75 free MCQs on Environmental Chemistry with worked answers and explanations. Understand how human activities alter the atmosphere, water, and soil. Covers air pollutants, smog types, ozone depletion, acid rain, greenhouse effect, water and soil pollution, and the principles of green chemistry.
Take the timed Environmental Chemistry chapterwise test →Below are 75 practice questions on Environmental Chemistry, sorted Easy → Hard. Tap “Show answer & explanation” under any question to check yourself. Want the full theory first? Read the Environmental Chemistry notes.
Classical smog is a reducing mixture of SO₂ and fog, while photochemical smog is an oxidising mixture generated by UV-driven reactions of NOx and hydrocarbons.
Q1.
A secondary air pollutant is one that:
Answer: A. Is formed in the atmosphere by reaction between primary pollutants
Why: Secondary pollutants (e.g., ozone, PAN) are not emitted directly but are produced by chemical reactions among primary pollutants in the atmosphere.
Q2.
Which soil/water condition directly results from acid rain falling on a region?
Answer: B. Leaching of toxic metal ions and lowering of soil and water pH
Why: Acid rain lowers the pH of soil and water and mobilises (leaches) toxic metal ions such as aluminium, harming plants and aquatic life.
Q3.
Which region of the atmosphere shows temperature increasing with altitude due to absorption of UV radiation by ozone?
Answer: C. Stratosphere
Why: In the stratosphere ozone absorbs UV radiation and releases heat, so temperature rises with altitude, unlike the troposphere where it falls.
Q4.
A pollutant that cannot be degraded by natural processes and persists in the environment is termed:
Answer: D. Non-biodegradable
Why: Non-biodegradable pollutants such as DDT, plastics and heavy metals are not broken down by microorganisms and accumulate in the environment.
Q5.
Which of the following is NOT considered a greenhouse gas?
Answer: A. Nitrogen (N<sub>2</sub>)
Why: Diatomic nitrogen does not absorb infrared radiation and is not a greenhouse gas, whereas CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O, ozone and water vapour are.
Q6.
Which atmospheric layer contains the ozone layer that protects the Earth from harmful UV radiation?
Answer: B. Stratosphere
Why: The ozone layer is found in the stratosphere, concentrated at about 23-25 km altitude. It absorbs harmful UV-B and UV-C radiation before it reaches Earth's surface. The troposphere lies below it and contains all weather phenomena.
Q7.
A high BOD (Biochemical Oxygen Demand) value in a water sample indicates:
Answer: D. Heavily polluted water with a high organic load
Why: BOD measures oxygen consumed by microbes decomposing organic matter in water over 5 days at 20 degrees C. A high BOD means large amounts of organic waste are present. Clean water has BOD below 5 ppm; very polluted water exceeds 17 ppm.
Q8.
Which disease is caused by methylmercury poisoning from consuming contaminated fish?
Answer: C. Minamata disease
Why: Minamata disease was first identified in Minamata Bay, Japan in the 1950s. Methylmercury discharged from a chemical plant accumulated in fish; people who ate the fish suffered irreversible neurological damage, tremors, and birth defects. Itai-itai disease is caused by cadmium, not mercury.
Q9.
Which gas is the single largest contributor to the enhanced greenhouse effect when total radiative forcing is considered?
Answer: B. Carbon dioxide (CO<sub>2</sub>)
Why: Although methane and CFCs are far more potent per molecule, CO<sub>2</sub> is present at far greater concentrations (over 420 ppm currently) and therefore contributes the largest share of total enhanced greenhouse forcing. CH<sub>4</sub> and N<sub>2</sub>O are also significant but at much lower concentrations.
Q10.
Acid rain forms when oxides of which two elements react with atmospheric moisture?
Answer: C. Sulphur and nitrogen
Why: SO<sub>2</sub> (from burning sulphur-containing fuels) and NO<sub>2</sub> (from high-temperature combustion) dissolve in rainwater to form H<sub>2</sub>SO<sub>4</sub> and HNO<sub>3</sub> respectively. This produces acid rain with pH typically between 4 and 5.6, compared to the normal value of about 5.6 for CO<sub>2</sub>-saturated rain.
Q11.
Which of the following is a PRIMARY air pollutant (directly emitted into the atmosphere)?
