NCERT Exemplar: Environmental Chemistry (Old NCERT) | Chemistry Class 11 - NEET PDF Download

MULTIPLE CHOICE QUESTIONS - I

Q.1. Which of the following gases is not a green house gas?
(1) CO
(2) O₃
(3) CH₄
(4) H₂O vapour
Ans.(1) CO
Solution.
CO does not absorb sunlight near the earth’s surface and then it is not radiated back to the earth. Greenhouse gases have this property.

Q.2. Photochemical smog occurs in warm, dry and sunny climate. One of the following is not amongst the components of photochemical smog, identify it.
(1) NO₂
(2) O₃
(3) SO₂
(4) Unsaturated hydrocarbon
Ans. (3) SO₂
Solution.
Photochemical fog is formed in the presence of sunlight in summer when NO and hydrocarbons are present in large amounts in atmosphere. SO₂ is not responsible for photochemical fog.

Q.3. Which of the following statements is not true about classical smog?
(1) Its main components are produced by the action of sunlight on emissions of automobiles and factories.
(2) Produced in cold and humid climate.
(3) It contains compounds of reducing nature.
(4) It contains smoke, fog and sulphur dioxide
Ans. (1)
Solution.
Classical smog is initiated by a mixture of S0₂, particulates and high humidity in the atmosphere in cold conditions. A fog of H₂S0₄ droplets formed condenses on the particulates to form the smog. It is of reducing nature. The gases released by automobiles and factories are not responsible for classical fog.

Q.4. Biochemical Oxygen Demand, (BOD)  is a measure of organic material present in water. BOD value less than 5 ppm indicates a water sample to be __________.
(1) Rich in dissolved oxygen.
(2) Poor in dissolved oxygen.
(3) Highly polluted.
(4) Not suitable for aquatic life
Ans. (1)
Solution.
The total amount of oxygen consumed by microorganism in decomposing organic matter present in certain volume of water is called Biochemical Oxygen Demand (BOD) of water. If BOD value is less than 5 ppm, then sample of water is considered to be pure. It is rich in dissolved oxygen.

Q.5. Which of the following statements is wrong?
(1) Ozone is not responsible for green house effect.
(2) Ozone can oxidise sulphur dioxide present in the atmosphere to sulphur trioxide.
(3) Ozone hole is thinning of ozone layer present in stratosphere.
(4) Ozone is produced in upper stratosphere by the action of UV rays on oxygen.
Ans. (1)
Solution.
O₃ is responsible for greenhouse effect, its contribution is about 8 to 10%. About 75% of the solar energy reaching the earth is absorbed by the earth’s surface, which increases its temperature. The rest of the heat radiates back to the atmosphere. Some of the heat is trapped by gases such as carbon dioxide, methane, ozone, chlorofluorocarbon compounds (CFCs) and water vapour in the atmosphere. Thus, they add to the heating of the atmosphere. This causes global warming.

Q.6. Sewage containing organic waste should not be disposed in water bodies because it causes major water pollution. Fishes in such a polluted water die because of
(1) Large number of mosquitoes.
(2) Increase in the amount of dissolved oxygen.
(3) Decrease in the amount of dissolved oxygen in water.
(4) Clogging of gills by mud.
Ans. (3)
Solution.
Organic waste is oxidized by microorganisms in presence of dissolved oxygen. Hence, oxygen decreases in water as a result it is harmful for aquatic life.

Q.7. Which of the following statements about photochemical smog is wrong?
(1) It has high concentration of oxidising agents.
(2) It has low concentration of oxidising agent.
(3) It can be controlled by controlling the release of NO₂, hydrocarbons, ozone etc.
(4) Plantation of some plants like pinus helps in controlling photochemical smog.
Ans. (2)
Solution.
The common components of photochemical smog are ozone, nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate (PAN). Photochemical smog causes serious health problems. Both ozone and PAN act as powerful eye irritants. Photochemical smog has high concentration of oxidants.

Q.8. The gaseous envelope around the earth is known as atmosphere. The lowest layer of this is extended upto 10 km from sea level, this layer is _________.
(1) Stratosphere
(2) Troposphere
(3) Mesosphere
(4) Hydrosphere
Ans. (2)
Solution.
The lowest region of atmosphere in which the human beings along with other organisms live is called troposphere. It extends up to the height of ~10 km from sea level.

