NCERT Solutions for Class 11 Science Chemistry Chapter 14 Environmental Chemistry are provided here with simple step-by-step explanations. These solutions for Environmental Chemistry are extremely popular among class 11 Science students for Chemistry Environmental Chemistry Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the NCERT Book of class 11 Science Chemistry Chapter 14 are provided here for you for free. You will also love the ad-free experience on Meritnation’s NCERT Solutions. All NCERT Solutions for class 11 Science Chemistry are prepared by experts and are 100% accurate.
Page No 176:
Question 1:
Which of the following gases is not a green house gas?
(i) CO
(ii) O3
(iii) CH4
(iv) H2O vapour
Answer:
The atmosphere traps the sun’s heat near the earth’s surface and keeps it warm. This is called natural greenhouse effect because it maintains the temperature and makes the earth perfect for life. CO is not a greenhouse gas. Greenhouse gases include carbon dioxide (CO2), methane (CH4), ozone (O3), chlorofluorocarbon compounds (CFCs) and water vapour in the atmosphere. Carbon monoxide (CO) is not a greenhouse gas.
Hence the correct answer is option A.
Page No 176:
Question 2:
Photochemical smog occurs in warm, dry and sunny climate. One of the following is not amongst the components of photochemical smog, identify it.
(i) NO2
(ii) O3
(iii) SO2
(iv) Unsaturated hydrocarbon
Answer:
SO2 is not a component of photochemical smog. It is a component of London/Classical smog which occurs in cool humid climate.
Hence the correct answer is option C.
Page No 176:
Question 3:
Which of the following statements is not true about classical smog?
(i) Its main components are produced by the action of sunlight on emissions of automobiles and factories.
(ii) Produced in cold and humid climate.
(iii) It contains compounds of reducing nature.
(iv) It contains smoke, fog and sulphur dioxide.
Answer:
Classical smog contains a mixture of SO2, particulates and high humidity in the atmosphere in cold conditions. A fog of H2SO4 droplets formed condenses on the particulates to form the smog. The classical smog is reducing in nature and the gases released by automobiles and factories are not responsible for classical smog.
Hence, the correct answer is option A.
Page No 176:
Question 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 __________.
(i) rich in dissolved oxygen.
(ii) poor in dissolved oxygen.
(iii) highly polluted.
(iv) not suitable for aquatic life.
Answer:
Biochemical oxygen demand (BOD) represents the amount of oxygen consumed by bacteria and other microorganisms in decomposing the organic matter in a certain volume of sample of water. Water is considered to be pure if it has BOD less than 5 ppm, whereas highly polluted water has BOD value of 17 ppm or more. Therefore, BOD value less than 5 ppm indicates that a water sample is rich in dissolved oxygen.
Hence, the correct answer is option A.
Page No 177:
Question 5:
​Which of the following statements is wrong?
(i) Ozone is not responsible for green house effect.
(ii) Ozone can oxidise sulphur dioxide present in the atmosphere to sulphur trioxide.
(iii) Ozone hole is thinning of ozone layer present in stratosphere.
(iv) Ozone is produced in upper stratosphere by the action of UV rays on oxygen.
Answer:
Ozone is one of the greenhouse gases which is responsible for greenhouse effect and is produced in the upper stratosphere by the action of UV rays on oxygen. Ozone can oxidise sulphur dioxide present in atmosphere to sulphur trioxide. Also, ozone hole is the thinning of ozone layer present in stratosphere.
Hence, the correct answer is option A.
Page No 177:
Question 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
(i) Large number of mosquitoes.
(ii) Increase in the amount of dissolved oxygen.
(iii) Decrease in the amount of dissolved oxygen in water.
(iv) Clogging of gills by mud.
Answer:
The decomposition of organic matter consumes oxygen and therefore, dissolved oxygen in water decreases and fish in such polluted water die.
​Hence, the correct answer is option C.
Page No 177:
Question 7:
Which of the following statements about photochemical smog is wrong?
(i) It has high concentration of oxidising agents.
(ii) It has low concentration of oxidising agent.
(iii) It can be controlled by controlling the release of NO2, hydrocarbons, ozone etc.
