NCERT Exemplar Class 11 Chemistry Solutions Chapter 14 Environmental Chemistry

Question 1: Which of the following gases is not a greenhouse gas?
(i) CO
$(ii) O_{3}$
$(iii) CH_{4}$
(iv)$H_{2}O$ vapour
Answer:

The answer is option (i) CO
Greenhouse gases have a property of not absorbing sunlight near the Earth’s surface and not radiating back to the Earth. CO shows the same traits; hence, it is a greenhouse gas.

Question 2: Photochemical smog occurs in warm, dry and sunny climates. One of the following is not amongst the components of photochemical smog, identify it.
$(i) NO_{2}$
$(ii) O_{3}$
$(iii) SO_{2}$
(iv) Unsaturated hydrocarbon
Answer:

The answer is the option $(iii) SO_{2}$
When NO and hydrocarbons are present in large amounts in the atmosphere in the presence of sunlight in summer, photochemical smog is formed. $SO_{2}$ is not responsible for the formation of photochemical smog.

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 a cold and humid climate.
(iii) It contains compounds of reducing nature.
(iv) It contains smoke, fog and sulphur dioxide.
Answer:

The answer is option (I). Its main components are produced by the action of sunlight on emissions of automobiles and factories.
Explanation: Classical smog occurs in cold, humid climates due to the gases released by automobiles and factories. A mixture of smoke, fog and $SO_{2}$ forms a classic smog.

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:

The answer is option (I), Rich in dissolved oxygen
BOD refers to the amount of oxygen consumed by microorganisms in water. If the value of BOD is less than 5 ppm, then the water is considered to be pure or rich in dissolved oxygen.

Question 5 Which of the following statements is wrong?
(i) Ozone is not responsible for green greenhouse effect.
(ii) Ozone can oxidise sulphur dioxide present in the atmosphere to sulphur trioxide.
(iii) The ozone hole is a thinning of the ozone layer present in the stratosphere.
(iv) Ozone is produced in the upper stratosphere by the action of UV rays on oxygen.
Answer:

The answer is option (I). Ozone is not responsible for the greenhouse effect.
The contribution of $O_3$ to the greenhouse effect is about 8 to 10%. About 75% of solar energy is absorbed by the surface of the earth, and the rest is radiated back to the atmosphere. This heat traps gases like $CO_{2}, CH_{4}, O_{3}, CFCs \;and\; H_{2}O$ present in the atmosphere and adds to the heat of the atmosphere, causing global warming.

Question 6 Sewage containing organic waste should not be disposed in water bodies because it causes major water pollution. Fishes in such 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 answer is option (iii) decrease in the amount of dissolved oxygen in the water.
Microorganisms oxidise organic waste in the presence of dissolved oxygen present in water; thus, decreasing the amount of dissolved oxygen present in water.

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 $NO_{2}$, hydrocarbons, ozone etc.
(iv) Plantation of some plants like Pinus, helps in controlling photochemical smog.
Answer:

The answer is option (ii) It has a low concentration of oxidising agents.
Photochemical smog has a high concentration of oxidants since it comprises $O_{3}$, nitric oxide, formaldehyde, peroxyacetyl nitrate & acrolein. Photochemical smog causes serious health issues.

Question 8 The gaseous envelope around the Earth is known as the atmosphere. The lowest layer of this is extended up to 10 km from sea level, this layer is _________.
(i) Stratosphere
(ii) Troposphere
(iii) Mesosphere
(iv) Hydrosphere
Answer:

The answer is option (ii) Troposphere
It extends up to approx. 10 km of height from sea level, and it is the lowest region of the atmosphere, where humans live.

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 a very high temperature.
(ii) The reaction can be initiated only in the presence of a catalyst.
(iii) Oxides of nitrogen are unstable.
(iv) $N_{2}$ and $O_{2}$ are unreactive.
Answer:

The answer is option (i) the reaction is endothermic, and it requires a very high temperature.
The dissociation energy of $N_{2}$ is very high due to the presence of a triple bond, and it is very stable; therefore, these gases do not react with each other at room temperature.

