Chemistry Objective Questions and Answers: Environmental Chemistry
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Multiple Choice Questions
(a) fermentation
(b) biogas generation
(c) biosynthetic pathway
(d) biological oxygen demand
Show Answer & Explanation
Answer: (d) biological oxygen demand
Explanation:
1. BOD is the oxygen used by microorganisms to decompose organic matter in a given volume of water.
2. This is measured over a fixed period.
Teacher's Note:
1. BOD is usually measured over five days.
2. High BOD means polluted water.
(a) pathogens
(b) clogging of gills by silt
(c) reduction in dissolved oxygen
(d) foul smell
Show Answer & Explanation
Answer: (c) reduction in dissolved oxygen
Explanation:
1. Bacteria decomposing the sewage use up dissolved oxygen.
2. Fish cannot survive when dissolved oxygen falls below about 6 ppm.
Teacher's Note:
1. Dissolved oxygen in cold water is at most about 10 ppm.
2. Oxygen depletion is the cause, not the smell.
(a) Industrial and domestic sewage discharge is the main reason for river water pollution.
(b) Surface water contains much organic matter and mineral nutrients.
(c) Oil spills in sea water cause heavy damage to fishery.
(d) Oil slick in sea water increases dissolved oxygen.
Show Answer & Explanation
Answer: (d) Oil slick in sea water increases dissolved oxygen.
Explanation:
1. An oil slick forms a film on the water surface.
2. It cuts off oxygen exchange with the air and so lowers dissolved oxygen. Statement (4) is false.
Teacher's Note:
1. Oil spills harm marine life and birds.
2. A film on the surface reduces gas exchange.
(a) The lower the concentration of dissolved oxygen, the more polluted is the water sample.
(b) The tolerable limit of lead in drinking water is about 50 ppb.
(c) Water is considered clean if its BOD is less than 5 ppm.
(d) The safe limit of copper in drinking water is 10 ppm.
Show Answer & Explanation
Answer: (d) The safe limit of copper in drinking water is 10 ppm.
Explanation:
1. The permitted limit of copper in drinking water is 3 ppm, not 10 ppm.
2. The other three statements are correct.
Teacher's Note:
1. Limits: Fe 0.2 ppm, Mn 0.05 ppm, Cu 3 ppm, Zn 5 ppm.
2. Learn these values as they are asked directly.
(a) weathering of phosphate rock only
(b) agricultural fertilisers only
(c) phosphate rocks and sewage
(d) sewage and agricultural fertilisers
Show Answer & Explanation
Answer: (d) sewage and agricultural fertilisers
Explanation:
1. Detergents and human waste in sewage contain phosphates.
2. Phosphate fertilisers wash into water bodies from fields.
Teacher's Note:
1. Phosphates promote algal bloom.
2. Algal bloom leads to eutrophication.
(a) cleanliness and less use of insecticides
(b) proper disposal of organic wastes, sewage and industrial effluents
(c) use of liquefied CO\(_2\) with a suitable detergent in place of tetrachloroethene for dry cleaning
(d) all of the above
Show Answer & Explanation
Answer: (d) all of the above
Explanation:
1. All of the listed measures reduce pollution.
2. Liquefied CO\(_2\) with detergent replaces the carcinogenic tetrachloroethene in dry cleaning.
Teacher's Note:
1. This is an example of green chemistry.
2. Tetrachloroethene contaminates ground water.
(a) produce colour during reactions
(b) reduce the use and production of hazardous chemicals
(c) are related to depletion of the ozone layer
(d) study the reactions in plants
Show Answer & Explanation
Answer: (b) reduce the use and production of hazardous chemicals
Explanation:
1. Green chemistry aims at reducing the use and generation of hazardous substances.
2. It lowers material use, energy use and waste.
Teacher's Note:
1. Examples: H\(_2\)O\(_2\) bleaching instead of chlorine, liquid CO\(_2\) dry cleaning.
2. Water is preferred as solvent.
(a) increase in concentration of insecticide in water
(b) increase in concentration of fluoride ion in water
(c) reduction of dissolved oxygen in water due to phosphate pollution
(d) attack of younger leaves of a plant by peroxyacetyl nitrate
Show Answer & Explanation
Answer: (c) reduction of dissolved oxygen in water due to phosphate pollution
Explanation:
1. Phosphate enrichment causes excessive algal growth.
2. Decay of the algae reduces dissolved oxygen.
Teacher's Note:
1. Eutrophication kills aquatic life.
2. It reduces biodiversity.
Assertion (A): The pH of rain water is 5.6.