Answer: C. Sulphur dioxide (SO<sub>2</sub>)
Why: Primary pollutants are emitted directly from identifiable sources. SO<sub>2</sub> is directly released from burning sulphur-containing coal and petroleum. Ozone, PAN, and sulphuric acid aerosol are secondary pollutants formed in the atmosphere by reactions among primary pollutants.
Q12.
Which class of compounds is primarily responsible for catalytic depletion of the stratospheric ozone layer?
Answer: A. Chlorofluorocarbons (CFCs)
Why: CFCs (chlorofluorocarbons, also called Freons) migrate to the stratosphere where short-wavelength UV breaks the C-Cl bond, releasing Cl radicals. Each Cl radical destroys approximately 10<sup>5</sup> ozone molecules in a catalytic cycle before being removed. CO<sub>2</sub> and CH<sub>4</sub> are greenhouse gases but do not deplete ozone.
Q13.
Eutrophication in lakes and rivers is mainly triggered by excess:
Answer: B. Nitrates and phosphates from agricultural runoff and sewage
Why: Eutrophication is driven by excessive nitrogen and phosphorus from fertilisers and sewage. These nutrients trigger algal blooms, which deplete dissolved oxygen when the algae die and decompose, causing hypoxia and death of aquatic organisms.
Q14.
Itai-itai disease ('ouch-ouch') is caused by chronic poisoning from which heavy metal?
Answer: D. Cadmium (Cd)
Why: Itai-itai disease occurred in Japan due to cadmium from zinc-smelting operations contaminating irrigation water. Cadmium replaces calcium in bones, causing softening, painful fractures, and renal failure. Do not confuse it with Minamata disease, which is caused by mercury.
Q15.
Green chemistry primarily aims to:
Answer: B. Design processes that reduce or eliminate hazardous substances
Why: Green chemistry, as defined by Anastas and Warner (1998), focuses on designing chemical products and processes that minimise or eliminate hazardous substances. Its 12 principles include atom economy, waste prevention, safer solvents, catalysis, and use of renewable feedstocks.
Q16.
The layer of the atmosphere nearest the Earth’s surface is the:
Answer: B. troposphere
Why: Weather and most air pollution occur in the troposphere, the lowest atmospheric layer.
Q17.
Which gas is the chief contributor to the greenhouse effect?
Answer: B. carbon dioxide
Why: Carbon dioxide is the principal greenhouse gas released by burning fossil fuels.
Q18.
Acid rain is caused mainly by oxides of sulfur together with oxides of:
Answer: B. nitrogen
Why: SO₂ and NOₓ dissolve in rain to form sulfuric and nitric acids, lowering its pH.
Q19.
Depletion of the stratospheric ozone layer is caused chiefly by:
Answer: B. chlorofluorocarbons
Why: CFCs release chlorine atoms that catalytically destroy ozone.
Q20.
In water quality, BOD stands for Biochemical Oxygen:
Answer: B. Demand
Why: Biochemical Oxygen Demand measures the oxygen needed by microbes to decompose organic matter in water.
Q21.
A common toxic air pollutant emitted by motor vehicles is:
Answer: B. carbon monoxide
Why: Incomplete combustion in engines releases carbon monoxide, a poisonous gas.
Q22.
The ozone layer shields living things by absorbing harmful:
Answer: B. ultraviolet radiation
Why: Stratospheric ozone absorbs most of the Sun’s damaging ultraviolet radiation.
Q23.
Photochemical smog is formed in the presence of:
Answer: B. sunlight
Why: Photochemical smog forms when sunlight drives reactions among NOₓ and hydrocarbons.
Q24.
Which of these is a greenhouse gas?
Answer: B. methane
Why: Methane is a potent greenhouse gas, trapping heat far more effectively than CO₂ per molecule.
Q25.
Which pollutant is chiefly responsible for global warming?
Answer: B. carbon dioxide
Why: Carbon dioxide from fossil-fuel combustion is the dominant driver of global warming.
Q26.
Photochemical smog is formed mainly by the action of sunlight on:
Answer: B. Nitrogen oxides and hydrocarbons
Why: Sunlight acting on NO<sub>x</sub> and unburnt hydrocarbons from vehicles produces oxidising photochemical smog (ozone, PAN, aldehydes).
Q27.
Water suitable for drinking (potable) should generally have a dissolved oxygen level and a BOD such that:
Answer: B. Clean water has high dissolved oxygen and low BOD
Why: Clean water holds ample dissolved oxygen and has a low BOD; a high BOD signals heavy organic pollution that depletes the dissolved oxygen.
Q28.
