxide and methane, water vapour is a potent greenhouse gas.
Answer: water vapour
Water vapour helps trap heat in the atmosphere, maintaining the Earth's warmth. It is also a key part of the weather cycle as it forms clouds and precipitation.
Teacher's Tip: Think of greenhouse gases as heat-trapping sponges in the sky.
Exam Tip: When asked for examples of greenhouse gases, always include water vapour along with CO₂ .

Question 3: The rate at which the temperature drops in the troposphere is 1^∘C per 165 metres.
Answer: troposphere
The troposphere is the layer where we live and where all weather events take place. As you climb higher in this layer, the air becomes thinner and much colder.
Teacher's Tip: "Tropo" means change, and this is where all the weather changes happen.
Exam Tip: Remember that temperature decreases with height only in the troposphere and mesosphere.

Question 4: The thermosphere is the layer of the atmosphere that help in radio transmission.
Answer: thermosphere
The thermosphere contains ions that reflect radio waves back to Earth, allowing long-distance communication. It is also the layer where temperatures can reach extremely high levels due to solar radiation.
Teacher's Tip: Associate "Thermo" with heat and "Radio" with communication signals.
Exam Tip: Mention the reflection of radio waves if asked why this layer is important for technology.

Question 5: The upper part of the thermosphere is called the ionosphere.
Answer: ionosphere
The ionosphere is filled with electrically charged particles that are essential for global communication. It sits within the thermosphere and serves as a bridge to the exosphere.
Teacher's Tip: "Ion" refers to the charged particles found in this specific part of the atmosphere.
Exam Tip: Differentiate clearly between the thermosphere (the whole layer) and the ionosphere (the active part for signals).

B. Match the following.

Question: Match Column A with Column B.

Answer:
1. Troposphere - (v) Densest layer of atmosphere
2. Stratosphere - (i) Ozone
3. Thermosphere - (ii) Ions
4. Exosphere - (iii) Outer space
5. Mesosphere - (iv) Lowest temperature
Each atmospheric layer has a unique characteristic that defines its role in protecting the Earth. Matching these correctly helps in understanding how the atmosphere is organized by altitude.
Teacher's Tip: Use the first letter of each layer (T-S-M-T-E) to remember the order from the ground up.
Exam Tip: In matching questions, double-check that every item in Column A has a unique partner in Column B.

C. Choose the correct answer.

Question 1: This per cent of the atmosphere is composed of nitrogen.
1. 99
2. 78
3. 21
4. 76
Answer: 2. 78
Nitrogen is the dominant gas in our atmosphere, making up nearly four-fifths of the air. It is essential for various biological processes despite being chemically inactive in its gas form.
Teacher's Tip: 78 (Nitrogen) + 21 (Oxygen) = 99% of the atmosphere.
Exam Tip: Read the options carefully to avoid picking 21 (Oxygen) by mistake.

Question 2: 21 per cent of the atmosphere is composed of
1. Helium
2. Oxygen
3. Nitrogen
4. Hydrogen
Answer: 2. Oxygen
Oxygen is the second most abundant gas and is vital for the survival of most living organisms. It supports combustion and is necessary for cellular respiration in animals.
Teacher's Tip: Oxygen is "O₂" and it makes up "21" percent - both start with similar shapes.
Exam Tip: Remember that while Nitrogen is the most abundant, Oxygen is the most critical for life.

Question 3: This gas protects us from the harmful ultraviolet radiation.
1. Oxygen
2. Ozone
3. Helium
4. Hydrogen
Answer: 2. Ozone
Ozone is a special form of oxygen located primarily in the stratosphere that acts as a solar filter. It absorbs the majority of the sun's dangerous UV rays before they reach the surface.
Teacher's Tip: Think of Ozone as "Sunscreen for the Earth."
Exam Tip: Always associate Ozone with the Stratosphere layer in your answers.