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:

The answer is option (iii), Peroxyacetyl nitrate
Ozone is produced when oxygen atoms combine with $O_{2}$ in the air
$O(g) + O_{2}(g) \rightarrow O_{3}(g)$ ………. (ii)
Now, this O3 reacts with NO to regenerate NO₂, viz., a brown gas and contributes to haze at high levels.
$NO(g) + O_{3}(g) \rightarrow NO_{2}(g) + O_{2}(g)$ ………….. (iii)
Ozone is toxic. Chemicals such as formaldehyde, Acrolein and PAN are produced due to the reaction of $NO_{2}$ and $O_{3}$ with unburnt hydrocarbons in the polluted air.
(Note: $NO_{2}$ and $O_{3}$ are strong oxidising agents).
$3CH_{4} + 2O_{3} \rightarrow 3CH_{2}=O + 3H_{2}O$

Question 11 Which of the following statements is correct?
(i) The ozone hole is a hole formed in the stratosphere from which ozone oozes out.
(ii) The ozone hole is a hole formed in the troposphere from which ozone oozes out.
(iii) The ozone hole is a thinning of the ozone layer of the stratosphere in some places.
(iv) Ozone hole means the vanishing of the ozone layer around the Earth.
Answer:

The answer is option (iii) Ozone hole is a thinning of the ozone layer of the stratosphere at some places.
There are some unique situations according to the season which is responsible for the zone hole.
In Summer:$NO_{2}$ and $CH_{4}$ react with Chlorine monoxide and chlorine atoms, forming chlorine sinks. It prevents much of the ozone depletion.
In Winter, Polar stratospheric clouds are formed over Antarctica. The following reaction occurs there:
$NO+O_{3}\rightarrow NO_{2}+O_{2}$
$CF_{2}Cl_{2}\overset{h\nu }{\rightarrow}\dot{C}F_{2}Cl+\dot{C}l$
$\dot{C}l+O_{3}\rightarrow Cl\dot{O}+O_{2}$
$Cl\dot{O}+O\rightarrow \dot{Cl}+O_{2}$
These reactions occur in the stratosphere.

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 $H_{2}O_{2}$ for bleaching purposes instead of using chlorine-based bleaching agents.
(iii) Using a bicycle for travelling small distances instead of using petrol/diesel-based vehicles.
(iv) Using plastic cans for neatly storing substances.
Answer:

The answer is option (iv) using plastic cans for neatly storing substances.
Plastic is non-biodegradable and causes pollution. Hence, using plastic cans won’t come under green chemistry.

Question 14 Phosphate-containing fertilisers cause water pollution. Addition of such compounds in water bodies causes __________.
(i) enhanced growth of algae.
(ii) decrease in the amount of dissolved oxygen in water.
(iii) deposition of calcium phosphate.
(iv) increase in fish population.
Answer:

The answer is the option (i) enhanced growth of algae & (ii) decrease in the amount of dissolved oxygen in the water
Phosphate enhances algae growth when added in water, and this algae growth covers the water surface and decreases the amount of dissolved oxygen in the water.

Question 15 The acids present in acid rain are _________.
(i) Peroxyacetylnitrate
(ii) $H_{2}CO_{3}$
(iii) $HNO_{3}$
(iv) $H_{2}SO_{4}$
Answer:

The answer is the option (ii) $H_{2}CO_{3}$ (iii) $HNO_{3}$ and (iv) $H_{2}SO_{4}$
The pH of rainwater is 5.6 because of the presence of H⁺ ions; they are formed by the reaction of $CO_{2}$ present in the atmosphere with rainwater
$H_{2}O(l) + CO_{2}(g) \rightarrow H_{2}CO_{3}(aq)$
$H_{2}CO_{3}(aq) \rightarrow H^{+}(aq) +HCO_{3}(aq)$
Acid rain is the one where the pH of rainwater drops below 5.6. Its major contributors are $SO_{2}$ and $NO_{2}$ after reaction and oxidation with rainwater,
$2SO_{2}(g) + O_{2}(g) + 2H_{2}O(l) \rightarrow 2H_{2}SO_4(aq)$
$4NO_{2}(g) + O_{2}(g) + 2H_{2}O(l) \rightarrow 4HNO_{2}^{-}(aq)$