Assertion: Carbon dioxide is a greenhouse gas yet it does not deplete the ozone layer. Reason: CO<sub>2</sub> absorbs infrared radiation but does not release ozone-destroying free radicals.
Answer: D. Both assertion and reason are true and the reason explains the assertion
Why: CO<sub>2</sub> traps outgoing IR (greenhouse effect) but, unlike CFCs, it produces no chlorine or other radicals that catalyse ozone breakdown, so the reason correctly explains the assertion.
Q29.
Photochemical smog damages plants and irritates eyes largely because it contains oxidants. A characteristic component of photochemical smog is:
Answer: A. Peroxyacetyl nitrate (PAN)
Why: Photochemical smog is oxidising and contains ozone, PAN and aldehydes formed from NO<sub>x</sub> and hydrocarbons in sunlight; PAN is a typical eye-irritant component.
Q30.
The addition of excess phosphate-containing detergents and fertilisers to a lake leads to eutrophication, whose immediate chemical consequence is:
Answer: B. Rapid algal growth followed by oxygen depletion when the algae decay
Why: Nutrient enrichment causes algal blooms; when the algae die, their decomposition by bacteria consumes dissolved oxygen, killing fish and other aquatic organisms.
Q31.
Photochemical smog is described as 'oxidising', whereas classical London smog is 'reducing'. The oxidising character of photochemical smog arises mainly from:
Answer: A. Ground-level ozone (O<sub>3</sub>) and peroxyacetyl nitrate (PAN), both powerful oxidants formed by UV-driven reactions
Why: Photochemical smog contains O<sub>3</sub> and PAN (peroxyacetyl nitrate), both strong oxidising agents formed when UV drives reactions between NOx and unburnt hydrocarbons. These oxidants crack rubber, damage plant tissue, and cause eye irritation. Classical smog is reducing because SO<sub>2</sub> and carbon particles act as reducing agents.
Q32.
Why can a single CFC-derived Cl radical destroy approximately 10<sup>5</sup> ozone molecules?
Answer: C. Cl is regenerated unchanged at the end of each catalytic cycle, repeating the process thousands of times
Why: The cycle is: Cl + O<sub>3</sub> → ClO + O<sub>2</sub> (step 1); ClO + O → Cl + O<sub>2</sub> (step 2). Cl is consumed in step 1 but regenerated in step 2 - it acts as a catalyst. A single Cl atom repeats this cycle approximately 10<sup>5</sup> times before being removed by other atmospheric reactions, making the damage enormous.
Q33.
Which reaction is responsible for acid rain damaging marble monuments such as the Taj Mahal?
Answer: A. CaCO<sub>3</sub> + H<sub>2</sub>SO<sub>4</sub> → CaSO<sub>4</sub> + H<sub>2</sub>O + CO<sub>2</sub>
Why: Sulphuric acid from acid rain reacts with marble (CaCO<sub>3</sub>) to produce calcium sulphate (gypsum), water, and CO<sub>2</sub>. Gypsum is soft, powdery, and partly soluble, so it crumbles away from the surface - a process known as 'stone cancer'. This is the specific reaction threatening the Taj Mahal's marble.
Q34.
Why is CO particularly dangerous to humans even at low atmospheric concentrations?
Answer: A. CO binds haemoglobin about 300 times more strongly than O<sub>2</sub>, forming carboxyhaemoglobin that cannot carry oxygen
Why: CO has an affinity for haemoglobin approximately 240-300 times greater than O<sub>2</sub>, forming stable carboxyhaemoglobin (COHb) that cannot transport oxygen. This causes cellular hypoxia even at a few hundred ppm. CO poisoning is particularly dangerous because it produces no irritation, so victims may be unaware of exposure until incapacitated.
Q35.
The standard BOD test is conducted at 20 degrees C for exactly 5 days. What is the best explanation for these standardised conditions?
Answer: C. 20 degrees C approximates temperate river temperature and supports active decomposer bacteria; 5 days oxidises about 70-80% of biodegradable organics without requiring an impractically long incubation
Why: BOD<sub>5</sub> at 20 degrees C is internationally standardised because 20 degrees C is typical of temperate rivers and supports active growth of mixed sewage bacteria. Five days allows ~70-80% of readily biodegradable organic matter to be consumed, providing a reproducible pollution index without requiring weeks of incubation.
Q36.