Q.9. Dinitrogen and dioxygen are main constituents of air but these do not react with each other to form oxides of nitrogen because _________.
(i) The reaction is endothermic and requires very high temperature.
(ii) The reaction can be initiated only in presence of a catalyst.
(iii) Oxides of nitrogen are unstable.
(iv) N₂ and O₂ are unreactive.
Ans. (a)
Solution.
Dinitrogen and dioxygen are the main constituents of air. These gases do not react with each other at a normal temperature. The dissociation energy of N₂ is very high due to the presence of triple bond and it is very stable.

Q.10. The pollutants which come directly in the air from sources are called primary pollutants. Primary pollutants are sometimes converted into secondary pollutants. Which of the following belongs to secondary air pollutants?
(1) CO
(2) Hydrocarbon
(3) Peroxyacetyl nitrate
(4) NO
Ans. (3)
Solution.
Hydrocarbons present in atmosphere combine with oxygen atom produced by the photolysis of N0₂ to form highly reactive intermediate called free  radical. Free radical initiates a series of reactions. Peroxyacetyl nitrate is formed, which Gan be said as secondary pollutant.


Q.11. Which of the following statements is correct?
(1) Ozone hole is a hole formed in stratosphere from which ozone oozes out.
(2) Ozone hole is a hole formed in the troposphere from which ozone oozes out.
(3) Ozone hole is thinning of ozone layer of stratosphere at some places.
(4) Ozone hole means vanishing of ozone layer around the earth completely.
Ans. (3)
Solution.
It was found that a unique set of conditions was responsible for the ozone hole. In summer season, nitrogen dioxide and methane react with chlorine monoxide and chlorine atoms forming chlorine sinks, preventing much ozone depletion, whereas in winter, special type of clouds called polar stratospheric clouds are formed over Antarctica. Following reactions occur in stratosphere.


ClONO₂(g) + H₂O(g) → HOCl(g) + HNO₃(g)
ClONO₂(g) + HCl(g) → Cl₂(g) + HNO₃(g)

Q.12. Which of the following practices will not come  under green chemistry?
(1) If possible, making use of soap made of vegetable oils instead of using synthetic detergents.
(2) Using H₂O₂ for bleaching purpose instead of using chlorine based bleaching agents.
(3) Using bicycle for travelling small distances instead of using petrol/ diesel based vehicles.
(4) Using plastic cans for neatly storing substances.
Ans. (4)
Solution.
Plastic is non-biodegradable polymer. Hence, it does not come under green chemistry. Green chemistry includes processes which lead to minimum pollution and less harm to the environment.

MULTIPLE CHOICE QUESTIONS -II

Q.13. Which of the following conditions shows the polluted environment.
(1) pH of rain water is 5.6.
(2) Amount of carbon-dioxide in the atmosphere is 0.03%.
(3) Biochemical oxygen demand 10 ppm.
(4) Eutrophication.
Ans. (3) and (4)
Solution.
Clean water would have BOD less than 5 ppm and polluted water has BOD value higher than 5 ppm. 10 BOD means highly polluted water. This process in which nutrient enriched water bodies support a dense plant population, which kills animal life by depriving it of oxygen and results in subsequent loss of biodiversity is known as Eutrophication.

Q.14. Phosphate containing fertilisers cause water pollution. Addition of such compounds in water bodies causes __________.
(1) Enhanced growth of algae.
(2) Decrease in amount of dissolved oxygen in water.
(3) Deposition of calcium phosphate.
(4) Increase in fish population.
Ans. (1) and (2)
Solution.
Fertilizers contain phosphates as additives. The addition of phosphates in water enhances algae growth. Such profuse growth of algae, covers the water surface and reduces the oxygen concentration in water.

Q.15. The acids present in acid rain are _________.
(1) Peroxyacetylnitrate
(2) H₂CO₃
(3) HNO₃
(4) H₂SO₄
Ans. (2, 3, 4)  
Solution.
CO₂ is slightly soluble in water forming carbonic acid.
CO₂ + H₂O → H₂CO₃
The oxides of nitrogen undergo oxidation reaction followed by reaction with water vapours to form nitric acid.
2NO + O₂→ 2NO₂
2NO₂ + H₂0 → HNO₃ + HNO₂
The oxidation of SO₂ into SO₃ occurs in the presence of dust particles or metal ions. The SO₃ then reacts with water vapours to form H₂SO₄.

