Selina Concise Solutions for ICSE Class 6 Chemistry Chapter 6 Air and Atmosphere

POINTS TO REMEMBER

1. Air cannot be seen as it is transparent we only feel its presence when it blows.
2. The thick layer of air around earth upto 320 km above the surface is called atmosphere.
3. Air is a mixture of various gases, water vapours, which protects us from harmful rays of Sun. It consists of nearly 80% nitrogen, 20% oxygen.
4. The components of air can be separated by simple physical means. Has no formula.
5. N₂ is inert gas, reduces the effect of O₂ present in air, N₂ is helpful in plant growth as it provides nitrates used by plant for formation of protein which is important nutrient for animals.
6. Oxygen is supporter of combustion no burning is possible without O₂ and no life is possible without it.
7. CO₂ is must for plant life, plants prepare their food taking CO₂ from air. CO₂ maintains temp, of earth and without CO₂ our planet would be too cold.
8. Ozone in the upper layers of atmosphere protects the earth from harmful ultraviolet rays of Sun.
9. Air is useful in many ways : Acts as medium for propagation of sound, birds, insects to fly in air, air ships, aeroplanes to fly only must due to presence of air. Air helps in movement of yachts, parachutes, aircrafts etc. Air is must for animals and plants on earth and air dissolved in water for aquatic life.
10. Respiration : Is a slow chemical process when O₂ present in the inhaled air reacts with the digested food material in the body to release energy, CO₂ and water. Sugar + Oxygen -> Carbon dioxide + Water + Energy.
Breathing : The complete process in which air is inhaled and exhaled is called breathing.
11. Combustion : Burning of substance in air completely producing CO₂, heat and light.
12. Photosynthesis : Preparation of food by green plants taking CO₂ from air, water from soil in presence of sun light.
13. Oxygen in air remains constant it is not depleted as plants during day time produce lot of O₂ in air.
14. Soil contains air trapped in between its particle and organisms living there breath this oxygen. Due to this earth worms come out of soil during heavy rains as water gets into the pores in the soil.
15. Common pollutant of air are smoke, dust, coal, emitted by industries and automobiles.
16. Gases like SO₂, NO₂ also pollute air as they form corresponding acids when they come in contact with water vapours in air and fall down in the form of acid rain with rain water.
17. These pollutants reduce the visibility which lead to accidents, cause global warming.

EXERCISE-I

Question 1: What is atmosphere ?

Answer:
The earth is surrounded by a thick layer of air called the atmosphere that extends upto a height of about 320 kilometres above the surface of the earth. This blanket of air is held in place by the Earth's gravity. It is essential for protecting life from the harsh environment of outer space.
Teacher's Tip: Think of the atmosphere as a giant, invisible shield that wraps around the Earth.
Exam Tip: Mention the specific height of 320 km to get full marks for accuracy.

Question 2: Why can't we see air ?

Answer:
We can't see air because it is colourless, odourless and transparent gaseous matter. Because air is a gas, its molecules are spread very far apart, allowing light to pass right through it. This transparency is why the world around us looks clear despite being filled with air.
Teacher's Tip: Remember that "transparent" means light can pass through it completely, making it invisible to our eyes.
Exam Tip: Always include the three keywords "colourless," "odourless," and "transparent" in your answer.

Question 3: What is wind ?

Answer:
Fast moving air is called wind. When air moves from a high-pressure area to a low-pressure area, we feel it as a breeze or wind. This movement is caused by the uneven heating of the Earth's surface by the sun.
Teacher's Tip: Just remember: Air at rest is just air; air in motion is wind.
Exam Tip: Define wind simply as "moving air" to ensure you provide the core scientific concept.

Question 4: What would have happened if there would have been no atmosphere around the earth ?

Answer:
Without atmosphere life would not be possible as atmosphere protects us from harmful gases. We could not live without air present in atmosphere. In absence of the atmopshere, the earth would get so cold at night that we would not be able to survive. No CO₂ and N₂ for plants without atmosphere. Furthermore, there would be no oxygen for us to breathe and no protection from dangerous solar radiation.
Teacher's Tip: Imagine the Moon; it has no atmosphere, which is why it is silent, has no weather, and cannot support life.
Exam Tip: List at least three distinct consequences (no breathing, temperature extremes, no plant life) for a complete answer.

P.Q: When water is heated, we see bubbles rising up. Why ?

Answer:
Water has a lot of air dissolved on it. This is what allows breathing to fishes and other aquatic beings. The solubility of gases decreases when the temperature is raised, and that is why the dissolved air bubbles out from the water. As the water gets hotter, the gas molecules move faster and escape the liquid. This process continues until the water eventually reaches its boiling point.
Teacher's Tip: Think of a soda bottle; when it gets warm, it loses its "fizz" (dissolved gas) much faster.
Exam Tip: Use the term "solubility" to explain why the air escapes when heat is applied.

Question 5: Why is air called a mixture ? Give five facts in support of your answer.

Answer:
Air is a mixture because :
1. Air has no formula, a mixture has no formula whereas compound has a formula.
2. No energy changes are involved to form air from various gases.
3. When air is formed out of its constituents no change in mass and no change in volume takes place.
4. Properties of air vary from place to place and time to time. i.e. there is more CO₂ in towns as compared to villages where more oxygen prevails as compared to towns.
5. Components of air can be separated by simple physical methods.
In a mixture like air, the gases are just hanging out together without being chemically bonded. This is why we can pull them apart using physical tricks like cooling and compression.
Teacher's Tip: Remember "Mixture = No Formula." If you can't write a single chemical formula for it, it's a mixture!
Exam Tip: When listing the five facts, make sure to contrast air with a "compound" to show you understand the difference.

Question 6: What are the main components of air ? Write down the composition of three main gases present in air by volume.

Answer:
Main components of air are :
(i) Nitrogen (ii) Oxygen
Composition of air by volume :
Nitrogen : ~78%
Oxygen : 21%
Carbondioxide : 0.03 - 0.04%
Inert gases : 0.9%
Water vapours : Variable
Dust particles : Variable
Other impurities : Variable
Nitrogen makes up the vast majority of our air, providing a stable environment for life. Oxygen, though less abundant than nitrogen, is the gas we actually need to stay alive and burn fuel.
Teacher's Tip: Remember the 78-21 rule: 78% for Nitrogen and 21% for Oxygen covers 99% of the air!
Exam Tip: Always specify that these percentages are "by volume" to be scientifically accurate.

Question 7: What do you observe when
(a) Ice cold water is filled in a glass tumbler.
(b) A burning candle is covered with an inverted jar.
(c) Carbon dioxide gas is passed through lime water.
(d) A beam of light is allowed to enter in a closed dark room through a small hole.

Answer:
(a) We will observe that fine water droplets get deposited on the outer wall of the glass tumbler.
(b) The candle burn more brightly because candle gets oxygen support in burning. (Note: Eventually, the candle will extinguish once the oxygen is used up).
(c) When CO₂ gas is passed through lime water it forms calcium carbonate which is white precipitate (turbidity). This gives the milky white appearance to the solution.
(d) We will observe randomly moving dust particles in the beam of light. This confirms the presence of dust particles in the air.
These observations prove that air contains invisible components like water vapour and dust. They also show how air interacts chemically with other substances, like lime water.
Teacher's Tip: For part (c), just remember: CO₂ + Lime Water = Milky White Magic!
Exam Tip: Use scientific terms like "condensation" for part (a) and "precipitation" for part (c) to show deeper understanding.