(iv) Plantation of some plants like pinus helps in controlling photochemical smog.
Answer:
Photochemical smog is oxidizing in nature as it contains large concentration of oxidizing agents NO2 and O3.
​Hence, the correct answer is option B.
Page No 177:
Question 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 _________.
(i) Stratosphere
(ii) Troposphere
(iii) Mesosphere
(iv) Hydrosphere
Answer:
Troposphere is extended upto 10 km from sea level. It is the domain of all living organisms including plants and animals.
Hence, the correct answer is option B.
Page No 177:
Question 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.
Answer:
Dinitrogen and oxygen do not react with each other as the triple bond in N2 is very stable and has a very high bond dissociation enthalpy. Therefore, large amount of energy and high temperature is required for the reaction to take place.
Hence, the correct answer is option A.
Page No 177:
Question 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?
(i) CO
(ii) Hydrocarbon
(iii) Peroxyacetyl nitrate
(iv) NO
Answer:
Unburnt hydrocarbons, ozone and oxides of nitrogen which are primary pollutants in the atmosphere react together in the presence of sunlight to form certain compounds e.g. PAN (Peroxyacetyl nitrates) which are harmful and known as secondary pollutants. Carbon monoxide is also a primary air pollutant.
Hence the correct option is option C.
Page No 178:
Question 11:
Which of the following statements is correct?
(i) Ozone hole is a hole formed in stratosphere from which ozone oozes out.
(ii) Ozone hole is a hole formed in the troposphere from which ozone oozes out.
(iii) Ozone hole is thinning of ozone layer of stratosphere at some places.
(iv) Ozone hole means vanishing of ozone layer around the earth completely.
Answer:
Ozone hole is thinning of ozone layer of stratosphere at some places. The two types of compounds that have been most responsible for the depletion of ozone layer are NO and chlorofluorocarbons (CFC's).
Hence, the correct answer is option C.
Page No 178:
Question 12:
Which of the following practices will not come under green chemistry?
(i) If possible, making use of soap made of vegetable oils instead of using synthetic detergents.
(ii) Using H2O2 for bleaching purpose instead of using chlorine based bleaching agents.
(iii) Using bicycle for travelling small distances instead of using petrol / diesel based vehicles.
(iv) Using plastic cans for neatly storing substances.
Answer:
Plastic is non-biodegradable polymer and therefore, it's use does not come under green chemistry. Green chemistry includes processes which lead to minimum pollution and less harm to the environment.
Hence, the correct answer is option D.
Page No 178:
Question 13:
In the given question two or more options may be correct.
Which of the following conditions shows the polluted environment.
(i) pH of rain water is 5.6.
(ii) amount of carbon dioxide in the atmosphere is 0.03%.
(iii) biochemical oxygen demand 10 ppm.
(iv) eutrophication.
Answer:
Normally rain water has a pH of 5.6 and when the pH of the rain water drops below 5.6, it is called acid rain. The normal amount of carbon dioxide in the atmosphere is 0.03%. The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of sample of water is called Biochemical Oxygen Demand. Clean water has a BOD value less than 5 ppm whereas polluted water has a BOD value greater than 5 ppm.
The process in which nutrient rich water bodies support a dense plant population, which kills animal life by depriving it of oxygen and results in the loss of biodiversity is known as eutrophication.
Hence, the correct answers are options C and D.
Page No 178:
Question 14:
In the given question two or more options may be correct.
Phosphate containing fertilisers cause water pollution. Addition of such compounds in water bodies causes __________.
(i) enhanced growth of algae.
(ii) decrease in amount of dissolved oxygen in water.
(iii) deposition of calcium phosphate.
(iv) increase in fish population.
Answer:
Fertilizers contain phosphates as additives. The addition of phosphates in water enhances algae growth which covers the surface of water and reduces the oxygen concentration in water. This leads to anaerobic conditions, commonly with accumulation of abnoxious decay and animal death. Thus, bloom-infested water inhibits the growth of other living organisms in the water body.
Hence, the correct answers are options A and B.
Page No 178:
Question 15:
​In the given question two or more options may be correct.
The acids present in acid rain are _________.