SO₃ + H₂O → H₂SO₄

Q.16. The consequences of global warming may be _________.
(1) Increase in average temperature of the earth
(2) Melting of Himalayan Glaciers.
(3) Increased biochemical oxygen demand.
(4) Eutrophication.
Ans. (1) and (2)
Solution.
About 75% of the solar energy reaching the earth is absorbed by the earth’s surface, which increases its temperature. The rest of the heat radiates back to the atmosphere. The average global temperature will increase to a level which may lead to melting of polar ice caps and flooding of low lying areas all over the earth. Increase in the global temperature increases the incidence of infectious diseases like dengue, malaria, yellow fever, sleeping sickness etc.

SHORT ANSWER TYPE QUESTIONS

Q.17. Green house effect leads to global warming. Which substances are responsible for green house effect?
Ans. Besides carbon dioxide, other greenhouse gases are methane, water vapour, nitrous oxide, oxides of sulphur, CFCs and ozone. Methane is produced naturally when vegetation is burnt, digested or rotted in the absence of oxygen. Large amounts of methane are released in paddy fields, coal mines, from rotting garbage dumps and by fossil fuels. Chlorofluorocarbons (CFCs) are man-made industrial chemicals used in air conditioning etc. CFCs are also damaging the ozone layer. Nitrous oxide occurs naturally in the environment but is increasing day by day by the human activities.

Q.18. Acid rain is known to contain some acids. Name these acids and where from they come in rain?
Ans. Acid rain contains acids such as HNO₃, H₂SO₄ and H₂CO₃ (along with small amount of HC1). HNO₃ is formed by the oxidation of NO present in air to NO₂ and NO₃ and subsequent dissolution in water. H₂SO₄ is formed by the oxidation of SO₂ present in air to SO₃ and subsequent dissolution in water. H₂CO₃ is formed by the dissolution of CO₂ of the air in water.

Q.19. Ozone is a toxic gas and is a strong oxidising agent even then its presence in the stratosphere is very important. Explain what would happen if ozone from this region is completely removed?
Ans. Ozone layer acts as a protective umbrella and does not allow the harmful UV radiations to reach the earth’s surface. If ozone is completely removed from the stratosphere, the UV radiations will fall directly on the humans causing skin cancer and on the plants affecting plant proteins.

Q.20. Dissolved oxygen in water is very important for aquatic life. What processes are responsible for the reduction of dissolved oxygen in water?
Ans. The discharge of human sewage and organic waste from pulp and paper industry and presence of leaves, grass, trash etc. in water due to run off result ‘ in phytoplankton growth. The microorganisms which decompose this organic matter need oxygen. Hence, the amount of oxygen in water of lakes etc. decreases.

Q.21. On the basis of chemical reactions involved, explain how do chlorofluorocarbons cause thinning of ozone layer in stratosphere.
Ans. Chlorofluorocarbons are stable compounds. They move to stratosphere by random diffusion. These undergo decomposition in the presence of sunlight to release Cl atoms. These Cl atoms cause catalytic chemical reactions and cause significant depletion of ozone layer as shown below:


Since the free radicals use ozone and convert it to oxygen, they cause thinning of ozone layer in stratosphere.

Q.22. What could be the harmful effects of improper management of industrial and domestic solid waste in a city?
Ans. If domestic waste in a city is not properly managed, it may find its way into. sewers or may be eaten up by the cattle. The non-biodegradable waste like polythene bags, metal scrap etc. choke the sewers. The polythene bags, if swallowed by the cattle, can result into their death. Similarly, if industrial waste is not properly managed, it will cause pollution of the air, soil and water.

Q.23. During an educational trip, a student of botany saw a beautiful lake in a village. She collected many plants from that area. She noticed that villagers were washing clothes around the lake and at some places waste material from houses was destroying its beauty. After few years, she visited the same lake again. She was surprised to find that the lake was covered with algae, stinking smell was coming out and its water had become unusable.Can you explain the reason for this condition of the lake?
Ans. The domestic waste and organic compounds like detergents can provide nutrients to enhance the growth of algae and aquatic plants. These are decomposed by bacteria and give unpleasant odor. This process is called eutrophication.