Question 8: Write the chemical name of -
(a) Lime water
(b) The white insoluble solid formed on reaction of carbon dioxide with lime water.

Answer:
(a) Calcium hydroxide Ca(OH)₂
(b) Calcium carbonate - CaCO₃
Calcium hydroxide is a clear solution that acts as a sensor for carbon dioxide. When they meet, they create calcium carbonate, which doesn't dissolve in water and looks like white clouds.
Teacher's Tip: Notice how "hydroxide" turns into "carbonate" when you add carbon dioxide!
Exam Tip: Always write both the full chemical name and the formula to ensure full marks.

EXERCISE-II

Question 1: Name two important processes supported by oxygen present in air.

Answer:
Two processes supported by oxygen present in air are :
1. Combustion
2. Respiration
Combustion is the scientific name for burning, which provides us with heat and energy. Respiration is how our cells use oxygen to "burn" food and keep us alive.
Teacher's Tip: Think of oxygen as the "life gas" and the "fire gas."
Exam Tip: If the question asks for "processes," providing these two is the standard requirement in science papers.

Question 2: Give two uses of the following components present in air:
(a) oxygen
(b) nitrogen
(c) carbon dioxide
(d) water vapour

Answer:
(a) Oxygen: For respiration and breathing no life is possible without oxygen. Oxygen is the most vital component which is responsible for two most important processes, (i) Respiration and (ii) Combustion.
(b) Nitrogen : Present in air gets fixed up in the soil as nitrates used by plants for their growth and for the formation of protein an important nutrient for animals. It is used to make fertilizers and various nitrogenous products.
(c) Carbon dioxide : Carbon dioxide is used by plants to prepare their food by photosynthesis. Dry ice (solid carbon-dioxide) is used as a refrigerant. Carbon dioxide is used to prepare fizzy drinks like soda water.
(d) Water vapour : Water vapors present in air provide moisture for both plants and animals. It also helps in predicting climatic conditions of a particular area as its amount varies from place to place and time to time.
Each gas in the air has a specific job that keeps the Earth's systems running smoothly. For instance, without water vapour, we would have no rain or snow to water our crops.
Teacher's Tip: Associate Nitrogen with "Growth" (protein/fertilizers) and CO₂ with "Plants" and "Fizz."
Exam Tip: Provide distinct uses (e.g., one biological and one industrial) for each gas to show a broad understanding.

Question 3: Define the following:
(a) Respiration
(b) Photosynthesis
(c) Combustion.

Answer:
(a) Respiration : Respiration is a chemical process that takes place in all living beings (slow in plants). In this process, oxygen present in inhaled air reacts with the digested food material in the body. This results in release of energy, carbon-dioxide and water. Sugar + Oxygen -> CO₂ + Water + Energy.
(b) Photosynthesis : Process of producing food by green plants taking CO₂ from air, water from soil by roots and in presence of sun light and chlorophyll is called photosynthesis.
(c) Combustion : Combustion also called burning "is burning of substance in oxygen of air completely producing heat and light is called combustion."
These three processes are the "circular" ways that gases like oxygen and carbon dioxide move through our world. Plants take CO₂ and give O₂, while we and our fires take O₂ and give back CO₂.
Teacher's Tip: Think of Photosynthesis and Respiration as opposites: one builds food, the other uses it!
Exam Tip: For any definition, try to include the "ingredients" needed for the process to happen.

Question 4: What are fuels ? Give two examples of modern fuels.

Answer:
Fuels : The substances which burn in air to produce large amount of energy in the form of heat and light are called fuels.
Examples :
1. CNG (Compressed natural gas)
2. LPG (Liquefied natural gas)
Fuels are essentially stored energy that we release through the process of combustion. Modern fuels like CNG and LPG are preferred because they burn more cleanly than older fuels like coal or wood.
Teacher's Tip: A fuel is like a battery made of chemicals that waits for a spark to release its power.
Exam Tip: When giving examples of "modern" fuels, always choose CNG and LPG as they are the standard answers for this grade level.

Question 5: Give reasons :
(a) Aquatic animals and plants are able to survive in matter.
(b) A burning candle stops burning if covered with a glass tumbler.
(c) Mountaineers and divers carry oxygen cylinders with them.
(d) When water is heated, we see bubbles rising up.

Answer:
(a) Aquatic animals and plants use oxygen dissolved in water and survive.
(b) Oxygen is necessary for combustion when covered with a glass tumbler supply of oxygen stops.
(c) For artificial respiration mountaineers carry oxygen cylinders as at high altitudes, the air is thin and breathing becomes difficult. Also divers carry oxygen cylinder for artificial respiration as there is less oxygen dissolved in water (less dense) and breathing becomes difficult.
(d) These bubbles come from the air dissolved in water. The marine life uses air dissolved in water.
Oxygen is so important that when it's not naturally available (like under the sea or high in the mountains), we have to bring our own. Even fire needs a constant "breath" of air to keep going.
Teacher's Tip: Remember the "Triangle of Fire": Heat, Fuel, and Oxygen. Take one away, and the fire dies!
Exam Tip: Use the term "dissolved oxygen" specifically when talking about aquatic life to be more scientific.

Question 6: Name the processes which maintain the balance between oxygen and carbon dioxide in the air. How is it done?

Answer:
Respiration and combustion are the processes which maintain the balance between oxygen and CO₂ in the air.
1. Respiration : Respiration is a chemical process that takes place in all living beings. In this process, oxygen present in the inhaled air reacts with the digested food material in the body. This results in the release of energy, carbon dioxide and water.
2. Combustion : Burning or combustion is a process in which a substance reacts chemically with oxygen and gets oxidised, with the release of energy in the form of heat and light. It is a fast process. During the process of burning, along with energy, carbon dioxide and water vapour are also produced.
By combining these processes with photosynthesis, the Earth keeps its air perfect for everyone. While we use oxygen, plants are constantly putting it back into the atmosphere.
Teacher's Tip: Imagine a balance scale: Respiration and Combustion go on one side, and Photosynthesis goes on the other.
Exam Tip: Ensure you mention that photosynthesis is the key process that *renews* the oxygen used by the other two.

Question 7: State two similarities and two differences between respiration and burning.

Answer:
Similarities:
1. Oxygen is needed to combine with carbon and hydrogen in compound.
2. CO₂ and H₂O are formed with release of energy.
Differences :
Burning
1. It occurs at higher temperature.
2. Is fast process.
3. A natural and continous process. (Note: Often considered artificial/uncontrolled).
Respiration
1. It occurs at body temperature.
2. Is slow process.
3. An artificial - discont inuous process. (Note: Actually a natural/continuous biological process).
Both processes are essentially the same chemical reaction: mixing fuel with oxygen to get energy. The main difference is that respiration is a carefully controlled "slow burn" inside our bodies, while burning is fast and hot.
Teacher's Tip: Think of burning as a "wildfire" and respiration as a "dimmer switch" for energy.
Exam Tip: Use a table format for the differences section to make your answer clear for the examiner.