Question 16 The consequences of global warming may be _________.
(i) an increase in the average temperature of the Earth
(ii) melting of Himalayan Glaciers.
(iii) increased biochemical oxygen demand.
(iv) eutrophication.
Answer:

The answer is the option (i) increase in the average temperature of the Earth and (ii) melting of the Himalayan Glaciers
We know that the Earth absorbed 75% of the solar energy and radiated the rest of it back to the atmosphere. This increases the global temperature and may result in the melting of glaciers and polar ice caps, due to which the low-lying areas would be flooded. There will be an increase in infectious diseases like dengue, sleeping sickness, yellow fever, etc.

Question 18 Acid rain is known to contain some acids. Name these acids and where from they come in rain?
Answer:

The pH of rainwater is 5.6 because of the presence of H⁺ ions; they are formed by the reaction of CO₂ present in the atmosphere with rainwater
$H_{2}O(l) + CO_{2}(g) \rightarrow H_{2}CO_{3}(aq)$
$H_{2}CO_{3}(aq) \rightarrow H^{+}(aq) +HCO_{3}(aq)$
Acid rain is the one where the pH of rainwater drops below 5.6. Its major contributors are $SO_{2}$ and $NO_{2}$ after reaction and oxidation with rainwater; polluted air contains particulate matter that catalyses the oxidation,
$2SO_{2}(g) + O_{2}(g) + 2H_{2}O(l) \rightarrow 2H_{2}SO_{4}(aq)$
$4NO_{2}(g) + O_{2}(g) + 2H_{2}O(l) \rightarrow 4HNO_{3}^{-}(aq)$

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:

The stratosphere consists of Ozone gas, which protects us from harmful UV radiation ($\lambda =$ 225 nm) coming from the sun. UV radiation can be harmful to humans and cause melanoma, cataract formation in ethe yes, genetic mutation, and destruction of crops. It can also cause harm to aquatic plants and animals. The main reason for ozone depletion is the release of chlorofluorocarbons (CFCs) in the atmosphere.
Hence, we can say that although O₃ is dangerous for us if it is present in the troposphere but in the stratosphere, it protects us from harmful radiations.

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 factors responsible for the reduction of dissolved oxygen in water are -
(i) It is said that the bacteria responsible for degrading biodegradable detergent feed on it and can also use up all the dissolved oxygen and grow rapidly.
(ii) The addition of fertilisers containing additives such as phosphates can enhance the growth of algae which may cover up the surface of the water and reduce the concentration of oxygen in the water.

Question 21 On the basis of chemical reactions involved, explain how do chlorofluorocarbons cause thinning of the ozone layer in the stratosphere.

Answer:

Chlorofluorocarbons break down in the stratosphere by the powerful UV radiation and release free radicals of chlorine,
$CF_{2}Cl_{2}(g)\overset{UV}{\rightarrow} \dot{Cl}(g) + \dot{C} F_{2}Cl(g)$
This chlorine radical reacts with stratospheric ozone and forms chlorine monoxide radicals and molecular oxygen,
$\dot{Cl}(g) + O_{3}(g) \rightarrow Cl\dot{O}(g) + O_{2}(g)$
Now chlorine monoxide radical produced with atomic oxygen produces more chlorine radicals.
$Cl\dot{O}(g) + O(g) \rightarrow \dot{C}l(g) + O_{2}(g)$
The breakdown of ozone is caused by the continuous regeneration of chlorine radicals. Therefore, we can say that CFCs are transporting agents that continuously generate chlorine radicals into the stratosphere and cause damage to the ozone layer.

Question 22

What could be the harmful effects of improper management of industrial and domestic solid waste in a city?
Answer:

(i) If domestic waste is not disposed of properly, it may go to sewers and be eaten by cattle, and if they swallow polythene bags, it may result in their death.
(ii) Non-biodegradable waste can choke the sewers.

(iii) Improper disposal of waste is considered as the major cause of environmental pollution.
(iv) Similarly, improper management of industrial waste can lead to air, water, and soil pollution.

