Selina Concise Solutions for ICSE Class 9 Chemistry Chapter 10 Atmospheric Pollution

Exercise 10

Question 1. What are pollutants?
Answer: The toxic substances that have an undesirable impact on different components of the environment and are injurious to life and property are known as pollutants.
In simple words: Pollutants are harmful substances that mess up nature and can hurt people, animals, and even buildings. Think of them as "environmental poison."

📝 Teacher's Note: Differentiate between natural pollutants (like volcanic ash) and man-made ones (like plastic or factory smoke) to help students see the full scope of the problem.

🎯 Exam Tip: Use the keywords "undesirable impact" and "injurious to life" to get full marks for this definition.

Question 2. Explain the effects of specific pollutants on living beings.
Answer: Effect of Pollutant on living beings:

• Fluorides: Effects teeth and bones.
• Smoke Particles: Cause asthma and lung diseases.
• Lead: Damages the nervous and digestive systems and can cause cancer.
• Mercury compounds: They cause disease like Minamata commonly found in fishermen.
• Smog: It reduces visibility and induces respiratory troubles.
• Nitrogen Oxide: Causes death of many plants.

In simple words: Different kinds of pollution hurt different parts of the body—for example, smoke hurts your breathing, while lead can damage your brain.

📝 Teacher's Note: Minamata disease is a great case study for class discussion about mercury poisoning in water chains. It helps students understand biological magnification.

🎯 Exam Tip: Matching the pollutant to the specific organ or system it damages (e.g., Lead → Nervous System) is a frequent exam question.

Question 3. Define air pollution and state how it takes place.
Answer: Air pollution: It is defined as the presence of a contaminant in the atmosphere in a concentration large enough to injure human, plant and animal life.
Air pollution takes place due to the presence of gaseous pollutants like oxides of sulphur, hydrocarbons, smoke, oxides of carbon, oxides of nitrogen, dust, particulate pollutants like mist, spray and fume.
In simple words: Air pollution happens when there is so much "junk" in the air (like smoke or chemicals) that it starts making living things sick.

📝 Teacher's Note: Emphasize the word "concentration." Most things in the air aren't harmful in tiny amounts; they only become "pollution" when there's too much of them.

🎯 Exam Tip: Remember to list both gaseous pollutants (like oxides of nitrogen) and particulate pollutants (like dust) when describing how air pollution occurs.

Question 4. List the components of pure air and their concentrations.
Answer: Components of air is:

In simple words: This table shows that air is mostly Nitrogen and Oxygen. Other gases like Carbon Dioxide or Neon are there, but in very tiny amounts.

📝 Teacher's Note: Help students convert between percentage and ppm. 1% is the same as 10,000 ppm. This makes the large numbers in the table less intimidating.

🎯 Exam Tip: Knowing that Nitrogen (\( \sim 78\% \)) and Oxygen (\( \sim 21\% \)) are the main components is basic knowledge, but memorizing the \( \text{CO}_2 \) level (\( 0.03\% \)) is often required for higher marks.

Question 5. Name some particulate pollutants.
Answer: Particulate Pollutant are dust, smoke, mist, spray and fume.
In simple words: These are tiny solid or liquid bits floating in the air that you can sometimes see, like dust or smoke.

📝 Teacher's Note: Use a flashlight in a dark room to show students "dust" particles. This makes the concept of particulate matter visible and real.

🎯 Exam Tip: Categorize these: Smoke and dust are solid particulates, while mist and spray are liquid particulates.

Question 6. Why is cigarette smoking harmful?
Answer: Cigarette smoking is harmful not only for one who is smoking but also for sitting nearby and so one should avoid it. Tobacco smoke causes lung cancer and asthma.
In simple words: Smoking doesn't just hurt the smoker; the "second-hand" smoke can also make people nearby very sick with things like asthma or cancer.

📝 Teacher's Note: This is a good opportunity to talk about "Passive Smoking." It emphasizes the social responsibility of not polluting common air spaces.

🎯 Exam Tip: Mentioning "Lung Cancer" and "Asthma" as specific health risks will strengthen your answer.

Question 7. What is smog and what are its effects?
Answer: Smog: A smog is a pollutant which is a combination of oxides of sulphur and nitrogen, partially oxidized hydrocarbons and their derivatives produced by industries and automobiles from a dark, thick dust and soot laden fog known as smog.
Damaging Effect: Smog is noxious and irritating. It reduces visibility, induce respiratory troubles and can cause death by suffocation. Photochemical Smog damages the tissues of certain plants and even decreases the yield of citrus fruits and grapes.
In simple words: Smog is a "poisonous fog" made of smoke and chemicals. It makes it hard to see and hard for people and plants to breathe.

📝 Teacher's Note: The word "smog" is a portmanteau of "smoke" and "fog." Explaining this origin helps students remember exactly what it is made of.