Question 4: The rate at which the temperature drops in the troposphere is called
1. lapse rate
2. loss rate
3. less rate
4. drop rate
Answer: 1. lapse rate
The normal lapse rate describes how temperature predictably falls as we move higher into the troposphere. This phenomenon explains why mountain peaks are often covered in snow even in summer.
Teacher's Tip: Think of the temperature "lapsing" or falling as you go up.
Exam Tip: Use the term "Normal Lapse Rate" for extra marks when describing temperature changes.

Question 5: This layer of the atmosphere has the lowest temperature.
1. Mesosphere
2. Thermosphere
3. Tropopause
4. Stratosphere
Answer: 1. Mesosphere
The mesosphere is the coldest layer of the atmosphere, where temperatures can drop below -100^∘C. It is also the layer where most meteors burn up upon entry.
Teacher's Tip: "Meso" is for Middle and "Meso" is for Minimum temperature.
Exam Tip: Don't confuse the Thermosphere (hottest) with the Mesosphere (coldest).

D. State whether the following are true or false.

Question 1: The earth’s atmosphere contains 99 per cent oxygen, which is essential for life.
Answer: False.
Correct : The earth’s atmosphere contains 21% per cent oxygen, which is essential for life.
Oxygen is a vital but smaller portion of the atmosphere compared to nitrogen. If oxygen were 99%, fires would be uncontrollable and life would be very different.
Teacher's Tip: Remember that Nitrogen is the "big" gas and Oxygen is the "smaller" important gas.
Exam Tip: When correcting a false statement, always provide the accurate numerical figure.

Question 2: The stratosphere is the lowest layer of the atmosphere.
Answer: False.
Correct : The troposphere is the lowest layer of the atmosphere.
The troposphere is the layer in direct contact with the Earth's surface. It contains almost all the water vapour and dust particles in the atmosphere.
Teacher's Tip: We live in the Troposphere; think "T" for "Territory" where we stay.
Exam Tip: Always list the layers in order starting from the Earth's surface to avoid confusion.

Question 3: The temperature drops at the rate of 1^∘C per 185 metres in the atmosphere.
Answer: False.
Correct : The temperature drops at the rate of 1^∘ C per 165 metres in the atmosphere.
This specific measurement is the standard rate of cooling observed as altitude increases. It is a fundamental rule used in meteorology and aviation.
Teacher's Tip: Memorize the number 165; it’s a key figure for atmospheric science.
Exam Tip: Be precise with units like "metres" and "^∘C" in your corrections.

Question 4: The stratosphere extends from the top of the troposphere up to 80 km above the surface of the earth.
Answer: False.
Correct : The stratosphere extends from the top of the troposphere up to 50 km above the surface of the earth.
The stratosphere is relatively thin compared to the upper layers but contains the vital ozone layer. Above 50 km, the mesosphere begins.
Teacher's Tip: 50 km is the limit for the stratosphere; 80 km is for the mesosphere.
Exam Tip: Learn the height limits for at least the first three layers of the atmosphere.

Question 5: Communication satellites orbit in thermosphere.
Answer: True.
The thermosphere provides a stable environment for satellites to orbit high above the Earth's weather. Its properties help in the transmission of signals across the globe.
Teacher's Tip: The thermosphere is the "gateway" for human technology in space.
Exam Tip: If asked why satellites are there, mention the lack of air resistance and ionosphere interaction.

E. Answer the following questions in brief.

Question 1: How is the atmosphere kept in place around the earth?
Answer:
The atmosphere is kept in place around the planet by another invisible entity called gravity.
Gravity acts as a giant magnet that prevents atmospheric gases from escaping into the vacuum of space. This force ensures that the air remains dense enough at the surface for us to breathe.
Teacher's Tip: Think of gravity as the glue that sticks the air to our planet.
Exam Tip: Use the word "force" or "gravitational pull" to make your answer sound more scientific.

Question 2: Name the important gases that are found in the atmosphere.
Answer:
The important gases that are found in atmosphere are nitrogen, oxygen, carbon-dioxide, hydrogen, helium and argon.
While nitrogen and oxygen are the main components, other trace gases play critical roles in climate and biology. For example, carbon dioxide is essential for plant life and regulating Earth's temperature.
Teacher's Tip: Remember the big two (N and O) and then the "tiny" ones like Argon and CO₂.
Exam Tip: List the gases in descending order of their abundance if possible.