Q.24. What are biodegradable and non-biodegradable pollutants?
Ans. Those pollutants which are decomposed by bacteria like, waste of vegetable and fruits, sewage, cow dung etc. are called biodegradable pollutants. Non-biodegradable pollutants are those which cannot be decomposed by bacteria e.g., mercury, polythene, aluminum, DDT etc.

Q.25. What are the sources of dissolved oxygen in water?
Ans. Oxygen reaches water either through atmosphere or from the process of photosynthesis carried out by many aquatic green plants during day light. However, during night, photosynthesis stops but the plants continue to respire, resulting in reduction of dissolved oxygen. The dissolved oxygen is also used by microorganisms to oxidize organic matter.
In brief, sources of dissolved oxygen in water are:
(i) Photosynthesis by aquatic plants.
(ii) Due to the direct contact of water surface with air.
(iii) Mechanical aeriation.

Q.26. What is the importance of measuring BOD of a water body?
Ans. Presence of excessive algal growth shows that water contains a lot of phosphate due to inflow of fertilizers, etc from the surroundings. Hence, such a sample of water is polluted.

Q.27. Why does water covered with excessive algal growth become polluted?
Ans. Presence of excessive algal growth shows that water contains a lot of phosphate due to inflow of fertilizers, etc from the surroundings. Hence, such a sample of water is polluted.

Q.28. A factory was started near a village. Suddenly villagers started feeling the presence of irritating vapours in the village and cases of headache, chest pain, cough, dryness of throat and breathing problems increased. Villagers blamed the emissions from the chimney of the factory for such problems. Explain what could have happened. Give chemical reactions for the support of your explanation.
Ans. The symptoms observed in the villagers show that oxides of nitrogen and sulphur must be coming out of the chimney. This is due to combustion of fossil fuels like coal, oil, natural gas, gasoline, etc. to produce high temperatures at which oxidation of atmospheric nitrogen takes place forming NO and NO₂:

SO₂ is produced due to combustion of sulphur containing coal and fuel oil or roasting of sulphide ores like iron pyrites (FeS₂), copper pyrites (CuFeS₂), etc.
Cu₂S + O₂→2Cu + SO₂

Q.29. Oxidation of sulphur dioxide into sulphur trioxide in the absence of a catalyst is a slow process but this oxidation occurs easily in the atmosphere. Explain how does this happen. Give chemical reactions for the conversion of SO₂ into SO₃.
Ans. The presence of particulate matter in polluted air catalyses the oxidation of SO₂ to SO₃. The reaction is also promoted by ozone and hydrogen peroxide.




Q.30. From where does ozone come in the photochemical smog?
Ans. When fossil fuels are burnt, nitric oxide and hydrocarbons from unburnt fuels are produced. In sunlight, nitric oxide is converted to nitrogen dioxide. NO₂  absorbs energy from sunlight and breaks up into NO and free oxygen atoms which are very reactive and combine with O₂ to form O₃, which reacts with NO to form NO₂ and O₂.

NO + O₃ → NO₂ + O₂

Q.31. How is ozone produced in stratosphere?
Ans. The formation of ozone in the stratosphere takes place in two steps. In the first step, ultraviolet radiation coming from the sun have sufficient energy to split dioxygen into two oxygen atoms. In the second step, the oxygen atoms react with more of dioxygen to form ozone.

O + O₂ → O₃

Q.32. Ozone is a gas heavier than air. Why does ozone layer not settle down near the earth?
Ans. In stratosphere, the formation of 0₃ gas goes on continuously, but 0₃ is also decomposed by UV radiation between 240-360 nm.

The O-atom reacts with second 0₃ molecule
O₃ + O → 2O₂
Net reaction 2O₃ —> 3O₂
Thus, the reaction forms a delicate balance in which the rate of O₃ decomposition matches the rate of O₃ formation, i.e., a dynamic equilibrium exists and maintains a constant concentration of O₃.