Among common greenhouse gases, N<sub>2</sub>O (nitrous oxide) is environmentally significant because:
Answer: B. It has a global warming potential about 298 times that of CO<sub>2</sub> over a 100-year period
Why: N<sub>2</sub>O has a GWP of approximately 298 times CO<sub>2</sub> over 100 years, making it the third most important greenhouse gas by radiative forcing despite low concentrations. Its main anthropogenic sources are nitrogen fertilisers and livestock manure. It also destroys stratospheric ozone by reacting with O atoms to produce NO.
Q37.
During photochemical smog formation, NO<sub>2</sub> plays a key initiating role. What is this specific role?
Answer: A. NO<sub>2</sub> absorbs UV light (wavelength less than 420 nm) and dissociates into NO and a reactive oxygen atom (O), which reacts with O<sub>2</sub> to form ground-level O<sub>3</sub>
Why: The initiating step is: NO<sub>2</sub> + UV (wavelength < 420 nm) → NO + O (atomic oxygen). The atomic oxygen then reacts: O + O<sub>2</sub> → O<sub>3</sub>. This ground-level O<sub>3</sub> oxidises unburnt hydrocarbons, producing radical species that ultimately form PAN, acrolein, formaldehyde, and the other toxic components of photochemical smog.
Q38.
The primary ecological hazard of persistent organochlorine pesticides like DDT is:
Answer: B. Being fat-soluble and metabolically resistant, they bioaccumulate in lipid tissues and biomagnify at each trophic level, reaching toxic concentrations in top predators
Why: DDT is lipophilic and resists metabolic degradation, so it accumulates in fatty tissues (bioaccumulation). At each trophic level the concentration multiplies (biomagnification) - concentrations in raptors can be 10<sup>7</sup> times those in ambient water. This caused fatal eggshell thinning in birds like the bald eagle, dramatically reducing their populations.
Q39.
How does biomagnification differ from bioaccumulation?
Answer: C. Bioaccumulation is build-up of a substance within one organism over time; biomagnification is the progressive increase in concentration at each higher trophic level in a food chain
Why: Bioaccumulation is the net build-up of a chemical within an individual organism (intake exceeds excretion rate). Biomagnification is the increase in concentration at successively higher trophic levels, because each predator ingests and retains the total pollutant burden from many prey organisms. Mercury and DDT undergo both processes simultaneously.
Q40.
In green chemistry, 'atom economy' is defined as:
Answer: B. The ratio of molecular mass of the desired product to the total molecular mass of all products, expressed as a percentage
Why: Atom economy = (molecular mass of desired product / sum of molecular masses of all products including by-products) x 100%. It measures what fraction of reactant atoms end up in the target product. Addition and rearrangement reactions achieve 100% atom economy; substitution reactions generate by-products, lowering atom economy. Introduced by Barry Trost (1991).
Q41.
The pH of normal, unpolluted rain water is about 5.6 because of dissolved:
Answer: B. carbon dioxide
Why: Dissolved CO₂ forms weak carbonic acid, giving natural rain a slightly acidic pH of about 5.6.
Q42.
Rain is classified as acid rain when its pH is:
Answer: B. below 5.6
Why: Acid rain has a pH below the natural 5.6 owing to dissolved SO₂ and NOₓ.
Q43.
The chief danger of carbon monoxide to humans is that it:
Answer: B. binds strongly to haemoglobin
Why: CO binds to haemoglobin far more tightly than O₂, blocking oxygen transport in the blood.
Q44.
Chlorofluorocarbons destroy ozone by releasing which reactive species?
Answer: B. chlorine atoms
Why: UV light frees chlorine atoms from CFCs, and these radicals catalytically break down ozone.
Q45.
Photochemical smog is oxidising and contains ozone, aldehydes and:
Answer: B. peroxyacetyl nitrate (PAN)
Why: PAN (peroxyacetyl nitrate) is a characteristic irritant component of photochemical smog.
Q46.
A high BOD value for a sample of water indicates:
Answer: B. heavy organic pollution
Why: A high BOD means microbes are consuming a lot of oxygen to break down abundant organic waste.
Q47.
Greenhouse gases warm the planet by trapping:
Answer: B. infrared radiation
Why: Greenhouse gases absorb and re-emit the infrared (heat) radiation given off by the Earth’s surface.
Q48.
Eutrophication of a lake is caused by an excess of:
Answer: B. nitrates and phosphates
Why: Nutrient enrichment triggers algal blooms; their decay depletes oxygen and harms aquatic life.