Question 3: How is carbon dioxide a useful gas for life on earth?
Answer:
Carbon dioxide exists in the earth’s atmosphere as a colourless and odourless gas. Carbon dioxide is produced by all animals, fungi and microorganisms during respiration and is used by plants during photosynthesis. It is also useful gas as it creates the greenhouse effect. It would have been too cold for life to exist on this planet without greenhouse effect.
Plants require carbon dioxide to create food and oxygen, which supports the entire food chain. Additionally, its role in the greenhouse effect keeps the Earth's climate warm enough for water to remain liquid.
Teacher's Tip: CO₂ is like plant food and Earth's heater combined.
Exam Tip: Mention both "Photosynthesis" and "Greenhouse Effect" to get full marks.

Question 4: What is the troposphere?
Answer:
Troposphere is the lowest layer of the atmosphere. It protects us from the heat of sun during day and keeps the earth warm at night ozone. This is the densest layer of the atmosphere and its thickness varies from 8 km over the poles to 18 km over the equator.
This is the layer where all weather events like rain, storms, and snow occur. It contains about 75% of the total mass of the atmosphere and nearly all of its water vapour.
Teacher's Tip: The Troposphere is the "Weather Layer."
Exam Tip: Don't forget to mention that its height varies at the poles and the equator.

Question 5: What is the difference between the troposphere and the tropopause?
Answer:
The lowest layer of atmosphere is called troposphere where as the boundary between the troposphere and the stratosphere is called tropopause.
The troposphere is a thick region where temperature changes, while the tropopause is just a thin transitional line. At the tropopause, the decrease in temperature with altitude finally stops.
Teacher's Tip: Think of the Tropopause as the "ceiling" of the Troposphere.
Exam Tip: Use the word "boundary" or "interface" when defining any "-pause" in the atmosphere.

F. Answer the following questions in one or two paragraphs.

Question 1: Write briefly about the layers of the atmosphere.
Answer:
The atmosphere is divided into five layers starting from the Earth’s surface :
1. Troposphere - Its average height is 13 km. The air we breathe exists here. Almost all the weather phenomena like rainfall, fog and hailstorm occur in this layer.
2. Stratosphere - It lies above the troposphere which extends up to a height of 50 km. This layer is almost free from clouds and associated weather phenomenon, making conditions most ideal for flying aeroplanes. It contains a layer of ozone gas.
3. Mesosphere - It lies above the stratosphere. It extends up to the height of 80 km. Meteorites bum up in this layer on entering from the space.
4. Thermosphere - In thermosphere, temperature rises very rapidly with increasing height. Ionosphere is a part of this layer. It extends between 80-400 km. This layer helps in radio transmission. In fact, radio waves transmitted from the Earth are reflected back to the Earth by this layer.
5. Exosphere - This upper most layer. This layer has very thin air. Light gases like helium and hydrogen float into the space from here.
The layers are organized by temperature and density, creating a complex system that supports life. Each layer serves a specific protective or functional purpose, from providing oxygen to reflecting radio waves.
Teacher's Tip: Visualize the atmosphere as an onion with many different layers protecting the center.
Exam Tip: For long answers, use a numbered list or bullet points for each layer to make it readable.

Question 2: State any three characteristics of the stratosphere.
Answer:
The main characteristics of stratosphere are:
(a) There are no clouds or weather changes so it is safe for air travel.
(b) It extends from the top of troposphere up to 50 km above the surface of earth.
(c) This layer has a band of ozone gas which protects us from sun’s harmful ultraviolet rays.
The stratosphere is much calmer than the troposphere because it lacks the rising air currents that cause weather. Its ozone content is vital for life as it prevents DNA damage from solar radiation.
Teacher's Tip: Pilots love the Stratosphere because it's smooth and clear for flying.
Exam Tip: Be sure to mention "Ozone" as it is the most defining feature of this layer.