(i) Peroxyacetylnitrate
(ii) H2CO3
(iii) HNO3
(iv) H2SO4
Answer:
The oxides of carbon, nitrogen and sulphur react with rainwater to give carbonic acid (H2CO3), nitric acid (HNO3) and sulphuric acid (H2SO4) which result in acid rain.
Hence correct answers are options B, C and D.
Page No 179:
Question 16:
In the given question two or more options may be correct.
​The consequences of global warming may be _________.
(i) increase in average temperature of the earth
(ii) melting of Himalayan Glaciers.
(iii) increased biochemical oxygen demand.
(iv) eutrophication.
Answer:
Global warming leads to an average increase in temperature of the earth. This increase in temperature disturbs the thermal balance on earth and leads to the melting of glaciers.
Hence, the correct answers are options A and B.
Page No 179:
Question 17:
Green house effect leads to global warming. Which substances are responsible for green house effect?
Answer:
CO2 is mainly responsible for green house effect. Other green house gases are methane, nitrous oxide, water vapours, ozone and cholorofluorocarbons.
Page No 179:
Question 18:
Acid rain is known to contain some acids. Name these acids and where from they come in rain?
Answer:
Acid rain contains sulphuric acid (H2SO4), nitric acid (HNO3) and carbonic acid (H2CO3). Burning of fossil fuels (which contain sulphur and nitrogenous matter) such as coal and oil in power stations and furnaces or petrol and diesel in motor engines produce sulphur dioxide and nitrogen oxides. H2SO4 is formed due to oxidation of SO2 present in the air to SO3 and then dissolution in water. H2CO3 is formed due to dissolution of carbon dioxide in water. HNO3 is formed by oxidation of NO to NO2 and NO3 and subsequent dissolution in water.
Page No 179:
Question 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?
Answer:
Considerable amount of ozone is present in the Stratosphere which protects us from harmful ultraviolet radiations coming from the sun. These radiations can cause skin cancer (melanoma) in humans, it can lead to genetic mutation and destroy plants, animals and aquatic life. Therefore, ozone presence is important.
Page No 179:
Question 20:
Dissolved oxygen in water is very important for aquatic life. What processes are responsible for the reduction of dissolved oxygen in water?
Answer:
The processes responsible for the reduction of dissolved oxygen in water are:
(i) Use of phosphate and nitrate fertilizers.
(ii) Detergents, the discharge of human sewage and organic waste from food, paper and pulp industries.
(iii) The large population of bacteria which decomposes organic matter present in water also utilize oxygen dissolved in water.
(iv) During night, photosynthesis stops but the aquatic plants continue to respire resulting in reduction of dissolved oxygen.
Page No 179:
Question 21:
On the basis of chemical reactions involved, explain how do chlorofluorocarbons cause thinning of ozone layer in stratosphere.
Answer:
Chlorofluorocarbons are stable compounds. They move to stratosphere by random diffusion and 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. The chemical reactions involved are as given below:
Since chlorine free radicals use ozone and convert it into oxygen, it leads to thinning of ozone layer in stratosphere.
Page No 179:
Question 22:
What could be the harmful effects of improper management of industrial and domestic solid waste in a city?
Answer:
The improper management of domestic waste in a city 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.
Page No 179:
Question 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?
Answer:
The reasons for this condition of the lake are:
(i) Disposing of waste material and washing clothes in lake water makes the water rich in nutrients like phosphate. It enhances algae growth.
(ii) Algal growth covers the water surface which reduces oxygen concentration in water which leads to anaerobic conditions with accumulation of dead and decaying water animals which leaves the water with a stinking smell and making it unusable.
Page No 179:
Question 24:
What are biodegradable and non-biodegradable pollutants?
Answer:
Biodegradable pollutants are those which are decomposed by microbes For example: sewage, cow dung, food wastes etc.
Non-biodegradable pollutants are those which cannot be decomposed easily and take a very long time for its decomposition. For example: rubber, plastic, chemicals etc.
Page No 179:
Question 25:
What are the sources of dissolved oxygen in water?
Answer:
The sources of dissolved oxygen in water are atmospheric oxygen, photosynthesis or mechanical aeration.