Q49.
Classical (London) smog is chemically:
Answer: B. reducing
Why: Classical smog contains SO₂ and smoke and is reducing, unlike the oxidising photochemical smog.
Q50.
The reaction of sulfur trioxide with rain water produces:
Answer: B. sulfuric acid
Why: SO₃ + H₂O → H₂SO₄, a major component of acid rain.
Q51.
The Biochemical Oxygen Demand (BOD) of clean water is:
Answer: A. Less than 5 ppm
Why: Clean water has a BOD below 5 ppm; a high BOD indicates heavy organic pollution.
Q52.
Over Antarctica, special conditions let ozone depletion accelerate sharply in spring. The key role of polar stratospheric clouds is that they:
Answer: C. Provide surfaces that convert inactive chlorine reservoirs to Cl<sub>2</sub>
Why: On ice-cloud surfaces, reservoir species (ClONO<sub>2</sub>, HCl) are converted to Cl<sub>2</sub> during the polar winter; returning spring sunlight photolyses Cl<sub>2</sub> to Cl radicals that catalytically destroy ozone.
Q53.
In the CFC-driven chain Cl• + O<sub>3</sub> → ClO• + O<sub>2</sub> followed by ClO• + O → Cl• + O<sub>2</sub>, the species ClO• is best described as:
Answer: D. A reaction intermediate, while Cl• is the regenerated catalyst
Why: Cl• is consumed in step one and regenerated in step two, so it acts as a catalyst; ClO• is formed and then used up, making it a reaction intermediate.
Q54.
Methylmercury formed from inorganic mercury in sediments is far more hazardous to humans than Hg<sup>2+</sup> itself mainly because methylmercury:
Answer: B. Is lipid soluble, so it is absorbed and biomagnified up the aquatic food chain
Why: Lipid-soluble methylmercury passes easily through membranes, is retained in tissues and becomes concentrated at each trophic level (biomagnification), reaching toxic levels in fish consumed by humans, causing Minamata disease.
Q55.
The green-chemistry concept of atom economy rates a synthesis more favourably when:
Answer: C. A larger fraction of all reactant atoms ends up in the desired product
Why: Atom economy measures how much of the reactant mass is incorporated into the useful product; a high atom economy means less waste, independent of the conventional percentage yield.
Q56.
In the CFC-driven ozone depletion cycle: Step 1: Cl• + O₃ → ClO• + O₂; Step 2: ClO• + O → Cl• + O₂. What is the net equation, and what is the catalytic role of Cl•?
Answer: A. Net: O₃ + O → 2O₂; Cl• is a catalyst - consumed in step 1 but regenerated in step 2, allowing ~10⁵ ozone destructions per Cl• atom
Why: Adding the two steps: (Cl• + O₃ → ClO• + O₂) + (ClO• + O → Cl• + O₂) → net reaction: O₃ + O → 2O₂. Cl• appears on both sides and cancels - it is a catalyst. This chain repeats ~10⁵ times before Cl• is scavenged by other reactions (e.g., forming HCl with CH₄). This catalytic multiplication is why a tiny amount of CFC causes enormous ozone loss.
Q57.
Why does the Antarctic ozone hole develop specifically in the Southern Hemisphere spring (September–October) rather than continuously throughout the year?
Answer: C. During winter, polar stratospheric clouds (PSCs) convert inactive chlorine reservoirs to Cl₂ and HOCl; when spring UV arrives, these photolyse rapidly to Cl•, triggering explosive catalytic ozone destruction in the isolated polar vortex
Why: During the Antarctic winter, temperatures drop below −78°C and the polar vortex isolates the stratospheric air mass. Polar stratospheric clouds (PSCs) form, and on their surfaces, reservoir species (HCl, ClONO₂) undergo heterogeneous reactions producing Cl₂ and HOCl. When spring sunlight returns, these are rapidly photolysed to Cl• radicals. Within the still-isolated polar vortex, Cl• destroys ozone explosively before mid-summer warming disperses the vortex and allows ozone-rich air from lower latitudes to mix in.
Q58.
PAN (peroxyacetyl nitrate, CH₃C(O)OONO₂) is a secondary pollutant that also acts as a NOx reservoir. Which correctly describes its formation mechanism and dual role?