Q.33. Some time ago formation of polar stratospheric clouds was reported over Antarctica. Why were these formed? What happens when such clouds break up by warmth of sunlight?
Ans. In summer season, nitrogen dioxide and methane react with chlorine monoxide and chlorine atoms forming chlorine sinks, preventing much ozone depletion, whereas in winter, special type of clouds called polar stratospheric clouds are formed over Antarctica. These polar stratospheric clouds provide surface on which chlorine nitrate gets hydrolysed to form hypochlorous acid. It also reacts with hydrogen chloride to give molecular chlorine.




When sunlight returns to the Antarctica in spring, the sun’s warmth breaks up the clouds and HOC1, Cl₂ are photolysed by sunlight.


The chlorine radicals thus formed initiate the chain reaction for ozone depletion.

Q.34. A person was using water supplied by Municipality. Due to shortage of water he started using underground water. He felt laxative effect. What could be the cause?
Ans. The laxative effect is observed only when the concentration of sulphates in water is greater than 500 ppm. Sulphate is harmless at moderate concentration but concentration above 500 ppm produces laxative effects and hypertension.

MATCHING TYPE

In the following questions more than one option of Column I and Column II may match.
Q.35. Match the terms given in Column I with the compounds given in Column II.

Ans. (a →3, 4); (b → 4, 5); (c → 2); (d → 1)
Solution.
(a) Acid rain is caused due to oxides of carbon, sulphur and nitrogen.
(b) Photochemical smog is formed by unburnt fuel (unsaturated hydrocarbons). *
(c) Carbon monoxide with haemoglobin is poisonous.
(d) Chlorofluorocarbons (CHC1₂ – CHF₂) cause ozone depletion.

Q.36. Match the pollutant(s) in Column I with the effect(s) in Column II.

Ans. (a → 4); (b → 5); (c →1); (d →3); (e → 2)
Solution.
(a) Low concentration of sulphur dioxide causes respiratory disease, e.g., asthma, bronchitis etc.
(b) The irritant red haze in traffic and congested places is due to oxides of nitrogen.
(c) The increased amount of CO₂ in air is mainly responsible for global warming.
(d) Excess’nitrate in drinking water cause methemoglobinemia (blue baby syndrome)
(e) Lead can damage kidney, liver, reproductive system etc.

Q.37. Match the activity given in Column I with the type of pollution created by it given in Column II.

Ans. (1 → v); (2 → iv); (3→ i); (4 → ii); (5→ iii)
Solution.
(1) Low concentration of sulphur dioxide causes respiratory disease, e.g., asthma, bronchitis etc.
(2) The irritant red haze in traffic and congested places is due to oxides of nitrogen.
(3) The increased amount of CO₂ in air is mainly responsible for global warming.
(4) Excess’nitrate in drinking water cause methemoglobinemia (blue baby syndrome)
(5) Lead can damage kidney, liver, reproductive system etc.

Q.38. Match the pollutants given in Column I  with their effects given in Column II.

Ans. (i) (a,d):(ii) (c); (iii) (a); (iv) (b)
Solution.
Phosphates in water enhances algae growth. Such profuse growth of algae, covers the water surface and reduces the oxygen concentration in water. BOD level of water increases. This process in which nutrients enriched water bodies support a dense plant population, which kills animal life by depriving it of oxygen and results in subsequent loss of biodiversity is known as Eutrophication.

ASSERTION AND REASON TYPE

In the following questions a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the choices given below each question.

Q.39. Assertion (A): Green house effect was observed in houses used to grow plants and these are made of green glass.
Reason (R) : Green house name has been given because glass houses are made of green glass.
(1) Both A and R are correct and R is the correct explanation of A.
(2) Both A and R are correct but R is not the correct explanation of A.
(3) Both A and R are not correct.
(4) A is not correct but R is correct
Ans. (3)
Solution.
A and R both are not correct. In cold places, sunlight required to grow plants which is less. Hence, plants are kept in a glass house so that sunlight enters in the green house to heat up the soil and plants. The warm soil and plants emit infrared radiations which are partially absorbed and partially reflected by the glass.

Q.40. Assertion (A) : The pH of acid rain is less than 5.6.
Reason (R) : Carbon dioxide present in the atmosphere dissolves in rain water and forms carbonic acid.
(i) Both A and R are correct and R is the correct explanation of A.
(ii) Both A and R are correct but R is not the correct explanation of A.
(iii) Both A and R are not correct.
(iv) A is not correct but R is correct.
Ans. (ii)
Solution.
Both A and R are correct but R is not the correct explanation of A.