Page No 179:
Question 26:
What is the importance of measuring BOD of a water body?
Answer:
Biochemical oxygen demand (BOD) represents the amount of oxygen consumed by bacteria and other microorganisms while they decompose organic matter under aerobic (oxygen is present) conditions at a specified temperature. BOD is used to measure the level of pollution caused by organic biodegradable material in terms of how much oxygen will be required to break down the organic material biologically. Clean water would have BOD values less than 5 ppm whereas BOD values greater than 5 ppm indicate that the water is polluted.
Page No 179:
Question 27:
Why does water covered with excessive algal growth become polluted?
Answer:
The water covered with excessive algal growth shows that water contains a lot of phosphate due to use of fertilizers, etc from the surroundings. The addition of phosphates in water enhances algae growth which covers the water surface and reduces the oxygen concentration in water. This leads to anaerobic conditions, commonly with accumulation of abnoxious decay and animal death. Thus, bloom-infested water inhibits the growth of other living organisms in the water body. Therefore, excessive algal growth leads to water pollution.
Page No 180:
Question 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.
Answer:
The combustion of fossil fuels like coal, petroleum etc. produces oxides of nitrogen and sulphur that leads to the symptoms observed by the villagers.
When fossil fuel is burnt at high temperature, dinitrogen and dioxygen combine to yield NO, i.e., nitric oxide.
is produced by burning of sulphur containing fossil fuels or by roasting of sulphide ores such a iron pyrites, copper pyrites etc.
The presence of particulate matter in polluted air catalyses the oxidation of sulphur dioxide to sulphur trioxide.
2SO2 + O2 → 2SO3
Page No 180:
Question 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.
Answer:
The presence of particulate matter in polluted air catalyses the oxidation of SO2 to SO3. The conversion of SO2 to SO3​ can also be catalysed photochemically in the presence of ozone or hydrogen peroxide. The chemical reactions involved in the conversion of SO2 to SO3 are:
Page No 180:
Question 30:
From where does ozone come in the photochemical smog?
Answer:
Nitric oxide and hydrocarbons from unburnt fuels are produced on burning of fossil fuels. Nitric oxide i.e., NO is converted to nitrogen dioxide in the presence of sunlight. NO2 absorbs energy from sunlight and breaks into NO and free oxygen atoms which are very reactive and combine with O2 to form O3, which then reacts with NO to form NO2 and O2.
​
Page No 180:
Question 31:
How is ozone produced in stratosphere?
Answer:
Two steps are involved in the formation of ozone in the stratosphere:
(i) The sufficient energy from the ultraviolet rays of the sun splits the dioxygen into two oxygen atoms.
(ii) The oxygen atoms then react with dioxygen to form ozone.
Page No 180:
Question 32:
Ozone is a gas heavier than air. Why does ozone layer not settle down near the earth?
Answer:
Ozone is thermodynamically unstable and decomposes into molecular oxygen. Therefore, a dynamic equilibrium exists between the production and decomposition of ozone and it does not settle down near the earth.
Page No 180:
Question 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?
Answer:
In summers, nitrogen dioxide and methane react with chlorine monoxide and chlorine atoms forming chlorine sinks, preventing ozone depletion to a large extent. Whereas, in winter, special type of clouds called polar stratospheric clouds are formed over Antarctica.
After the return of sunlight in Antarctica in spring, sunlight breaks up the clouds and HOCl, Cl2 are photolysed by sunlight.
The chlorine free radical formed initiates the chain reaction for ozone depletion.
Page No 180:
Question 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?
Answer:
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.
Page No 180:
Question 35:
In the given question more than one option of Column I and Column II may match.
Match the terms given in Column I with the compounds given in Column II.
| Column I | Column II | ||
| (i) | Acid rain | (a) | CHCl2 – CHF2 |
| (ii) | Photochemical smog | (b) | CO |
| (iii) | Combination with haemoglobin | (c) | CO2 |
| (iv) | Depletion of ozone layer | (d) | SO2 |
| (e) | Unsaturated hydrocarbons |
Answer:
(i)-(c), (d), (ii)-(e), (iii)-(b), (iv)-(a)
Acid rain is due to presence of CO2, NO2, SO2 in the air.These gases combine with rain water to give carbonic acid, nirtic acid, sulphuric acid
Photochemical smog is formed by NO2, unburnt fuel(hydrocarbons) present in the air are exposed to sunlight
CO combines with haemoglobin to give carboxyhaemoglobin which is poisonous as it does not allow oxygen to bind with haemoglobin.