Question 8: Define rusting ? What are the two necessary conditions for rusting of iron. Give the chemical name of rust.

Answer:
Rusting : Slow conversion of iron into its hydrated oxide in the presence of moisture and air is called rusting.
Conditions for rusting:
1. Presence of moisture (water).
2. Presence of oxygen (air).
Chemical name of rust is hydrated iron oxide [Fe₂O₃ \cdot xH₂O]
Rusting is like a slow-motion fire that eats away at metal. Without both water and air present together, iron can stay shiny and clean for a very long time.
Teacher's Tip: Remember the formula for rust: "Iron + Water + Air = Ugly Brown Flakes."
Exam Tip: You must mention *both* air and water; mention only one, and you won't get full marks.

Question 9: How is air useful to :
(a) water boats
(b) agriculture
(c) windmills
(d) scooters and cars.

Answer:
(a) Air helps movement of water boats.
(b) Air speeds up drying up of agricultural products like grains, pulses fruits etc. Air helps in pollination of flowers and dispersel of seeds.
(c) Windmills work where there is sufficient movement of air.
(d) Air filled tyres of cars move smoothly on road as there is less friction.
Air isn't just for breathing; its physical pressure and movement do work for us every day. From helping plants reproduce to making our cars comfortable, air is a versatile tool.
Teacher's Tip: Think of air as a giant "invisible hand" that pushes sails and turns windmill blades.
Exam Tip: For part (b), mentioning "seed dispersal" is a great way to show your biological knowledge.

Question 10: State the full form of LPG and CNG ? How are the two different in their composition ?

Answer:
LPG (Liquefied Petroleum Gas): It is obtained from crude petroleum oil. It mainly contains gaseous compounds known as isobutane and butane. Popularly it is known as cooking gas. It is the best fuel for domestic purposes and in laboratories. It is available in cylinders. It is also supplied through pipes in big cities.
CNG (Compressed Natural Gas): It is produced along with crude oil. It mainly contains methane gas. It has become a popular fuel for vehicles like three wheeler scooters, cars and buses. It is a cheap fuel as well as pollution free. It is used as a substitute of petrol.
Difference in composition:
LPG is obtained from crude petroleum oil. It mainly contains gaseous compounds known as isobutane and butane. While CNG is produced along with crude oil. It mainly contains methane gas.
LPG is the gas we use to cook our dinner, while CNG is often what powers the buses and taxis we ride in. The main difference is the type of gas molecule inside the tank: Butane for LPG and Methane for CNG.
Teacher's Tip: LPG = L for "Liquefied" and CNG = C for "Compressed."
Exam Tip: Be sure to mention "Methane" for CNG and "Butane" for LPG to get the composition marks.

Question 11: (a) Why is nitrogen important to all living beings ?
(b) What is nitrogen fixation ?

Answer:
(a) Nitrogen constitutes 78% of air by volume. It is of vital importance to the plants, animals and human beings as it is needed to prepare vital nutrient 'protein' to every living being which is necessary for their growth.
(b) Nitrogen cannot be absorbed directly by plants. It is first fixed up in the soil as nitrites and nitrates and then absorbed by the plants in soluble forms. This phenomenon is called nitrogen fixation.
Even though we are surrounded by nitrogen, we can't just breathe it in to get what we need. We rely on special bacteria and lightning to "fix" it into a form that plants can use, which we then eat.
Teacher's Tip: Nitrogen is like the "construction worker" of the body; it helps build proteins for muscles and growth.
Exam Tip: Use the term "Nitrates" when explaining how plants eventually take in nitrogen.

EXERCISE-III

Question 1: What is air pollution ?

Answer:
Air Pollution : "Mixing of UNWANTED and HARMFUL SUBSTANCES in air is called AIR POLLUTION." These substances can be solid particles, liquids, or gases that aren't supposed to be there. They can hurt our lungs, damage buildings, and even change the Earth's climate.
Teacher's Tip: Think of pollution as "dirt" in the air that makes it unsafe to use.
Exam Tip: Quote the definition verbatim but highlight the words "unwanted" and "harmful" for emphasis.

Question 2: Mention five causes of air pollution.

Answer:
CAUSES OF AIR POLLUTION ARE:
1. Burning of FOSSIL FUELS and FIBRES.
2. Cutting of forests.
3. Erruption of volcanoes.
4. Increase in POPULATION.
5. Agricultural activities like use of chemical fertilisers, insecticides, pesticides and burning of husks etc.
Human activities like driving cars and farming are the biggest reasons the air gets dirty. However, nature can also pollute the air when a volcano explodes and fills the sky with ash.
Teacher's Tip: Remember: Factories, Cars, Fires, Cutting Trees, and Volcanoes.
Exam Tip: Provide a mix of man-made and natural causes to show you understand all sources of pollution.

Question 3: Name two air pollutants which
(a) affect our health
(b) cause acid rain
(c) cause global warming.

Answer:
(a) Two air pollutants that affect our health are :
1. DUST
2. SMOKE
(b) Two air pollutants that cause acid rain are :
1. SULPHUR DIOXIDE (SO₂) and
2. NITROGEN DIOXIDE (NO₂)
(c) Two air pollutants that cause global warming are :
1. CARBON DIOXIDE (CO₂)
2. METHANE (CH₄)
Specific pollutants cause specific problems in our environment. While smoke makes us cough, gases like methane act like a warm blanket that traps too much heat around our planet.
Teacher's Tip: Think of SO₂ as "Sour" because it makes acid, and CO₂ as "Cozy" because it keeps the heat in.
Exam Tip: Always provide the chemical formula (SO₂, CO₂, etc.) next to the name of the pollutant.

Question 4: What is meant by ozone depletion?

Answer:
Ozone is present in the upper layer of atomosphere called stratosphere. It protects the earth from harmful ultraviolet rays of the sun. Some air pollutants like chlorofluorocarbon (CFC) react with ozone present in the atmosphere. These pollutants reduce the density of the air. The ultra-violet rays from the sun reach directly on the earth which affect human health, causing skin diseases and cancer and also increase the earth's temperature.
Depletion means the protective ozone layer is getting thinner because of chemicals we used to use in fridges and spray cans. This "hole" in our shield lets dangerous rays through that can burn our skin.
Teacher's Tip: Think of Ozone as Earth's "Sunscreen" that we are accidentally rubbing off.
Exam Tip: Mention "CFCs" specifically as the chemicals responsible for this problem.

Question 5: State four steps to be taken to control air pollution.

Answer:
Four steps taken to control air pollution :
1. Tall chimneys be installed in factories and power houses so that smoke and gases rise high and get diluted.
2. GROW more tall trees to absorb CO₂ Also plants help in bringing rains.
3. Use unleaded petrol and CNG in all public transport vehicles
4. Industries should be located far away from residential areas.
By changing how we build things and what we drive, we can significantly clean up the air. Planting trees is one of the easiest and best ways to naturally "filter" the atmosphere.
Teacher's Tip: Remember the 3 Ts: Tall chimneys, Trees, and Transport (CNG).
Exam Tip: Give practical, actionable points like "planting trees" to ensure your answer is realistic and earns full credit.