Q.41. Assertion (A) : Photochemical smog is oxidising in nature.
Reason (R) : Photochemical smog contains NO₂ and O₃, which are formed during the sequence of reactions.
(i) Both A and R are correct and R is the correct explanation of A.
(ii) Both A and R are correct but R is not the correct explanation of A.
(iii) Both A and R are not correct.
(iv) A is not correct but R is correct.
Ans. (i)
Solution.
Both A and R are correct and R is the correct explanation of A. Photochemical smog has high concentration of oxidizing agents and is, therefore, called as oxidizing smog. Two of the pollutants that are emitted are hydrocarbons (unburnt fuels) and nitric oxide (NO). When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sunlight in which NO is converted into nitrogen dioxide (NO₂). This NO₂ in turn absorbs energy from sunlight and breaks up into nitric oxide and free oxygen atom.
NO₂(g) → NO(g) + O(g)
O(g) + O₂(g) → O₃(g)

Q.42. Assertion (A) : Carbon dioxide is one of the important greenhouse gases.
Reason (R) : It is largely produced by respiratory function of animals and plants.
(i) Both A and R are correct and R is the correct explanation of A.
(ii) Both A and R are correct but R is not the correct explanation of A.
(iii) Both A and R are not correct.
(iv) A is not correct but R is correct.
Ans. (ii) Both A and R are correct but R is not the correct explanation of A.
Solution.
The increased amount of CO₂ in the air is mainly responsible for global warming. A large amount of carbon dioxide gets released into the atmosphere. Excess of CO₂ in the air is removed by green plants and this maintains an appropriate level of CO₂ in the atmosphere.

Q.43. Assertion (A) : Ozone is destroyed by solar radiation in upper stratosphere.
Reason (R) : Thinning of the ozone layer allows excessive UV radiations to reach the surface of earth.
(i) Both A and R are correct and R is the correct explanation of A.
(ii) Both A and R are correct but R is not the correct explanation of A.
(iii) Both A and R are not correct.
(iv) A is not correct but R is correct.
Ans. (iv) A is not correct but R is correct.
Solution.
A is not correct but R is correct. Ozone in the stratosphere is a product of UV radiations acting on dioxygen (O₂) molecules. The main reason of ozone layer depletion is believed to be the release of chlorofluorocarbon compounds (CFCs), also known as freons. With the depletion of ozone layer, more UV radiations filters into troposphere. UV radiations lead to ageing of skin, cataract, sunburn, skin cancer, killing of many phytoplankton’s, damage to fish productivity etc.

Q.44. Assertion (A) : Excessive use of chlorinated synthetic pesticides causes soil and water pollution.
Reason (R) : Such pesticides are non-biodegradable.
(i) Both A and R are correct and R is the correct explanation of A.
(ii) Both A and R are correct but R is not the correct explanation of A.
(iii) Both A and R are not correct.
(iv) A is not correct but R is correct.
Ans. (i) Both A and R are correct and R is the correct explanation of A.
Solution.
Both A and R are correct and R is the correct explanation of A. Pesticides are basically synthetic toxic chemicals with ecological repercussions. The repeated use of the same or similar pesticides give rise to pests that are resistant to that group of pesticides thus making the pesticides ineffective. Therefore, as insect resistance of DDT increased, other organic toxins such as Aldrin and Dieldrin were introduced in the market by pesticide industry. Most of the organic toxins are water insoluble and non-biodegradable.

Q.45. Assertion (A) : If BOD level of water in a reservoir is less than 5 ppm it is highly polluted.
Reason (R) : High biological oxygen demand means low activity of bacteria in water.
(i) Both A and R are correct and R is the correct explanation of A.
(ii) Both A and R are correct but R is not the correct explanation of A.
(iii) Both A and R are not correct.
(iv) A is not correct but R is correct.
Ans. (iii) Both A and R are not correct.
Solution.
The amount of oxygen required by bacteria to breakdown the organic matter present in a certain volume of a sample of water, is called Biochemical Oxygen Demand (BOD). If BOD of water is less than 5 ppm, it is clean. If BOD increases, water pollution increases.