🎯 Exam Tip: Be sure to mention that smog reduces "visibility" as well as causing health problems; this is a key physical effect.

Question 8. What does "ppm" mean?
Answer: ppm means parts per million. That is, share in 10,00,000.
In simple words: ppm is a way to measure really tiny amounts. It's like having one red marble in a giant box of a million blue ones.

📝 Teacher's Note: Use the analogy of a single drop of ink in a large backyard swimming pool to help students visualize how tiny 1 ppm actually is.

🎯 Exam Tip: Always write out "parts per million" in full before using the abbreviation.

Question 9. List some major air pollutants and secondary pollutants.
Answer: Major air pollutants are: Large amounts of Carbon monoxide, Sulphur dioxide, \( \text{H}_2\text{S} \), Chlorine, \( \text{HCl} \), Hydrocarbons and particulates. Particulate matter like sand, dust etc. Secondary pollutant like (PAN) peroxyaryl.
In simple words: Major pollutants come directly from things like car exhausts, while secondary ones are made when different chemicals mix together in the air.

📝 Teacher's Note: Explain that "Primary" (major) pollutants are emitted directly from a source, while "Secondary" pollutants form in the atmosphere through chemical reactions.

🎯 Exam Tip: PAN (Peroxyacetyl Nitrate) is the most common example of a secondary pollutant required for exams.

Question 10. How can we control emissions of Carbon monoxide and Oxides of Sulphur?
Answer: Control of:
(i) Carbon monoxide, \( \text{CO} \) emission:
Emission of \( \text{CO} \) can be controlled by:
• Switching over from internal combustion engine to electrically powered cars.
• Using alcohols, \( \text{CNG} \), \( \text{LNG} \) in place of gasoline.
• By using Catalytic Mufflers or Convertors.
• Using pollution control devices to burn gasoline completely.
• Using lead free petrol.

(ii) (\( \text{SO}_x \)) oxides of Sulphur emission: Oxides of sulphur (\( \text{SO}_2, \text{SO}_3 \)) emission can be reduced by-
(a) Using coal or oil that has low sulphur content.
(b) By using Scrubber, a device that absorbs gaseous pollutants.
In simple words: We can stop pollution by using cleaner fuels like natural gas, driving electric cars, and using special filters in factory chimneys called "scrubbers."

📝 Teacher's Note: Show diagrams of a catalytic converter and a scrubber. Understanding the physical hardware makes these technological solutions easier for students to remember.

🎯 Exam Tip: Knowing the difference between \( \text{CNG} \) and gasoline as a "cleaner alternative" is a standard answer for pollution control questions.

Question 11. What is acid rain and how is it formed?
Answer: Acid rain: Factories in big cities release nitrogen dioxide and sulphur dioxide as their wastes. These gases dissolve in rainwater during rains and form nitrous acid and sulphurous acid. As the rain falls, these acids come down to the ground as an acid rain.
The normal rain is slightly acidic having a \( \text{pH} \) about 5.6 as carbon dioxide gas reacts with it to form a weak carbonic acid.
\( \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \)
(Carbonic acid)
The \( \text{pH} \) of acid rain ranges between 5.6 – 3.5 and in some cases \( \text{pH} \) can go even lower than 2.

Formation of acid rain:
Acid rain refers to rain which has \( \text{pH} \) less than 5.6. It is mainly caused by atmospheric pollutants.
• Natural sources: Bacterial decomposition, forest fires, volcanic eruptions.
• Man made sources: Industries and smelting plants, automobile exhausts, power plants etc.
Oxides of nitrogen and sulphur interact with water vapour in presence of sunlight in the atmosphere to form nitric acid and sulphuric acid mist respectively.

Formation of Nitric acid and Nitrous acid:
Nitrogen and oxygen combines in the presence of thunder and lightning to form nitric acid.
\( \text{N}_2 + \text{O}_2 \rightarrow 2\text{NO} \)
(Nitrogen oxide)
\( 2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2 \)
(Nitrogen dioxide)
\( 2\text{NO}_2 + \text{H}_2\text{O} \rightarrow \text{HNO}_2 + \text{HNO}_3 \)
(Nitrous acid) (Nitric acid)

Formation of Sulphuric acid and Sulphurous acid:
Impurities in the coal: Coal used in power plants contains upto 4% sulphur. On combustion it forms pollutant sulphur dioxide.
\( \text{S} + \text{O}_2 \rightarrow \text{SO}_2 \)
(Sulphur dioxide)
\( \text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \)
(Sulphurous acid)
\( 2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3 \)
(Sulphur trioxide)
\( \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 \)
(Sulphuric acid)

Impact of Acid rain:
• Changes the acidity of soil: Increases soil acidity and removes essential minerals like calcium and potassium, making soil lose its fertility.
• Affects water bodies: Lakes and rivers become acidic, killing aquatic life.
• Material damage: Corrodes metals and weakens building materials like marble (The Taj Mahal faces this).
\( \text{CaCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{CO}_2 + \text{H}_2\text{O} \)
• Impact on living things: Damages forests and enters the food chain. Sulphur dioxide irritates the respiratory tract.
In simple words: When factory smoke mixes with rain clouds, the rain becomes acidic. This "sour rain" can kill fish, damage trees, and eat away at beautiful old marble buildings.