Question 6: Name three greenhouse gases.

Answer:
Carbon dioxide, methane, nitrous oxide, etc. These gases are like the glass in a greenhouse; they let sunlight in but stop the heat from bouncing back out into space. While we need some of them to stay warm, having too many causes the Earth to overheat.
Teacher's Tip: Just remember the "Gases of Warmth": CO₂, Methane, and Water Vapour.
Exam Tip: Listing these three will always fulfill the requirements for a greenhouse gas question.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks

(a) The layer of air around the earth is called the atmosphere.
(b) Although we cannot see air, we can feel it.
(c) Air is a mixture of gases.
(d) Plants and animals maintain the balance of carbon dioxide and oxygen in air.
(e) Polluted air is harmful for health.
(f) The supporter of combustion in air is oxygen.
(g) Green plants need CO₂ water and light to prepare food.
(h) Oxygen is used in burning the food to get energy.
(i) Aquatic plants and animals use dissolved oxygen in water.
(j) The reddish brown powder on the surface of iron nails exposed to air and moisture is called rust.

2. Write 'true' or 'false for the following

(a) Air is a compound.
Answer: False
Correct: Air is a mixture. Air doesn't have a fixed chemical formula and its parts can be separated physically.
Teacher's Tip: Remember, if you can change the "recipe" (like more CO₂ in a city), it's a mixture!
Exam Tip: Always provide the "Correct" statement for any "False" answer to get full points.

(b) Carbon dioxide is given out during photosynthesis.
Answer: False
Correct : Oxygen is given out during photosynthesis. Carbon dioxide is actually the "food ingredient" that plants take in.
Teacher's Tip: Plants are "Oxygen Factories" – they take the bad stuff and give us the good stuff!
Exam Tip: Don't confuse the gases used in photosynthesis with those used in respiration.

(c) Respiration needs nitrogen.
Answer: False
Correct: Respiration needs oxygen. Nitrogen is an inert gas that we breathe in and out without our bodies using it.
Teacher's Tip: Respiration is like a biological fire, and fires need oxygen to burn.
Exam Tip: Remember that oxygen is the "active" component of air used by living things.

(d) The composition of air was discovered by Lavoisier.
Answer: True. Lavoisier is known as the "Father of Modern Chemistry" because he correctly identified the gases in the air. His work proved that air was not just one single substance.
Teacher's Tip: Think of Lavoisier as the "Air Detective" who solved the mystery of what air is made of.
Exam Tip: Associating scientists with their specific discoveries is a common exam requirement.

(e) The major component of air is oxygen.
Answer: False
Correct: The major component of air is nitrogen. Nitrogen makes up 78% of the air, while oxygen is only 21%.
Teacher's Tip: Even though oxygen is more "famous," nitrogen is the "background" gas that fills up most of the sky.
Exam Tip: Memorize the 78/21 ratio so you never mix up which gas is the major component.

MULTIPLE CHOICE QUESTIONS

1. Air consists of
1. only oxygen
2. only nitrogen
3. only carbon dioxide
4. all of these

Answer: 4. all of these. Air is a complex mixture containing many different types of gas molecules at once. While oxygen and nitrogen are the main ones, CO₂ and inert gases are also present.
Teacher's Tip: Whenever you see "only," be careful—nature is usually a big mix!
Exam Tip: Read all the options before picking one; often the most inclusive answer is correct.

2. Air pollution is due to the
1. cutting of green plants
2. gases like carbon monoxide, sulphur dioxide etc.
3. smoke given out by factories
4. all of the above

Answer: 4. all of the above. Both losing the things that clean the air (plants) and adding things that dirty the air (gases and smoke) lead to pollution. This is why we need to both stop the bad activities and start good ones like planting trees.
Teacher's Tip: Pollution is a multi-step problem with many different causes working together.
Exam Tip: "All of the above" is a frequent correct choice in pollution-related questions because the causes are so varied.

3. The gases which cause acid rain are
1. sulphur dioxide and oxygen
2. nitrogen and oxygen
3. carbon dioxide and water vapour
4. nitrogen dioxide and sulphur dioxide

Answer: 4. nitrogen dioxide and sulphur dioxide. These gases turn into powerful nitric and sulphuric acids when they dissolve in rain clouds. When it rains, these acids can damage forests, statues, and aquatic life in lakes.
Teacher's Tip: Just remember: SO₂ and NO₂ are the "Acid Twins."
Exam Tip: Ensure you identify the *dioxides* specifically, as plain nitrogen or oxygen do not cause acid rain.

4. Rust is
1. hydrated iron oxide
2. hydrated copper sulphate
3. anhydrous iron oxide
4. none of the above

Answer: 1. hydrated iron oxide. The word "hydrated" means that water is chemically bonded to the iron oxide. This combination is what gives rust its crumbly, orange-brown texture.
Teacher's Tip: Rust = Iron + Water (Hydrate) + Oxygen (Oxide).
Exam Tip: Look for the word "hydrated" to distinguish rust from regular iron oxide.

5. Photosynthesis is a process in which plants
1. take in oxygen and give out carbon dioxide
2. take in carbondioxide and give out oxygen
3. take in nitrogen and give out oxygen .
4. none of the above.

Answer: 2. take in carbondioxide and give out oxygen. This process happens in the leaves using sunlight as the power source. It is the most important biological process for maintaining life on Earth.
Teacher's Tip: Plants are the world's "Oxygen Recyclers"!
Exam Tip: Remember that photosynthesis is the *reverse* of what humans do when we breathe.

6. Fuels which do not leave any residue on burning are
1. coal and wood
2. coal and LPG
3. wood and CNG
4. LPG and CNG

Answer: 4. LPG and CNG. Because these are gaseous fuels, they burn completely without leaving behind solid ash or soot. This makes them much cleaner and easier to use in our homes and cars.
Teacher's Tip: "Residue" means the leftovers (like ash); gas fuels leave nothing behind!
Exam Tip: In questions about clean energy, LPG and CNG are almost always the correct examples of clean-burning fuels.

7. Respiration
1. is a slow process
2. is a natural and continuous process
3. takes place at body temperature
4. all of the above

Answer: 4. all of the above. Respiration is a biological process that must happen every second of your life to keep you alive. It is perfectly designed to happen at your normal body temperature without burning you up.
Teacher's Tip: Respiration is like a slow-burning battery that never turns off.
Exam Tip: Contrast these points with "Combustion" which is fast, hot, and occurs outside the body.

8. Which of the following is common in combustion and respiration
1. oxygen
2. release of heat and light
3. natural process
4. nitrogen

Answer: 1. oxygen. Both processes are types of "oxidation," which means they require oxygen to break down fuel for energy. Oxygen acts as the key that unlocks the energy stored in sugar or wood.
Teacher's Tip: Oxygen is the "spark" needed for both the fire in a stove and the energy in your cells.
Exam Tip: While they both release energy, only combustion produces visible "light," so "oxygen" is the better common factor.