Answer: C. The peroxyacetyl radical (CH₃C(O)OO•) - formed by OH•-initiated oxidation of hydrocarbons - combines with NO₂ to give PAN; it is a strong lachrymator and, being thermally unstable, decomposes in warmer regions to re-release NO₂, transporting NOx over long distances
Why: PAN forms via: hydrocarbon + OH• → acetaldehyde → (OH•) → acetyl radical (CH₃CO•) → (O₂) → peroxyacetyl radical (CH₃C(O)OO•) → (NO₂) → PAN. PAN is a potent lachrymator, causing eye tearing, and inhibits photosynthesis at ppb levels. Crucially, PAN is thermally labile: it decomposes at warmer temperatures to release NO₂ again, acting as a long-range transporter of NOx pollution from urban sources to remote rural areas.
Q59.
Without anthropogenic CFCs and NOx from high-altitude aircraft, stratospheric ozone would reach a higher natural steady-state concentration. Why does adding catalytic CFC-derived Cl• and NOx cycles lower the ozone steady state?
Answer: D. Extra catalytic destruction pathways (Cl•/ClO• and NO/NO₂ cycles) increase the total O₃ removal rate without changing the formation rate, so the steady-state [O₃] shifts lower until formation again equals destruction
Why: The stratospheric ozone steady state is set by: rate of ozone formation = rate of ozone destruction. The natural destruction pathways (O₃ + O → 2O₂, etc.) plus the CFC-catalytic cycle (Cl•) and the NOx catalytic cycle (NO + O₃ → NO₂ + O₂; NO₂ + O → NO + O₂) all increase the total destruction rate. Since the formation rate (from O + O₂) is unchanged, the steady-state [O₃] must decrease until rates balance again at a new, lower equilibrium - this is ozone depletion.
Q60.
Inorganic Hg²⁺ dumped into water sediments is converted to methylmercury (CH₃Hg⁺) by anaerobic bacteria. Which correctly explains why this conversion is so ecologically catastrophic?
Answer: B. Methylmercury is lipophilic, bioaccumulates in organism tissues, and biomagnifies up food chains; humans consuming top-predator fish can receive doses causing irreversible neurological damage (Minamata disease)
Why: Methylmercury (CH₃Hg⁺), produced by methylcobalamin-dependent anaerobic bacteria in sediments, is far more dangerous than Hg²⁺ because it is lipophilic - it crosses biological membranes efficiently, is poorly excreted, and binds strongly to thiol groups in proteins. It biomagnifies dramatically up food chains (plankton → small fish → large fish → humans), and even small human doses cause permanent neurological damage, sensory loss, paralysis, and congenital defects, as seen in the 1950s Minamata Bay tragedy.
Q61.
A river water sample is diluted 20-fold before the BOD test. Initial dissolved oxygen = 9.0 mg/L; final DO after 5 days at 20°C = 3.5 mg/L. What is the BOD of the original (undiluted) river water?
Answer: A. 110 mg/L
Why: DO consumed by the diluted sample = 9.0 − 3.5 = 5.5 mg/L. Since this is the demand of the 1/20 diluted sample, the original water's BOD = 5.5 × 20 = 110 mg/L. This severely exceeds the 17 ppm threshold for very polluted water, indicating raw sewage or highly concentrated industrial effluent.
Q62.
In the aqueous-phase oxidation pathway of acid rain formation, SO₂ dissolves in cloud droplets to form H₂SO₃, which is then oxidised by H₂O₂. Why is this aqueous pathway particularly important for producing highly acidic rain?
Answer: C. The H₂SO₃ + H₂O₂ → H₂SO₄ + H₂O reaction in droplets proceeds rapidly even at low pH, so acidification is self-reinforcing - the more acidic the droplet becomes, the more this pathway (relative to pH-sensitive pathways) dominates, enabling very low pH values of 3–4
Why: Unlike some aqueous oxidants (e.g., O₃, which reacts with HSO₃⁻ at a rate that slows as pH decreases), the reaction H₂SO₃ + H₂O₂ → H₂SO₄ + H₂O involves the undissociated form of sulphurous acid and is relatively pH-independent at the pH range of interest (3–5). This makes it self-accelerating: as the droplet acidifies, H₂SO₃ (pKa 1.81) predominates over HSO₃⁻, and the H₂O₂ pathway continues unimpeded, allowing cloud droplets to reach pH 3–4 in heavily polluted regions.
Q63.