Question 3: What is ozone? What is the ozone layer? How is the presence of ozone layer essential to preserve life on earth?
Answer:
(0₃) is present in the earth’s atmosphere in stratosphere and helps to protect the earth from the ultraviolet radiation from the sun.
Ozone is a molecule made of three oxygen atoms that specifically filters out UV-B and UV-C rays. Without this layer, humans and animals would suffer from severe skin cancers and eye damage.
Teacher's Tip: Ozone is like a high-altitude security guard protecting us from the sun's rays.
Exam Tip: Mention that Ozone is found in the Stratosphere for a complete answer.

Question 4: State any three characteristics of the mesosphere.
Answer:
The main characteristics of mesosphere are :
1. It extends from the top of stratosphere upto a height of 80 km above the earth.
2. This layer has lowest temperature in the atmosphere reaches -100^∘C at its end.
3. Most meteors from space burn up in this layer.
The mesosphere is the "protective shield" that destroys space rocks before they can hit the ground. It is also the region where beautiful "shooting stars" are created by friction with air molecules.
Teacher's Tip: "M" for Mesosphere and "M" for Meteors.
Exam Tip: Always mention the extreme cold temperature as a key characteristic of this layer.

Question 5: Why is the thermosphere important ?
Answer:
The upper part of the thermosphere is called the ionosphere. The ionosphere contains electrically charged particles called ions, which help in transmitting communication signals.
This layer allows us to have global television, radio, and phone communications by bouncing signals across continents. It also protects the lower layers by absorbing high-energy X-rays and UV radiation.
Teacher's Tip: Think of the Thermosphere as Earth's natural satellite dish.
Exam Tip: Link the word "Ions" directly to "Radio Communication" to show you understand the mechanism.

Question 6: Explain the significance of the earth’s atmosphere.
Answer:
Significance of the Atmosphere :
1. It gives us air to breathe.
2. It forms a protective shield against extreme heat.
3. It also protects us against falling debris from space.
4. It supports the formation of clouds and rain.
The atmosphere is a dynamic system that makes the Earth habitable by providing life-sustaining gases and regulating climate. It acts as a barrier that keeps the planet safe from external space threats while maintaining the internal water cycle.
Teacher's Tip: Summarize significance into "Breathe, Protect, and Regulate."
Exam Tip: Providing a numbered list of significance points is an excellent way to organize your thoughts.

G Picture study :

Question: This is a picture of a supersonic aircraft. In which layer of the atmosphere does this airplane normally fly?
Answer:
A supersonic aircraft fly in stratosphere.
Supersonic planes fly here to avoid weather turbulence and because the thinner air provides less resistance for high-speed travel. This allows for faster and more fuel-efficient long-distance flights.
Teacher's Tip: Think of the Stratosphere as the "Expressway" for planes.
Exam Tip: If asked why, state that the absence of weather makes it the safest and fastest layer for flying.

Question: This layer of the atmosphere is also known to protect living beings from the harmful rays of the sun. How?
Answer:
It has a band of ozone gas which protects the sun’s harmful U.V. rays to reach the living beings.
The ozone gas acts as a filter, absorbing radiation that would otherwise destroy living tissue and plant cells. This protection is what allowed life to move from the oceans onto land millions of years ago.
Teacher's Tip: Ozone molecules are the "bodyguards" of life on land.
Exam Tip: Use the term "absorbs" when explaining how ozone deals with UV rays.

LET’S DO SOMETHING

Question: In your notebooks, draw a diagram showing the layers of the atmosphere. Mark clearly the ozone layer, the height at which a jet plane can fly and also the height up to which birds can fly.
Answer:
(Refer to the atmospheric layer diagram. Troposphere: 0-13km, birds fly here. Stratosphere: 13-50km, jets fly and ozone layer is located here. Mesosphere: 50-80km. Thermosphere: 80-400km. Exosphere: 400km+.)
A clear diagram helps visualize how incredibly thin the habitable part of our atmosphere is compared to the vastness of space. Marking specific points like where birds or planes fly provides a sense of scale for each layer's altitude.
Teacher's Tip: Use different colors for each layer in your notebook to make them stand out.
Exam Tip: Practice drawing this diagram, as "Label the diagram" is a very common exam question.