9. Which of the following is not a green house gas ?
1. carbondioxide
2. sulphur dioxide
3. methane
4. nitrogen

Answer: 4. nitrogen. Nitrogen is a very stable gas that does not trap heat in the atmosphere. Because it makes up 78% of the air, it's a good thing it doesn't cause warming, or Earth would be far too hot.
Teacher's Tip: The major gases (N₂ and O₂) are NOT greenhouse gases.
Exam Tip: Read carefully! The question asks which is **not** a greenhouse gas.

10. The substance which accelerates the speed of a reaction without itself undergoing any change is called
1. catalyst
2. pollutant
3. fuel
4. none of the above.

Answer: 1. catalyst. A catalyst is like a "chemical cheerleader" that helps a reaction happen faster but doesn't get used up itself. It lowers the energy needed for the reaction to start.
Teacher's Tip: A catalyst is like a shortcut on a map; it gets you to the finish line faster but doesn't change the destination.
Exam Tip: In biology, we call these catalysts "enzymes," and they are essential for processes like digestion.

ADDITIONAL QUESTIONS FOR PRACTICE

Question 1: State what do you understand by the term 'air'. Explain its importance to mankind.

Answer:
Air is a mixture whose composition varies at different intervals of time and different places of the world. Man cannot survive without air and water and hence air is very essential for survival. It provides the oxygen we need for respiration and regulates the Earth's temperature to keep it livable.
Teacher's Tip: Just remember: No air = No life. It's as simple as that!
Exam Tip: When explaining "importance," mention both breathing (biological) and temperature control (environmental).

Question 2: Give a brief account of the discovery of air and the scientists involved.

Answer:
(i) John Mayow (1674) proved air has two components - active and inactive components.
(ii) Lavoisier (1789) named active component as oxygen and inactive component as nitrogen
(iii) Other scientists (1800's) discovered noble gases, CO₂, water vapours.
It took hundreds of years of research for humans to realize that the "invisible" air was actually made of different parts. Each discovery helped us understand how we breathe and how engines work.
Teacher's Tip: Mayow found the "parts," Lavoisier gave them their "names."
Exam Tip: List the scientists in chronological order to show you understand the timeline of discovery.

Question 3: What is meant by the term 'atmosphere'. State the role played by the atmosphere for the survival of mankind.

Answer:
A blanket of air around earth is called atmosphere. Without atmosphere life would not be possible as atmosphere protects us from harmful gases. We could not live without air present in atmosphere. In absence of the atmopshere, the earth would get so cold at night that we would not be able to survive. No CO₂ and N₂ for plants without atmosphere.
The atmosphere acts like a thermal blanket that traps heat and a shield that stops dangerous rocks and radiation from space. It also provides the medium for sound to travel so we can hear each other.
Teacher's Tip: Without an atmosphere, you couldn't hear a shout even if someone was right next to you!
Exam Tip: Use the word "insulator" to describe how the atmosphere keeps the Earth warm.

Question 4: Explain in brief the different layers of the atmosphere and the basic functions of the main layers.

Answer:
(i) Troposphere: 0 to 10 kilometers. Contains most of the air and oxygen in the atmosphere. This air is fit for respiration and survival of living organisms. Weather conditions are formed in the troposphere.
(ii) Stratosphere: 10 to 50 kilometers. The stratosphere contains the ozone layer which prevents the harmful effects of the sun (U.V. rays) from reaching the earth.
(iii) Mesosphere: 50 to 80 kilometers. The mesosphere is a very cold layer and most meteors burn up in this layer and hence are prevented from entering the earth's atmosphere.
(iv) Thermosphere: 80 to 320 kilometers. (Function not specified in text, but known for absorbing high-energy solar radiation).
As you go higher up, the air gets thinner and colder until you reach the edge of space. Each layer has a specific job, like the stratosphere protecting us from the sun's burns.
Teacher's Tip: Remember "TSMT" for the layers: Troposphere, Stratosphere, Mesosphere, Thermosphere.
Exam Tip: Memorize the distance ranges for the first three layers as they are often tested.

Question 5: Describe a simple experiment with the help of a diagram to show that
(a) Air occupies space
(b) Air has mass
(c) Air exerts pressure
(d) Air is highly compressible

Answer:
(a) Air occupies space - Press an inverted glass vertically into water. Tilt the tumbler to one side. Observation : On tilting the tumbler, bubbles of air are seen coming out and water enters. Conclusion : The space in the tumbler was occupied by air.
(b) Air has mass - An inflated balloon is placed on one side of a scale. Observation : It is seen that the scale moves towards the side of the inflated balloon. Conclusion : Air has mass.
(c) Air exerts pressure - Take an open can, fill it with water and heat it until steam forms. Place the cap on tightly and keep the can aside. Observation : On condensation of the steam the pressure inside the can reduces. The air outside exerts pressure on the can which crumples it. Conclusion : Air exerts pressure.
(d) Air is highly compressible - A leak proof syringe filled with air is taken. Observation : On pressing the piston, it moves inwards easily because air has maximum inter-molecular space. Conclusion : Air is highly compressible.
These experiments prove that air is "stuff" just like solids and liquids. Even though we can't see it, it takes up room, weighs something, and can push on things quite hard.
Teacher's Tip: For (d), think of an air pump; you can squish the air inside until it's very tight.
Exam Tip: Draw simple, labeled sketches for each part of this answer to get full "experimental" marks.

Question 6: Name the components of air with their approximate percentage by volume in air. Does the percentage by volume of each component remain the same, in the atmosphere of different parts of the world. Explain with reasons.

Answer:
Main components of air are :
1. Nitrogen (78% or 4/5th)
2. Oxygen (21% or 1/5th)
3. Carbon dioxide (0.03 - 0.04%)
4. Inert gases (0.9%)
5. Water vapour, Dust particles, Other impurities (Variable)
Air is a mixture whose composition varies at different intervals of time and different places of the world. For example, there is more CO₂ in industrial towns compared to rural villages because of smoke and factories. Humidity (water vapour) also changes based on the weather and location.
Teacher's Tip: Air is like a soup; the basic ingredients are the same, but different cities add their own "spices" (pollutants/moisture).
Exam Tip: Use the term "variable composition" to explain why air isn't the same everywhere.

Question 7: With the help of a labeled diagram, describe a simple experiment to show the presence of oxygen and nitrogen in air using a piece of white phosphorus.

Answer:
Activity:
1. A trough is taken and filled with water.
2. Over it is placed a bell-jar marked with five equal parts.
3. A crucible containing white phosphorus is placed on a cork floating on the water.
4. The phosphorus is ignited with a heated rod. Observation : Dense white fumes of phosphorus pentoxide (P₂O₅) are formed when the phosphorus burns in oxygen. The rise in the level of water in the bell-jar is by ~ 1/5.
Conclusion : The active component of air i.e. oxygen (1/5 of air) is used up in burning. The remaining inactive component of air i.e. nitrogen (4/5 of air) remains behind.
This experiment visually shows us that oxygen is the "active" gas that gets used up, while nitrogen is the "lazy" gas that just stays in the jar. The water rises to fill the empty space left behind by the burned oxygen.
Teacher's Tip: Just remember: 1/5th of the air is the "burning gas" (Oxygen).
Exam Tip: Mention the white fumes (P₂O₅) as it is a key observation in this specific experiment.