Cholorofluorocarbons are responsible for ozone layer depletion.
Page No 180:
Question 36:
In the given question more than one option of Column I and Column II may match.
Match the pollutant(s) in Column I with the effect(s) in Column II.
| Column I | Column II | ||
| (i) | Oxides of sulphur | (a) | Global warming |
| (ii) | Nitrogen dioxide | (b) | Damage to kidney |
| (iii) | Carbon dioxide | (c) | ‘Blue baby’ syndrome |
| (iv) | Nitrate in drinking water | (d) | Respiratory diseases |
| (v) | Lead | (e) | Red haze in traffic and congested areas |
Answer:
The correct match between column I and column II is given as:
(i)-(d), (ii)-(e), (iii)-(a), (iv)-(c), (v)-(b)
Page No 181:
Question 37:
In the given question more than one option of Column I and Column II may match.
Match the activity given in Column I with the type of pollution created by it given in Column II.
| Column I (Activity) | Column II (Effect) | ||
| (i) | Releasing gases to the atmosphere after burning waste material containing sulphur. | (a) | Water pollution |
| (ii) | Using carbamates as pesticides | (b) | Photochemical smog, damage to plant life, corrosion to building material, induce breathing problems, water pollution |
| (iii) | Using synthetic detergents for washing clothes | (c) | Damaging ozone layer |
| (iv) | Releasing gases produced by automobiles and factories in the atmosphere. | (d) | May cause nerve diseases in human. |
| (v) | Using chlorofluorocarbon compounds for cleaning computer parts | (e) | Classical smog, acid rain, water pollution, induce breathing problems, damage to buildings, corrosion of metals. |
Answer:
The correct match between column I and column II is given as:
(i)-(e), (ii)-(d), (iii)-(a), (iv)-(b), (v)-(c)
Page No 181:
Question 38:
In the given question more than one option of Column I and Column II may match.
Match the pollutants given in Column I with their effects given in Column II.
| Column I | Column II | ||
| (i) | Phosphate fertilisers in water | (a) | BOD level of water increases |
| (ii) | Methane in air | (b) | Acid rain |
| (iii) | Synthetic detergents in water | (c) | Global warming |
| (iv) | Nitrogen oxides in air | (d) | Eutrophication |
Answer:
The correct match between column I and column II is given as:
(i)-(a), (d)
(ii)-(c)
(iii)-(a)
(iv)-(b)
Page No 181:
Question 39:
In the given question 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.
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.
(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.
Answer:
In a greenhouse, solar radiations pass through the transparent glass and heat up the soil and the plants that emit infrared radiations. Since glass is opaque to infrared radiations (thermal region), it partly reflects and partly absorbs these radiations. This mechanism keeps the energy of the sun trapped in the greenhouse. Similarly, atmosphere traps the sun’s heat near the earth’s surface and keeps it warm. This is called natural greenhouse effect because it maintains the temperature of the earth..
Hence, the correct answer is option C.
Page No 182:
Question 40:
In the given question 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.
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.
Answer:
Normal rain water has a pH of about 5.6 due to dissolution of carbon dioxide into it. When pH falls below 5.6 it becomes acid rain.
Acid rain contains sulphuric acid and nitric acid which are formed from reaction of the oxides of sulpur and nitrogen present in the air with rainwater and has pH of 4-5.
Hence the correct answer is option B.
Page No 182:
Question 41:
In the given question 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.
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.
Answer:
Photochemical smog is oxidising in nature because it contains high concentration of NO2 and O3 which are oxidising agents.
Hence, the correct answer is option A.
Page No 182:
Question 42:
In the given question 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.
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.
Answer:
Carbon dioxide is one of the important greenhouse gases because it traps the solar heat radiation falling on the surface of earth and add to the heating of the atmosphere which is responsible for global warming. It is produced during the respiratory function of plants and animals.