Reason (R): H\(^+\) ions are formed by the reaction of rain water with carbon dioxide present in the atmosphere.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Rain water dissolves CO\(_2\) to form carbonic acid, which ionises to give H\(^+\) and HCO\(_3^-\).
2. This gives a pH of about 5.6, so R explains A.
Teacher's Note:
1. Rain is called acid rain only when its pH falls below 5.6.
2. The natural acidity comes from CO\(_2\).
Assertion (A): Bacteria, fungi, moulds and algae are viable particulates.
Reason (R): Smoke particulates consist of solid or a mixture of solid and liquid particles formed during combustion of organic matter.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. Living organisms dispersed in air are viable particulates, so A is true.
2. R is also true, but it describes smoke, which is a non-viable particulate, so it does not explain A.
Teacher's Note:
1. Viable means living.
2. Smoke is non-viable.
Assertion (A): Photochemical smog results from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides liberated by automobiles and factories.
Reason (R): Classical smog is a mixture of smoke, fog and sulphur dioxide.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. Both statements are true.
2. But the composition of classical smog does not explain how photochemical smog is formed.
Teacher's Note:
1. Classical smog is reducing, photochemical smog is oxidising.
2. They differ in climate and composition.
Assertion (A): In the stratosphere, ozone is produced by the action of UV radiation on dioxygen.
Reason (R): UV radiation splits molecular oxygen into free oxygen atoms which combine with molecular oxygen to form ozone.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. O\(_2\) absorbs UV and gives 2O.
2. O + O\(_2\) \(\rightarrow\) O\(_3\), so R correctly explains A.
Teacher's Note:
1. Ozone formation and destruction are balanced naturally.
2. CFCs disturb this balance.
Assertion (A): Deficiency of fluoride in drinking water causes diseases such as tooth decay.
Reason (R): F\(^-\) ions make the enamel on teeth much harder by converting hydroxyapatite into the much harder fluorapatite.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Fluoride converts hydroxyapatite into fluorapatite, which resists decay, so lack of fluoride causes tooth decay.
2. R explains A.
Teacher's Note:
1. The recommended fluoride level is up to 1 ppm.
2. Above 2 ppm it causes mottling of teeth.
Assertion (A): Green plants maintain an appropriate level of CO\(_2\) in the atmosphere.
Reason (R): Green plants require CO\(_2\) for photosynthesis and they, in turn, release oxygen.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. Photosynthesis removes CO\(_2\) from the air and releases O\(_2\).
2. This is exactly how the CO\(_2\) level is kept in balance.
Teacher's Note:
1. Deforestation raises the CO\(_2\) level.
2. Higher CO\(_2\) increases global warming.
(a) CCl\(_4\)
(b) COCl\(_2\)
(c) C\(_3\)O\(_2\)
(d) CF\(_2\)Cl\(_2\)
Show Answer & Explanation
Answer: (d) CF\(_2\)Cl\(_2\)
Explanation:
1. Freons are chlorofluorocarbons.
2. CF\(_2\)Cl\(_2\) contains C, F and Cl, so it is a freon.
Teacher's Note:
1. Freon-12 is CF\(_2\)Cl\(_2\).
2. Freons are stable, non-toxic and non-flammable.
(a) high chemical stability
(b) good absorption of UV radiation
(c) polar nature
(d) high toxicity
Show Answer & Explanation
Answer: (a) high chemical stability
Explanation:
1. CFCs are chemically inert, non-flammable and non-toxic.
2. This stability makes them suitable as refrigerants, but also lets them reach the stratosphere.
Teacher's Note:
1. Their stability is also the reason they harm the ozone layer.
2. They are now being phased out.
(a) \(\cdot\)CH\(_3\)
(b) Cl\(\cdot\)
(c) F\(_2\)
(d) Cl\(_2\)
Show Answer & Explanation
Answer: (b) Cl\(\cdot\)
Explanation:
1. A chlorine free radical reacts with ozone: Cl\(\cdot\) + O\(_3\) \(\rightarrow\) ClO\(\cdot\) + O\(_2\).
2. ClO\(\cdot\) + O \(\rightarrow\) Cl\(\cdot\) + O\(_2\) regenerates Cl, so ozone is destroyed continuously.
Teacher's Note:
1. The chlorine radical acts like a catalyst.
2. Cl\(_2\) is a molecule, not a radical.
(a) SO\(_2\) and NO\(_2\)
(b) NO and NO\(_2\)
(c) CH\(_4\) and NO\(_2\)
(d) Cl\(_2\) and F\(_2\)
Show Answer & Explanation
Answer: (c) CH\(_4\) and NO\(_2\)
Explanation:
1. Cl + CH\(_4\) \(\rightarrow\) CH\(_3\) + HCl removes chlorine atoms.
2. ClO + NO\(_2\) \(\rightarrow\) ClONO\(_2\) removes ClO radicals. So the sinks are CH\(_4\) and NO\(_2\).
Teacher's Note:
1. HCl and ClONO\(_2\) are reservoir compounds.
2. They temporarily remove chlorine from the cycle.
(a) is low in nutrients
(b) is high in nutrients
(c) has high temperature
(d) has a large amount of dissolved oxygen
Show Answer & Explanation
Answer: (b) is high in nutrients
Explanation:
1. Nutrient-rich water supports excess growth of algae.
2. Their decay removes oxygen from the water.
Teacher's Note:
1. Nutrients here mainly mean phosphates and nitrates.
2. Low oxygen kills fish.
(a) Absorption of terrestrially radiated heat by CO\(_2\) is the main cause of global warming.
(b) Global warming increases the rate of melting of polar ice caps and raises the sea level.
(c) Global warming of the earth's surface is mainly due to reforestation.
(d) CO\(_2\), CH\(_4\), O\(_3\), CFCs and water vapour are greenhouse gases.
Show Answer & Explanation
Answer: (c) Global warming of the earth's surface is mainly due to reforestation.
Explanation:
1. Planting forests removes CO\(_2\) and reduces warming.
2. Global warming is caused mainly by deforestation and burning of fossil fuels, so statement (3) is false.
Teacher's Note:
1. Greenhouse gases trap infrared radiation from the earth.
2. Melting of ice caps raises sea level.
(a) Hydrocarbon
(b) Ozone
(c) PAN
(d) Water vapour
Show Answer & Explanation
Answer: (a) Hydrocarbon
Explanation:
1. Unburnt hydrocarbons and NO from vehicles start the smog reactions.
2. Ozone and PAN are products (secondary pollutants).
Teacher's Note:
1. Sunlight splits NO\(_2\) to give O atoms.
2. The O atoms form ozone.
(a) using catalytic converters in automobiles
(b) planting certain plants such as pinus and juniperus
(c) both (1) and (2)
(d) neither (1) nor (2)
Show Answer & Explanation
Answer: (c) both (1) and (2)
Explanation:
1. Catalytic converters stop NO and hydrocarbons from leaving the exhaust.
2. Plants such as pinus, juniperus and vitis metabolise NO. So both measures help.
Teacher's Note:
1. Another method is to spray compounds that give free radicals.
2. These radicals combine with smog-forming radicals.
(a) \(\cdot\)CH\(_3\)
(b) ClO\(\cdot\)
(c) Cl\(\cdot\)
(d) CFCl\(_2\)\(\cdot\)
Show Answer & Explanation
Answer: (c) Cl\(\cdot\)
Explanation:
1. For example CF\(_2\)Cl\(_2\) \(\xrightarrow{h\nu}\) Cl\(\cdot\) + \(\cdot\)CF\(_2\)Cl.
2. The chlorine free radical then attacks ozone.
Teacher's Note:
1. The C-Cl bond breaks first.
2. CFCs act as carriers of chlorine into the stratosphere.
(a) Pathogens
(b) Organic wastes
(c) Chemical pollutants
(d) None of these
Show Answer & Explanation
Answer: (d) None of these
Explanation:
1. Pathogens, organic wastes and chemical pollutants all pollute water.
2. So the option that does not contribute is "none of these".
Teacher's Note:
1. The main water pollutants are pathogens, organic wastes, plant nutrients, heavy metals, sediments, pesticides and heat.
2. Heat pollution comes from cooling water.
(a) Photochemical smog has a high concentration of reducing agents and is therefore called reducing smog.
(b) Non-viable particulates consist of smoke, dust, mist and fumes.
(c) Classical smog occurs in cool, humid climate and is a mixture of smoke, fog and SO\(_2\).
(d) Ozone reacts with unburnt hydrocarbons in polluted air to produce PAN.
Show Answer & Explanation
Answer: (a) Photochemical smog has a high concentration of reducing agents and is therefore called reducing smog.
Explanation:
1. Photochemical smog contains oxidising agents such as NO\(_2\) and O\(_3\); it is called oxidising smog.
2. So statement (1) is false and the others are true.
Teacher's Note:
1. Classical smog is the reducing smog.
2. Do not interchange the two types.
(a) Soot particles have a diameter of about 5 nm.
(b) H\(_2\)SO\(_4\) fog particles have a size of 500-1000 nm.
(c) Fly ash particles have a diameter of about \(5 \times 10^5\) nm.
(d) All particulates have the same size.
Show Answer & Explanation
Answer: (d) All particulates have the same size.
Explanation:
1. Particulates vary widely in size, from a few nm for soot to hundreds of micrometres for fly ash.
2. So the statement that all are the same size is incorrect.
Teacher's Note:
1. Particles above 5 micron lodge in the nasal passage.
2. Particles near 1 micron enter the lungs.
(a) O\(_3\) + h\(\nu\) \(\rightarrow\) O + O\(_2\)
(b) Cl\(\cdot\) + O\(_3\) \(\rightarrow\) ClO\(\cdot\) + O\(_2\)
(c) ClO\(\cdot\) + O \(\rightarrow\) Cl\(\cdot\) + O\(_2\)
(d) Reactions (2) and (3) are involved
Show Answer & Explanation
Answer: (d) Reactions (2) and (3) are involved
Explanation:
1. The chain is Cl + O\(_3\) giving ClO + O\(_2\), then ClO + O giving Cl + O\(_2\).
2. Photolysis of ozone alone is not the CFC mechanism, so only reactions (2) and (3) are involved.
Teacher's Note:
1. Cl is regenerated in the second step.
2. Net result: O\(_3\) + O \(\rightarrow\) 2O\(_2\).
(a) DDT
(b) Aldrin
(c) Dieldrin
(d) None of these
Show Answer & Explanation
Answer: (d) None of these
Explanation:
1. DDT, aldrin and dieldrin are persistent and non-biodegradable.
2. So none of the three is biodegradable.
Teacher's Note:
1. They pass along the food chain and build up in higher animals.
2. This is called biomagnification.
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Practice MCQs for JEE Chemistry Environmental Chemistry
JEE Chemistry Environmental Chemistry Objective Test Questions
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FAQs
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By solving our JEE Chemistry Environmental Chemistry MCQs Set 12, JEE students can improve their accuracy and speed which is important as objective questions provide a chance to secure 100% marks in the Chemistry.
Yes, Chemistry MCQs for JEE have answer key and brief explanations to help students understand logic behind the correct option as its important for 2026 competency-focused JEE exams.
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