Question 8: You are given a test tube with two outlets and a bottle of lime water. Usint same, how would you demonstrate experimentally the presence of carbon dioxide in air.

Answer:
Procedure:
1. Take lime water in a test tube with two outlets 'A' and 'B'.
2. Air is sucked slowly through the outlet 'B'.
3. Air moves in through end 'A' and passes through the lime water. Observation: The line water in the test tube turns milky. Conclusion: Air contains carbon dioxide which turns the lime water milky.
By bubbling air through lime water, we are using a chemical test to prove something invisible is there. The "milkiness" is the physical proof that CO₂ was present in the air we sucked through.
Teacher's Tip: Be careful! Suck slowly so you don't drink the lime water by mistake!
Exam Tip: The standard result for a CO₂ test is always that the solution "turns milky."

Question 9: Give a reason why water droplets appear on the outer surface of a tumbler containing ice.

Answer:
Water vapour present in air, condenses on the cooler surface of the tumbler. Hence they condenses into tiny water droplets. This happens because cold surfaces take energy away from the invisible gas, slowing the molecules down until they turn back into a liquid.
Teacher's Tip: This is called "condensation" – it's the exact same way clouds and rain form!
Exam Tip: Use the word "condensation" to describe the gas-to-liquid change for extra marks.

Question 10: Explain the importance of nitrogen of the air for plant growth.

Answer:
Nitrogen of air is converted to soluble nitrogenous compounds in the soil in the presence of air and moisture. These are absorbed by plants and converted to plant proteins. These proteins are then passed on to animals when they eat the plants, helping them grow and repair their bodies.
Teacher's Tip: Nitrogen = Protein = Muscle. It's the building block for all living things.
Exam Tip: Explain that plants cannot use nitrogen directly from the air; it must be "fixed" in the soil first.

Question 11: Give a reason why nitrogen is filled in food packets and not oxygen.

Answer:
Nitrogen being inert or unreactive is filled into food packets to drive out the oxygen and reduce bacterial growth. The food package then stays well preserved. Oxygen would cause the food to react (oxidize) and go stale or rot very quickly.
Teacher's Tip: Nitrogen acts like a "preservation shield" that keeps your chips crunchy and fresh.
Exam Tip: Use the word "inert" to describe nitrogen; it means it doesn't like to react with other things.

Question 12: State what would happen, if the air above the earth contained mainly oxygen and no nitrogen.

Answer:
Nitrogen is utilised by plants for their growth and development. Free nitrogen of the air is converted to nitrogen compounds which are absorbed by the plants and converted to plant proteins. In the absence of nitrogen no plants will be survived on the earth. Also, because oxygen supports burning, a world of pure oxygen would be extremely dangerous as any tiny spark could cause massive, unstoppable fires.
Teacher's Tip: Nitrogen is like the "water" in a strong drink; it dilutes the oxygen so it's safe for us to handle.
Exam Tip: Mention both the biological impact (no plants) and the safety impact (fire hazard) for a perfect answer.

Question 13: State the utility of oxygen for respiration in (a) living organisms (b) plants.

Answer:
(a) Respiration in living organisms : It is a process whereby living things 1. Use oxygen from their air to oxidize food substances mainly glucose, in their body cells. 2. Release energy in the form of heat. 3. Carbon dioxide and water vapour are also produced and released in the exhaled air. Glucose + Oxygen -> CO₂ + Water + Energy.
(b) Respiration in plants : 1. Plants respire during the day and night. 2. Plants- Respire through tiny pores on the surface of the leaves called 'stomata'. 3. The stomata help in taking in oxygen of the air and giving out carbon dioxide. 4. Plants do not perform bodily activities and hence need less energy than animals. The rate of respiration in plants is thereby slower than in animals.
Respiration is like a slow-burning engine inside every cell that turns food into the fuel we need to move and grow. While animals breathe through lungs, plants "breathe" through thousands of tiny mouths on their leaves.
Teacher's Tip: Remember: Humans breathe all the time, and so do plants—even at night!
Exam Tip: Be sure to mention "stomata" when describing how plants take in air.

Question 14: Compare respiration and combustion-both involving oxygen of the air.

Answer:
Respiration is a process whereby living organisms: 1. Use oxygen from the air to oxidize food substances mainly glucose, in their body cells. 2. Release energy in the form of heat. 3. Carbon dioxide and water vapour are also produced and released in the exhaled air.
Combustion or burning, involves oxidation i.e. combination of substances like fuels with oxygen or air generally resulting in production of heat and light. Carbon dioxide is released into the air as a result of all burning.
The biggest difference is speed and control. Respiration is a slow, controlled process that happens at body temperature, while combustion is a fast, hot reaction that releases a lot of light.
Teacher's Tip: Respiration is "Cell Fire," and Combustion is "Stove Fire."
Exam Tip: Use the term "oxidation" to show what both processes have in common.

Question 15: Explain the importance of carbon dioxide for (a) photosynthesis (b) warming the earth's environment. State what would happen if excess carbon dioxide as a pollutant is released into the atmosphere.

Answer:
(a) During photosynthesis which occurs during daytime, carbon dioxide and water in the presence of sunlight and chlorophyll is changed into carbohydrates and oxygen is released out. Hence oxygen in the air is renewed by photosynthesis.
(b) The reflected I.R. rays from the earth's surface are prevented by the carbon dioxide from being radiated out of the earth's surface. This results in the earth's environment staying warm. However, a thick CO₂ layer prevents too much heat from leaving, which results in a rise in global temperature (Global Warming).
CO₂ is like a thermostat for the Earth; just the right amount keeps us warm and helps plants grow. But too much CO₂ is like turning the heater up way too high, causing ice caps to melt and weather to go crazy.
Teacher's Tip: Think of CO₂ as a "Warm Blanket." A thin blanket is good, but a heavy blanket makes you sweat!
Exam Tip: Use the term "Greenhouse Effect" to describe how CO₂ traps heat.

Question 16: State in brief how water vapour in the atmosphere determines the climatic conditions.

Answer:
Water vapour in the atmosphere in excess : (a) Minimizes the rate of evaporation. (b) Produces rain, mist etc. thereby determining climatic conditions. The rain serves as a natural source of water for plants and animals.
Water vapour is the source of all our weather—from the morning dew to giant thunderstorms. It also helps control how fast things dry out; on a humid day, there's so much vapour in the air that your sweat can't evaporate!
Teacher's Tip: Vapour = Weather. No vapour means no rain and a permanent desert.
Exam Tip: Connect water vapour to "Humidity" and "Precipitation" for a complete climate answer.

Question 17: Give a reason why two different rare (inert) gases find application in advertisement signs and fluorescent bulbs.

Answer:
1. Neon - In neon sign advertisements A brilliant red glow is seen on passage of an electric current through neon gas at low pressure. The colour can be changed by mixing mercury vapour and argon with neon.
2. Xenon - In fluorescent bulbs flash bulbs and lasers. It emits intense white light in discharge tubes.
These gases are perfect for lighting because they don't react with anything, even when they get very hot from electricity. They can glow for thousands of hours without burning out.
Teacher's Tip: Just remember: Neon for Red signs and Xenon for White flashes!
Exam Tip: Specify the *colour* of the light produced by each gas to show detailed knowledge.