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 a few years, she visited the same lake again. She was surprised to find that the lake was covered with algae, the stinking smell was coming out and its water had become unusable. Can you explain the reason for this condition of the lake?
Answer:

The condition of the lake is a result of the process of eutrophication. We know that domestic waste and organic compounds like detergents enhance the growth of algae & aquatic plants in water. They are decomposed by bacteria and produce an unpleasant odour.

Question 24 What are biodegradable and non-biodegradable pollutants?
Answer:

(i) Biodegradables are the pollutants that can be decomposed by bacteria, e.g., cow dung, waste of vegetables and fruits, etc.
(ii) Non-biodegradable pollutants are the one which cannot be decomposed by bacteria, e.g., Mercury, polythene, DDT, etc.

Question 25 What are the sources of dissolved oxygen in water?
Answer:

Dissolved oxygen in the water can be used by microorganisms to oxidise organic matter. It reaches water either through the atmosphere or due to photosynthesis carried out by aquatic plants. During the night, photosynthesis cannot take place due to the absence of sunlight, but the plants continue to respire and decrease the amount of dissolved oxygen present in the water. The sources of dissolved oxygen in water are-

• Mechanical aeration

• Photosynthesis by many aquatic plants &

• Direct contact of air with the water surface

Question 26 What is the importance of measuring BOD of a water body?
Answer:

BOD is a measure of the amount of organic material present in water, in terms of how much oxygen it requires to break it down biologically. If the water has a value of BOD less than 5 ppm, it is considered clean water, whereas highly contaminated water has a value of BOD of 17 ppm or more.

Question 27 Why does water covered with excessive algal growth become polluted?
Answer:

When fertilizers with phosphate are added to water, they cause algae to grow quickly and cover the water’s surface. This makes the water look dirty, smell bad, and unsafe for swimming or boating. It also reduces the oxygen in the water, which can harm fish and other aquatic animals.

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 the 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:

In an automobile engine, fossil fuels are burnt at a high temperature which releases oxides of sulphur and nitrogen from the chimney which were experienced by the villagers, causing them breathing problems.
Here, dinitrogen and dioxygen combine and produce ample amounts of NO and $NO_{2}$ , as shown below:
$N_2(g) + O_{2}(g) \rightarrow 2NO(g)$ …. (at temperature 1483K)
$2NO(g) + O_{2}(g) \rightarrow 2NO_{2}(g)$
Combustion of sulphur-containing fossil fuel produces $SO_{2}$
$S + O_{2} \rightarrow SO_{2}(g)$

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 this happens. Give chemical reactions for the conversion of $SO_{2}$ into $SO_{3}$.
Answer:

Oxidation of sulphur without a catalyst can be remarkably slow; however, polluted air contains particulate matter which catalyses the reaction of $SO_{2}$ to $SO_{3}$.
$2SO_{2}(g) + O_{2}(g) \rightarrow 2SO_{3}(g)$
This reaction can also be promoted by hydrogen peroxide or ozone
$SO_{2}(g) + O_{3}(g) \rightarrow 2SO_{3}(g) + O_{2}(g)$
$SO_{2}(g) + H_{2}O_{2}(l) \rightarrow H_{2}SO_{4}(aq)$

Question 30 From where does ozone come in the photochemical smog?
Answer:

Nitric oxide and the free oxygen atom are formed by breaking $NO_2$ by absorbing sunlight
$NO_2 (g) \rightarrow NO (g) + O (g)$
We know that oxygen atoms are very reactive, and hence, they react with $O_{2}$ to form ozone
$O(g) + O_2(g) + M \rightarrow O_{3}(g) + M$
M can be an inert gas such as $N_{2}$ or $O_{3}$, which is produced during smog formation.