LONG ANSWER TYPE QUESTIONS

Q.46. How can you apply green chemistry for the following :
(i)  To control photochemical smog.
(ii) To avoid use of halogenated solvents in drycleaning and that of chlorine in bleaching.
(iii) To reduce use of synthetic detergents.
(iv) To reduce the consumption of petrol and diesel.
Ans. (i) (a) Many techniques are used to control or reduce the formation of photochemical smog. If we control the primary precursors of photochemical smog, such as NO₂ and hydrocarbons, the secondary precursors such as ozone and PAN of the photochemical smog will automatically be reduced.
(b) Usually, catalytic converters are used in the automobiles, which prevent the release of nitrogen oxide and hydrocarbons to the atmosphere.
(c) Certain plants e.g., Pinus, Juniparus, Quercus, Pyrus and Vitis can metabolise nitrogen
oxide and therefore, their plantation could help in this.
(ii) Organic solvents such as benzene, toluene, carbon tetrachloride etc., are highly toxic. One should be careful while using them. For bleaching the cloth H₂O₂ is used which is not harmful. Earlier Cl₂ was used for bleaching the cloth.
(iii) Synthetic detergents are non-biodegradable therefore their use in daily life is to be
reduced. Good quality soaps to be prepared from vegetable oil. Soaps are biodegradable.
(iv) In place of petrol and diesel oil the use of CNG and LNG has been preferred because
they are pollution free fuels. The other sources such as H₂, C₂H₅OH etc. can also be used in place of petrol and diesel.

Q.47. Green plants use carbon dioxide for photosynthesis and return oxygen to the atmosphere, even then carbon dioxide is considered to be responsible for green house effect. Explain why?
Ans. Carbon dioxide gas is confined to troposphere only. Normally it forms about 0.03 percent by volume of the atmosphere. With the increased use of fossil fuels, a large amount of carbon dioxide gets released into the atmosphere. Excess of CO₂ in the air is removed by green plants and this maintains an appropriate level of CO₂ in the atmosphere. Green plants require CO₂ for photosynthesis and they, in turn, emit oxygen, thus maintaining the delicate balance. As you know, deforestation and burning of fossil fuel increases the CO₂ level and disturb the balance in the atmosphere. The increased amount of CO₂ in the air is mainly responsible for global warming.

Q.48. Explain how does green house effect cause global warming.
Ans. Green chemistry is a way of thinking and is about utilizing the existing knowledge and principles of chemistry and other sciences to reduce the adverse impact on environment. Green chemistry is a production process that would bring about minimum pollution or deterioration to the environment.The byproducts generated during a process, if not used gainfully, add to the environmental pollution. Such processes are not only environmental unfriendly but also cost-ineffective. The waste generation and its disposal both are economically unsound. Utilization of existing knowledge base for reducing the chemical hazards along with the developmental activities is the foundation of green chemistry.

Q.49. A farmer was using pesticides on his farm. He used the produce of his farm as food for rearing fishes. He was told that fishes were not fit for human consumption because large amount of pesticides had accumulated in the tissues of fishes. Explain how did this happen?
Ans. Pesticides are basically synthetic toxic chemicals with ecological repercussions. Most of the organic toxins are water insoluble and non- biodegradable. These high persistent toxins are, therefore, transferred from lower tropic level to higher tropic level through food chain. Over the time, the concentration of toxins in higher animals reach a level which causes serious metabolic and physiological disorders.

Q.50. For dry cleaning, in the place of tetrachloroethane, liquefied carbon dioxide with suitable detergent is an alternative solvent. What type of harm to the environment will be prevented by stopping use of tetrachloroethane? Will use of liquefied carbon dioxide with detergent be completely safe from the point of view of pollution? Explain.
Ans. Tetrachlroroethene (Cl₂C=CCl₂) was earlier used as solvent for dry cleaning. The compound contaminates the groundwater and is also a suspected carcinogen. The process using this compound is now being replaced by a process, where liquefied carbon dioxide, with a suitable detergent is used. Replacement of halogenated solvent by liquid CO₂ will result in less harm to groundwater. These days, hydrogen peroxide (H₂O₂) is used for the purpose of bleaching clothes in the process of laundry, which gives better results and makes use of lesser amount of water.