Question 18: A mixture has a variable composition. Give three other reasons why air is considered a mixture and not a compound.

Answer:
Air is a mixture because :
1. Air has no formula, a mixture has no formula whereas compound has a formula.
2. No energy changes are involved to form air from various gases.
3. When air is formed out of its constituents no change in mass nor change in volume takes place. 4. Components of air can be separated by simple physical methods.
Unlike water (H₂O), air doesn't have a specific name tag made of letters and numbers. It's just a collection of different gas "friends" hanging out without being glued together.
Teacher's Tip: A mixture is like a salad; a compound is like a baked cake. You can't un-bake a cake!
Exam Tip: If you are asked for "three other reasons," don't include "variable composition" in your list, as it's already in the question.

Question 19: State a reason why there is a balance in the amount of carbon dioxide in the air, even though carbon dioxide is released into the atmopshere by various processes.

Answer:
By the way of photosynthesis in plants which which occurs during daytime, carbon dioxide and water in the presence of sunlight and chlorophyll is changed into carbohydrates and oxygen is released out. Hence, oxygen in the air is renewed by photosynthesis and therefore the amount of oxygen in air does not get depleted or removed from air. (Note: This also means CO₂ is absorbed by plants, balancing the CO₂ cycle).
Plants act as giant CO₂ sponges that clean the air for us. As long as we have enough forests, they can keep up with the CO₂ produced by our breathing and our cars.
Teacher's Tip: Imagine plants as the "Earth's Lungs" that breathe in what we breathe out.
Exam Tip: Always mention "Photosynthesis" as the primary balancing force in the atmosphere.

Question 20: Name three different appliances where air is utilized.

Answer:
1. Vehicles - Run on tyres which are inflated with air.
2. Brake mechanisms - Of trains and other machines, work on compressed air.
3. Pumps and siphons - Work on air pressure.
Because air can be squished (compressed) and it always pushes back, we can use it to stop heavy trains or lift water out of a well. It's an invisible tool that helps us move and work.
Teacher's Tip: Compressed air is like a "spring" made of gas that we can use to do work.
Exam Tip: Mention "inflated tyres" as a simple, everyday example to ensure your answer is relatable.

OBJECTIVE TYPE QUESTIONS

Q.I. Match the statements in List I with the correct answers in List II.

List I:
1. An impurity in air
2. The component of air which supports combustion
3. The inert component in air, slightly soluble in water
4. The component of air which minimizes the rate of evaporation
5. The process which involves oxidation of food substance in living organisms
6. The component of air which finds application in observation balloons
7. The process which involves oxidation of substances, liberating heat and light energy
8. The component of air which is heavier than air and fairly soluble in water
9. The process by which atmospheric carbon dioxide is absorbed by plants resulting in formation of carbohydrates
10. The component of air, which is lighter than air and less than 1% in air

List II:
A: Water vapour
B: Respiration
C: Rare gases
D: Sulphur dioxide
E: Helium
F: Carbon dioxide
G: Combustion
H: Nitrogen
I: Photosynthesis
J: Oxygen

Answer:
1 - D (Sulphur dioxide)
2 - J (Oxygen)
3 - H (Nitrogen)
4 - A (Water vapour)
5 - B (Respiration)
6 - E (Helium)
7 - G (Combustion)
8 - F (Carbon dioxide)
9 - I (Photosynthesis)
10 - C (Rare gases)

Q.2. The diagrams below represents an experiment to show the presence of a component of air.

Question 1: State why air is sucked from - outlet 'B' and not from outlet "A".

Answer:
Because if we suck from 'A', lime water will be sucked out. Outlet B is above the water level, allowing only air to be pulled through, while A is submerged in the liquid.
Teacher's Tip: Always suck from the "dry" pipe to pull air *through* the "wet" one.
Exam Tip: Use the diagram to identify which pipe is submerged to justify your answer.

Question 2: State why the air is sucked 'slowly' from the outlet "B"

Answer:
Because if air is sucked fast, lime water will also be sucked out. Sucking slowly also gives the CO₂ more time to react with the lime water so the milkiness becomes visible.
Teacher's Tip: Patience is key in chemistry—bubbling slowly gets the best results!
Exam Tip: Mention that slow sucking prevents "splashing" and ensures a complete reaction.

Question 3: State the observation seen after completion of the experiment.

Answer:
Lime water turns milky. This happens because a new, O₂ experiment.
Exam Tip: Use the word "turbid" or "milky" to describe the visual change.

Question 4: State the reason for the above observation seen.

Answer:
The reason is the formation of CO₂. (Specifically, the formation of insoluble calcium carbonate from the reaction of CO₂ with lime water).
Teacher's Tip: It's like making tiny white clouds inside a test tube!
Exam Tip: Mention "Calcium Carbonate" specifically to show you know the chemistry behind the milkiness.

Question 5: Name another component which if present in air as a pollutant - would have given a similar observation.

Answer:
Sulphur dioxide. Just like CO₂, SO₂ can react with lime water to create a cloudy precipitate, making it a "copycat" in this experiment.
Teacher's Tip: SO₂ is like the "evil twin" of CO₂ when it comes to this test.
Exam Tip: Memorize SO₂ as the alternative gas that can turn lime water milky.

Q.3. Complete the statements given below by filling in the blanks with the correct word/s from the word/s in bracket.

1. The stratosphere is a layer which extends upto about 10-50 kms. above the earth.
2. Hydrogen sulphide is an example of a pollutant present in air.
3. The component of air used in photosynthesis is carbon dioxide and the products of photosynthesis is oxygen.
4. The product formed which is common to both combustion and respiration is nitrogen dioxide. (Note: The text states nitrogen dioxide, but typically CO₂ and H₂O are the common products).
5. The component of air which is variable in air above sea level and in air in general water vapour.

Q.4. Name the following

Question 1: The product formed when phosphorus burns in oxygen.
Answer: Phosphorus pentoxide (P₂O₅). This is the thick white smoke you see in the bell-jar experiment. It is a solid made of phosphorus and oxygen atoms bonded together.
Teacher's Tip: Think of "Pent-" for five, as there are five oxygen atoms in the formula.
Exam Tip: Always provide the formula P₂O₅ along with the name.

Question 2: The layer of atmosphere which contains the ozone layer.
Answer: Stratosphere. This layer sits just above the troposphere where we live. It's home to the ozone molecules that keep us safe from getting too much sun damage.
Teacher's Tip: S for Stratosphere, S for "Sun protection."
Exam Tip: Be careful not to confuse it with the "Mesosphere" which is much higher up.

Question 3: The active component of air which supports combustion and is used up in burning.
Answer: Oxygen. Oxygen is highly reactive, meaning it loves to join up with other substances to create fire. Without it, the world would be much safer from fires, but we wouldn't be able to stay alive.
Teacher's Tip: Oxygen is the "fuel-helper" gas.
Exam Tip: Use the word "active" to describe oxygen in this context.

Question 4: The main, rare (inert) gas present in air.
Answer: Argon. Even though it's "rare," there is actually quite a lot of argon in the air—about 0.9%. It is used inside light bulbs because it doesn't react with the hot metal filaments.
Teacher's Tip: Argon is the "introverted" gas—it doesn't like to talk to or bond with anyone!
Exam Tip: Argon is the most common of the "rare" gases, so name it first.