The BHC Company redesigned ibuprofen synthesis using green chemistry, reducing steps from 6 to 3 and achieving ~99% atom economy. The old 6-step Boots synthesis had low atom economy because:
Answer: B. The 6-step process involved stoichiometric inorganic reagents (e.g., AlCl₃ as reagent, not catalyst) that became waste by-products not incorporated in ibuprofen, so most atom mass from reactants did not appear in the final product
Why: Atom economy = MW(desired product) / Σ MW(all products) × 100%. The Boots process used stoichiometric AlCl₃ as a Lewis acid (consumed, not recycled), and stoichiometric oxidants and reagents at multiple steps, each generating large amounts of inorganic waste salts that were not ibuprofen. This gave ~40% atom economy. The BHC process uses catalytic HF (recycled), three steps, and incorporates most atoms into the product - achieving ~99% atom economy. Atom economy is independent of yield.
Q64.
Strontium-90 (⁹⁰Sr, half-life ~29 years) from nuclear fallout is a long-term soil and food-chain hazard specifically because:
Answer: D. Being a Group 2 element chemically similar to calcium, ⁹⁰Sr is taken up by plants and deposited in animal bones in place of Ca²⁺, where its beta emissions irradiate bone marrow and raise the risk of leukaemia
Why: Strontium is in Group 2, directly below calcium in the periodic table, and mimics calcium in biological systems. After entering the food chain via plant uptake, ⁹⁰Sr is deposited in bones and teeth in place of calcium. Its half-life of ~29 years means contamination persists for centuries, and the continuous beta radiation emitted inside bone irradiates haematopoietic (blood-forming) tissue in the marrow, raising the risk of leukaemia.
Q65.
During a temperature inversion over a polluted city, classical smog can persist for days. What is the meteorological mechanism, and why does it trap pollutants so effectively?
Answer: B. A layer of warm air sits above cooler, denser surface air, creating stable atmospheric stratification that suppresses convective uplift of pollutants, causing them to accumulate near the ground
Why: Normally, air temperature decreases with altitude and warm, buoyant surface air rises, dispersing pollutants. During a temperature inversion, a layer of warmer, lighter air caps a cooler, denser layer near the surface. The surface air cannot rise through the warmer air above, so pollutants accumulate and intensify in the stable lower layer. This is what allowed the 1952 Great London Smog to reach lethal concentrations over four days, killing approximately 4,000 people.
Q66.
A single chlorine radical from a CFC can destroy ozone catalytically, meaning it can break down:
Answer: B. many O₃ molecules
Why: Because the Cl atom is regenerated in each cycle, one radical can destroy thousands of ozone molecules.
Q67.
The Antarctic ozone hole develops mainly during the:
Answer: A. spring
Why: Polar stratospheric clouds in the cold winter set up conditions so that ozone is destroyed rapidly in the returning spring sunlight.
Q68.
Green chemistry seeks to design chemical processes that:
Answer: B. minimise hazardous substances
Why: Green chemistry aims to prevent pollution by reducing or eliminating hazardous substances and waste.
Q69.
Which international agreement was designed specifically to protect the ozone layer?
Answer: B. Montreal Protocol
Why: The 1987 Montreal Protocol phased out ozone-depleting substances such as CFCs.
Q70.
Dissolved oxygen in a river or lake is essential mainly for:
Answer: B. aquatic life
Why: Fish and other aquatic organisms depend on dissolved oxygen for respiration.
Q71.
The nitrogen oxides that trigger photochemical smog come chiefly from:
Answer: B. vehicle exhaust
Why: High-temperature combustion in vehicle engines produces the NOₓ that seeds photochemical smog.
Q72.
A biodegradable pollutant is one that:
Answer: B. is decomposed by microorganisms
Why: Biodegradable pollutants (e.g. sewage) can be broken down naturally by microbes, unlike persistent pollutants.
Q73.
The primary pollutant that reacts in sunlight to generate photochemical smog is:
Answer: B. nitrogen dioxide
Why: NO₂ photodissociates in sunlight to start the chain of reactions producing ozone and other smog components.
Q74.
A greenhouse gas that also depletes ozone and has a very high warming potential per molecule is:
Answer: B. a chlorofluorocarbon
Why: CFCs are both potent greenhouse gases and ozone-depleting; each molecule traps far more heat than CO₂.
Q75.
The process by which excess plant nutrients cause dense algal growth and later oxygen depletion is:
Answer: A. eutrophication
Why: Eutrophication is nutrient over-enrichment leading to algal blooms and subsequent oxygen loss.