Hence, the correct answer is option B.
Page No 182:
Question 43:
In the given question 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.
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.
Answer:
Ozone is destroyed by CFCs released in the atmosphere where they mix with the normal atmospheric gases and eventually reach the stratosphere. In stratosphere, they get broken down by powerful UV radiations, releasing chlorine free radicals which are continuously regenerated and cause the breakdown of ozone. The thinning of ozone layer allows the excessive UV radiations to reach the surface of earth which cause skin cancer (melanoma) in humans.
Hence, the correct answer is option D.
Page No 182:
Question 44:
In the given question 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.
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.
Answer:
Chlorine containing insecticides and pesticides are non biodegradable and the repeated use of the same or similar pesticides give rise to pests that are resistant to them. These are high persistent toxins which pollute soil and water and are, therefore, transferred from lower trophic level to higher trophic level through food chain.
Hence, the correct answer is option A.
Page No 183:
Question 45:
In the given question 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.
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.
Answer:
The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water, is called Biochemical Oxygen Demand (BOD). When the biological oxygen demand i.e., BOD is greater than 5 ppm, water is highly polluted. High BOD means high activity of bacteria in water.
Hence, the correct answer is option C.
Page No 183:
Question 46:
How can you apply green chemistry for the following :
(i) to control photochemical smog.
(ii) to avoid use of halogenated solvents in dry cleaning and that of chlorine in bleaching.
(iii) to reduce use of synthetic detergents.
(iv) to reduce the consumption of petrol and diesel.
Answer:
(i) The primary precursors of photochemical smog, such as nitric oxides and unburnt unsaturated hydrocarbons produced by automobiles and factories can be controlled by fitting catalytic converters in automobiles for complete combustion of fuels and by installing smoke precipitators and chimneys in factories to curb the release of poisonous gases.
(ii) Replacement of halogenated solvent by liquid CO2 will result in less harm to ground water. Also, hydrogen peroxide (H2O2) can be used for the purpose of bleaching clothes in the process of laundary, which gives better results and makes use of lesser amount of water.
(iii) Synthetic detergents are non-biodegradable, therefore, their use in daily life should be reduced. Good quality soaps should be prepared from vegetable oil that is biodegradable.
(iv) In place of petrol and diesel, use of CNG has been preferred as they cause less or no pollution.
Page No 183:
Question 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?
Answer:
Carbondioxide present in nornal concentration helps to maintain the temperature on the earth required for the existence of living organisms. Due to burning of fuels and other human activities the amount of carbondioxide is increasing in the atmosphere. Green plants require carbondioxide for photosynthesis and helps to maintain the balance, but still amount of carbondioxide is much more than consumed in photosynthesis by plants .
Page No 183:
Question 48:
Explain how does green house effect cause global warming.
Answer:
The earth’s atmosphere allows most of the visible light from the sun to pass through and reach the earth’s surface. As the earth’s surface is heated by sunlight, it radiates a part of this energy back towards the space as longer IR wavelengths. Some of the heat is trapped by CH2, CO2 . CFCs and water vapor are present in the atmosphere. They absorb IR radiations and block a large portion of the earth’s emitted radiations. The radiations thus absorbed are partly remitted to the earth’s surface. Therefore the earth’s surface gets heated up by a phenomenon called the greenhouse effect. Thus greenhouse effect is defined as the heating up of the earth's surface due to the trapping of infrared radiations reflected by the earth’s surface by the CO2 layer in the atmosphere. The heating up of the earth through the greenhouse effect is called global warming.
Page No 183:
Question 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?
Answer:
Pesticides are 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 trophic levels to higher trophic levels through the food chain. Over time, the concentration of toxins in higher animals reaches a level that causes serious metabolic and physiological disorders.
Page No 183:
Question 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.
Answer:
Tetrachloroethene (Cl2​C=CCl2​) was earlier used as a 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 (H2​O2​) is used for the purpose of bleaching clothes in the process of laundry, which gives better results and makes use of the lesser amount of water.
View NCERT Solutions for all chapters of Class 11