Question 5: The component of air which is present more in industrial areas.
Answer: Carbon monoxide, sulphur dioxide, oxides of nitrogen, hydrogen sulphide, dust particles. These are the byproducts of burning coal, oil, and petrol in factories and cars. They change the air from being "fresh" to being "polluted."
Teacher's Tip: Industrial air is "Dirty Air" filled with chemical leftovers.
Exam Tip: Listing three of these (like SO₂, CO, and dust) is usually enough for a full-mark answer.

Question 6: The type of bacteria which directly absorb nitrogen from the air and converted soluble nitrates in the soil.
Answer: Symbiotic bacteria. (Specifically Rhizobium bacteria found in the roots of pulses). They have a "you help me, I help you" relationship with plants, trading nitrates for sugary food.
Teacher's Tip: Think of these bacteria as tiny "Nitrogen Fixers" living in root basements.
Exam Tip: Use the term "Nitrogen-fixing bacteria" if you can't remember "symbiotic."

Question 7: The product of respiration released in exhaled air, other than carbon dioxide.
Answer: Water vapour and energy. When you breathe on a cold window, the fog you see is the water vapour leaving your body. You also feel the warmth of the energy being released as heat.
Teacher's Tip: You're not just breathing out "bad air"; you're breathing out steam and heat!
Exam Tip: Mention "energy" specifically as it's the most important result of respiration.

Question 8: An inert gas which has a low boiling point and is used for producing very low temperatures.
Answer: Helium. Helium stays as a gas even when it's extremely cold. This makes it perfect for cooling down scientific instruments and MRI machines in hospitals.
Teacher's Tip: Helium is the gas that makes balloons float AND keeps things "super cold."
Exam Tip: Identify it as an "Inert gas" to show you know its chemical family.

Question 9: A process which removes carbon dioxide from the air.
Answer: Photosynthesis. This is the Earth's natural vacuum cleaner for CO₂. By taking in CO₂, plants prevent the atmosphere from becoming too thick and overheating.
Teacher's Tip: More trees = Faster CO₂ removal.
Exam Tip: Photosynthesis is the **only** major natural process that removes CO₂ from the air.

Question 10: The component of air which does not support combustion.
Answer: Nitrogen. Nitrogen is like a fire extinguisher that is always present in the air. It makes up 78% of the air and acts to "calm down" the active oxygen so fires don't get out of control easily.
Teacher's Tip: Nitrogen is the "Fire Preventer" gas.
Exam Tip: Contrast it with Oxygen, which **does** support combustion.

Q.5. State whether the following statements are 'true' or 'false'. If false write the correct statement.

1. The mesosphere in the atmosphere, contains most of the air, fit for respiration.
Answer: False.
Correct : The troposphere in the atmosphere, contains most of the air, fit for respiration. The mesosphere is too high up and the air there is way too thin for anyone to breathe.
Teacher's Tip: We live in the "Troposphere" – remember "T" for "Territory" where we live.
Exam Tip: The layer closest to Earth always has the most air due to gravity.

2. The percentage of carbon dioxide in the air is between 0.2 to 0.4%.
Answer: True. While it's a very small amount, it is exactly what plants need and what keeps our planet at the right temperature. Even a small increase in this number can cause big changes in our climate.
Teacher's Tip: CO₂ is a "tiny but mighty" gas!
Exam Tip: Note that the decimal points are crucial: it's not 2% or 4%, but much less.

3. Carbon dioxide, water vapour and energy in the form of heat are evolved during respiration.
Answer: True. These three things are the "waste products" of our cells turning sugar into power. This is why our breath is warm and why a crowd of people can heat up a small room.
Teacher's Tip: Respiration = Food + Oxygen -> Energy + Water + CO₂.
Exam Tip: If any one of these three is missing from the list, the statement might be considered incomplete.

4. Photosynthesis is a process by which green plants prepare their own food in the presence of sunlight and chlorophyll.
Answer: True. Chlorophyll is the "solar panel" in the leaf that catches sunlight and turns it into the energy needed to build food. This is why almost all plants are green!
Teacher's Tip: Green leaves = Active food factories.
Exam Tip: You must mention both sunlight and chlorophyll as the "power" and "machine" for photosynthesis.

5. Air is a mixture and not a compound since the components of air cannot be separated by physical methods.
Answer: False
Correct : Air is a mixture and not a compound since the components of air - can be separated by physical methods. We can use cooling and filtering to pull apart the oxygen from the nitrogen quite easily.
Teacher's Tip: Mixture = Physical Separation; Compound = Chemical Separation.
Exam Tip: The ability to separate parts "physically" is the primary proof that something is a mixture.

Q.6. Give word equations for the following conversions.

Question 1: Nitrogen of the air to nitric oxide.
Answer: Nitrogen + Oxygen (under high temp/lightning) -> Nitric oxide. The nitric oxide reacts with oxygen to give nitrogen dioxide. This reaction usually happens in nature during thunderstorms when lightning provides the massive energy needed.
Teacher's Tip: Think of lightning as a "chemical mixer" that forces nitrogen and oxygen to bond.
Exam Tip: Mention that high temperature or lightning is needed for this first step.

Question 2: Nitric oxide to nitrogen dioxide.
Answer: Nitric oxide + Oxygen -> Nitrogen dioxide. The nitrogen dioxide further reacts with oxygen and water vapour of the air to give nitric acid. This happens very quickly as nitric oxide is very eager to react with more oxygen.
Teacher's Tip: This is like a game of tag where oxygen keeps joining the group!
Exam Tip: Just add "Oxygen" to Nitric Oxide to get the Dioxide.

Question 3: Nitrogen dioxide to nitric acid.
Answer: Nitrogen dioxide + Oxygen + Water -> Nitric acid. The nitric acid formed comes down with the rain and combines with the carbonates in the soil to form soluble nitrates. This is the final step of turning "air" into "liquid fertilizer."
Teacher's Tip: This is how "Acid Rain" forms naturally, but it's also how plants get food!
Exam Tip: Remember that "Water" is the key ingredient needed to turn a gas into an acid.

Question 4: Nitric acid to soluble nitrates in the soil.
Answer: Calcium carbonate + Nitric acid -> Calcium nitrate + water + carbon dioxide. The water soluble calcium nitrates are absorbed by the plants and converted to plant proteins. This is the point where the nitrogen finally becomes a solid that roots can drink up.
Teacher's Tip: Soil minerals act like a "sponge" that catches the acid and turns it into safe food.
Exam Tip: Identify "Calcium nitrate" as the specific soluble nitrate that plants love.

Question 5: Carbon dioxide in air to carbohydrates in plants.
Answer: Carbon dioxide + Water (in sunlight/chlorophyll) -> Carbohydrates + Oxygen. Carbon dioxide and water in the presence of sunlight and chlorophyll is changed into carbohydrated and oxygen is released out. This is the magic of photosynthesis where air literally turns into solid food.
Teacher's Tip: This equation is the most important one in all of biology—memorize it!
Exam Tip: Write "sunlight" and "chlorophyll" over the arrow to show they are the conditions for the reaction.