Question 31 How is ozone produced in the stratosphere?
Answer:

The UV radiations that fall on the ozone layer break the oxygen molecule into free oxygen(O) atoms; these free atoms of oxygen then react with molecular oxygen to form ozone.
$O_{2} (g) \rightarrow O (g) + O (g)$
$O (g) + O_{2} (g) \rightarrow O_{3} (g)$

Question 32 Ozone is a gas heavier than air. Why does the ozone layer not settle down near the earth?
Answer:

Ozone is a thermodynamically unstable gas and can be decomposed into molecular oxygen; therefore, there exists a dynamic equilibrium between the production and decomposition of ozone.
$O_2 (g) \rightarrow O (g) + O (g)$
$O (g) + O_{2} (g) \rightarrow O_{3} (g)$

Question 33 Some time ago the formation of polar stratospheric clouds was reported over Antarctica. Why were these formed? What happens when such clouds break up by the warmth of sunlight?
Answer:

Some time ago the scientists working in Antarctica reported depletion of the ozone layer, or it can also be said that the presence of an ozone hole was reported over the South Pole in the Antarctic region.
It was found that a unique set of conditions was responsible for this ozone hole based on the seasons, viz.,
In summer, NO₂ reacts with CH₄ to form chlorine monoxide and chlorine atoms, forming chlorine sinks, which prevent much ozone depletion.
In winter, Polar stratospheric clouds are formed over Antarctica, which provides a surface on which chlorine nitrate formed gets hydrolysed and forms hypochlorous acid. It also reacts with HCl and produces molecular chlorine.
$ClO + NO_{2} (g) \rightarrow ClONO_{2}$
$Cl (g) + CH_{4}\rightarrow CH_{3}(g) + HCl (g)$
$ClONO_{2} (g) +H_{2}O \rightarrow HOCl (g) + HNO_{3}$
$ClONO_{2} (g) + HCl \rightarrow Cl_{2} (g) + HNO_{3} (g)$

When the sunlight returns there in spring, the warmth of the sun breaks the cloud and photolyses HOCl and $Cl_{2}$,
$HOCl(g)\overset{h\nu }{\rightarrow}\dot{O}H(g)\dot{C}l(g)$
$Cl_{2}\overset{h\nu }{\rightarrow}2\dot{C}l(g)$
Therefore, the chain reaction of ozone depletion initiates with the formation of chlorine radicals.

Question 34 A person was using water supplied by the Municipality. Due to the shortage of water, he started using underground water. He felt a laxative effect. What could be the cause?
Answer:

The laxative effect is caused by drinking water having excessive sulphate (>500 ppm); however, it is harmless at moderate levels.

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

Answer:

$(i) \rightarrow (d); (ii) \rightarrow (e); (iii) \rightarrow (a); (iv) \rightarrow (c); (v) \rightarrow (b)$
Explanation-
(i) $SO_{2}$ is poisonous, and even its low concentrations can cause respiratory diseases.
(ii) Oxides of nitrogen cause an irritant red haze effect in traffic and congested places.
(iii) $CO_{2}$’s increasing amounts in the air are responsible for global warming.
(iv) Excess nitrate in drinking water can cause methemoglobinemia, i.e., ‘blue baby’ syndrome.
(v) The maximum limit of lead concentration in drinking water is 50 ppb. Consuming it more than that can cause kidney damage.

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

Answer:

$(i) \rightarrow (e); (ii) \rightarrow (d); (iii) \rightarrow (a); (iv) \rightarrow (b); (v) \rightarrow (c)$
Explanation-
(i) Acid rain, smog formation, breathing problems, etc. are caused by $SO_{2}$.
(ii) Carbamates are biodegradable pesticides but can be severely neurotoxic and cause harm to humans.
(iii) Synthetic detergents cause water pollution since they are non-biodegradable.
(iv) Gases released from factories can cause photochemical smog formation and cause damage to plants, water life, etc.
(v) CFCs present in the atmosphere are responsible for damaging the ozone layer.

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

Answer:

$(i) \rightarrow (a, d); (ii) \rightarrow (c); (iii) \rightarrow (a); (iv) \rightarrow (b)$
Phosphates enhance the growth of algae in the water, which covers the surface of the water and reduces the concentration of oxygen in the water and thereby increasing the BOD level of the water. This causes harm or may even prove fatal to animal life by depriving them of oxygen, and this subsequent loss of biodiversity is known as eutrophication.