📝 Teacher's Note: The Taj Mahal is a powerful example of "Stone Leprosy" caused by acid rain. It helps students connect chemistry to national heritage.

🎯 Exam Tip: Memorize the equations for the formation of Sulphuric and Nitric acid. They are the core "how-it-works" parts of this topic.

Question 12. What are the effects of Sulphur dioxide as a pollutant?
Answer: Sulphur dioxide pollutes the air, effects the yield of crops and causes damage to lungs. \( \text{SO}_2 \) gets converted to \( \text{SO}_3 \) and combines with water drops of air and cause acid rain.
In simple words: Sulphur dioxide is a double-threat: it hurts your lungs directly and also creates acid rain that kills crops.

📝 Teacher's Note: Emphasize that \( \text{SO}_2 \) is a primary pollutant that can turn into a secondary one (\( \text{SO}_3 \)) before eventually becoming acid rain.

🎯 Exam Tip: Mention "lung damage" and "acid rain" as the two primary consequences of \( \text{SO}_2 \) pollution.

Question 13. How is ozone formed in the atmosphere?
Answer: In the atmosphere ozone is formed by the action of ultraviolet rays of the sun on oxygen.
\( 3\text{O}_2 (g) \rightarrow 2\text{O}_3 (g) \)
The high energy \( \text{UV} \) radiations break oxygen molecules into oxygen atoms.
\( \text{O}_2 + \text{Far UV} \rightarrow \text{O} + \text{O} \)
Oxygen atom reacts with oxygen molecule to form ozone.
\( \text{O} + \text{O}_2 \rightarrow \text{O}_3 \)
In simple words: Sunlight hits regular oxygen and splits it apart, then those loose bits join with other oxygen molecules to create Ozone.

📝 Teacher's Note: Clarify that ozone is "good" in the upper atmosphere (stratosphere) but "bad" near the ground (smog). This prevents confusion about whether ozone is a pollutant or a protector.

🎯 Exam Tip: Be ready to write the two-step chemical process: first, the splitting of \( \text{O}_2 \), then the formation of \( \text{O}_3 \).

Question 14. What is the importance of the ozone layer?
Answer: Ozone layer acts as a blanket in the atmosphere 16 km height above the Earths surface. It absorbs harmful ultraviolet rays (\( \text{UV} \) radiations) coming from the Sun and prevents them to reach the surface of the Earth. Ultraviolet rays have very harmful effects on living things. It causes skin cancer. It destroys many organic species which are necessary for life.
In simple words: The ozone layer is like "Earth's sunglasses." It blocks the most dangerous rays of the sun so they don't give us skin cancer or hurt plants.

📝 Teacher's Note: Compare the ozone layer to a filter. It doesn't block all sunlight, just the dangerous "short-wave" radiation that damages DNA.

🎯 Exam Tip: The height (16 km) and the specific health risk (skin cancer) are the two most important facts for this answer.

Question 15. Which chemicals are responsible for the depletion of the ozone layer?
Answer: Chemicals responsible for ozone destruction are free radical chlorine (\( \text{Cl} \)) and nitric oxide (\( \text{NO} \)).

Fuel of planes: Burning of fuels of planes emits large quantity of nitric oxide. Nitric oxide reacts with ozone and form nitrogen dioxide and nitrogen trioxide.
\( \text{NO} (g) + \text{O}_3 (g) \rightarrow \text{NO}_2 (g) + \text{O}_2 (g) \)
\( \text{NO}_2 (g) + \text{O}_3 (g) \rightarrow \text{NO}_3 (g) + \text{O}_2 (g) \)

Excessive use of chlorofluro carbon (CFC): It is released by refrigerators and air conditioning systems. The chlorofluro carbons are decomposed by ultraviolet rays to highly reactive chlorine free radicals.
\( \text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2 \)
\( \text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2 \)
Again this free radical destroys ozone and the process continues giving rise to depletion of ozone layer.
In simple words: Chemicals from old fridges (CFCs) and jet planes (Nitric Oxide) eat away at the ozone layer. One tiny chlorine bit can keep destroying ozone over and over again like a never-ending monster.

📝 Teacher's Note: Emphasize the "catalytic" nature of the chlorine radical. It isn't used up; it gets recycled to destroy more ozone, which is why even small amounts of CFCs are so dangerous.

🎯 Exam Tip: Always mention both CFCs (refrigerants) and Nitric Oxide (jets) as the two main culprits for ozone depletion.