Question 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.
Answer:

The answer is option (ii). Both A and R are correct, but R is not the correct explanation of A.
Explanation: Normally, the pH of rainwater is 5.6 due to the presence of H⁺ ions produced due to the dissolution of $CO_2$ in rainwater. If the pH drops below 5.6, it is known as acid rain.
$H_2O(l) + CO_2(g) \rightarrow H_2CO_3(aq)$
$H_2CO_3(aq) + H^{+}(aq) \rightarrow HCO_{3}^{-}(aq)$

Question 41 Assertion (A) : Photochemical smog is oxidising in nature.
Reason (R) : Photochemical smog contains $NO_{2}$ and $O_{3}$, 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:

The answer is the option (I). Both A and R are correct, and R is the correct explanation of A.
Explanation: Photochemical smog is also called oxidising smog since it has high concentrations of oxidising agents. When NO and hydrocarbons are built up in sufficiently high levels, a chain reaction occurs from their interaction with sunlight, where NO is converted to $NO_{2}$. This $NO_{2}$ absorbs sunlight and breaks into NO and a free oxygen atom.
$NO_{2}(g)\overset{h\nu }{\rightarrow}NO(g)+O(g)$
$O(g)+O_{2}(g)\rightarrow O_{3}(g)$

Question 42 Assertion (A): Carbon dioxide is one of the important greenhouse gases.
Reason (R): It is largely produced by the 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,t 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 answer is the option (ii). Both A and R are correct, but R is not the correct explanation of A.
An increased amount of $CO_{2}$ is responsible for global warming. It is removed by plants, and hence the balance is maintained.

Question 43 Assertion (A): Ozone is destroyed by solar radiation in the upper stratosphere.
Reason (R): Thinning of the ozone layer allows excessive UV radiation to reach the surface of the 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:

The answer is option (iv) A is not correct, but R is correct.
Ozone is produced in the stratosphere as UV rays act on $O_{2}$ molecules. The major reason for ozone depletion is CFCs. With the depletion of the ozone layer, UV rays filter into the troposphere and cause skin cancer, cataracts, sunburn, etc., it also damage fish productivity.

Question 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,t 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 answer is the option (I). Both A and R are correct, and R is the correct explanation of A.

Pesticides are basically synthetic toxic chemicals with ecological repercussions. Its repeated use gives rise to pests which are resistant to it. Hence, as insect resistance of DDT increases, other organic pesticides like Aldrin and Aieldrin were introduced. Organic toxins and water-insoluble and non-biodegradable.

Question 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 a 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 answer is option (iii). Both A and R are correct.
BOD is a measure of the amount of organic material present in water, in terms of how much oxygen it requires to break it down biologically. If the water has a value of BOD less than 5 ppm, it is considered clean water; however, water pollution increases if the BOD increases.

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 the greenhouse effect. Explain why?
Answer:

$CO_{2}$ increases global warming. It is produced by the burning of fossil fuels; however, plants consume $CO_{2}$ for photosynthesis and, in turn, emit oxygen, hence reducing global warming.
$CO_{2}$ is confined to the troposphere only and forms about 0.03% by volume in the atmosphere.
But as we know, deforestation and burning of fossil fuels is increasing $CO_{2}$­ and, hence, increasing global warming.

Question 48 Explain how does greenhouse effect causes global warming.
Answer:

The 'greenhouse effect' refers to the warming of the climate that results when the atmosphere traps heat radiating from Earth toward space. Certain gases in the atmosphere resemble glass in a greenhouse, allowing sunlight to pass into the 'greenhouse,' but blocking Earth's heat from escaping into space. The gases that contribute to the greenhouse effect include water vapour, carbon dioxide (CO₂), methane, nitrous oxides, and chlorofluorocarbons (CFCs).

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 organic toxins are non-biodegradable and water-insoluble. They are transferred from the lower tropic level to the higher tropic level because they are highly persistent. Over time, the concentration of toxins in higher animals increases and causes serious metabolic and physiological disorders. They enter into food chain and hence pose potential harm for human consumption.

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:

Tetrachloroethane was used as a solvent for cleaning; it contaminates groundwater and is suspected to be a carcinogen. It is now replaced with liquified $CO_2$ since it will be less harmful to the groundwater. Nowadays, $H_{2}O_{2}$ is used for bleaching, which gives better results and requires a lesser amount of water.