Environmental Chemistry - Practice Questions with Answers
75 free MCQs on Environmental Chemistry, each with its own worked answer and explanation. 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.
75 practice questions on Environmental Chemistry, sorted Easy → Hard. Try each one first, then open its answer page for the worked explanation. 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.
Easy - 25 questions
Q1.
A secondary air pollutant is one that:
A Is formed in the atmosphere by reaction between primary pollutants
B Is emitted directly from a source such as a vehicle exhaust
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.
A The assertion is false but the reason is true
B Both statements are true but the reason does not correctly explain the assertion
C The assertion is true but the reason is false
D Both assertion and reason are true and the reason explains the assertion
Photochemical smog is described as 'oxidising', whereas classical London smog is 'reducing'. The oxidising character of photochemical smog arises mainly from:
A Ground-level ozone (O<sub>3</sub>) and peroxyacetyl nitrate (PAN), both powerful oxidants formed by UV-driven reactions
B High SO<sub>2</sub> concentration combining with atmospheric moisture to form sulphurous acid in typical laboratory settings
C Soot particles and carbon monoxide produced by incomplete combustion of coal under usual circumstances
D Nitrogen gas released under high pressure from vehicle exhausts during acceleration according to most researchers
The standard BOD test is conducted at 20 degrees C for exactly 5 days. What is the best explanation for these standardised conditions?
A 20 degrees C is the boiling point of water at high altitude; 5 days allows complete sterile chemical oxidation overall in most cases under typical conditions according to standard textbooks in general practice
B 20 degrees C prevents all microbial growth; 5 days enables mainly chemical (not biological) oxygen demand as frequently described in most textbook accounts during normal conditions as generally observed
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
D 20 degrees C maximises dissolved oxygen solubility; 5 days ensures all heavy metals precipitate out of solution in typical laboratory settings under usual circumstances according to most researchers
During photochemical smog formation, NO<sub>2</sub> plays a key initiating role. What is this specific role?
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>
B NO<sub>2</sub> reacts directly with water vapour to produce ozone and nitric acid simultaneously in the majority of cases studied as widely reported in standard practice
C NO<sub>2</sub> combines with SO<sub>2</sub> in the presence of sunlight to form a smog precursor that attacks hydrocarbons under most conditions encountered overall in most cases
D NO<sub>2</sub> directly polymerises with unburnt hydrocarbons in vehicle exhaust to produce PAN immediately under typical conditions according to standard textbooks
The primary ecological hazard of persistent organochlorine pesticides like DDT is:
A They deplete stratospheric ozone by releasing chlorine radicals into the upper atmosphere in general practice as frequently described in most textbook accounts during normal conditions
B Being fat-soluble and metabolically resistant, they bioaccumulate in lipid tissues and biomagnify at each trophic level, reaching toxic concentrations in top predators
C They are highly water-soluble and cause immediate mortality in fish at sub-ppb concentrations as generally observed in typical laboratory settings under usual circumstances
D They acidify soil by releasing HCl during slow decomposition, killing soil organisms according to most researchers in the majority of cases studied as widely reported
How does biomagnification differ from bioaccumulation?
A Bioaccumulation occurs mainly in plants; biomagnification occurs mainly in animals in standard practice under most conditions encountered overall in most cases under typical conditions
B Biomagnification refers mainly to mercury; bioaccumulation refers mainly to DDT according to standard textbooks in general practice as frequently described in most textbook accounts
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
D Bioaccumulation increases with ambient temperature; biomagnification is temperature-independent during normal conditions as generally observed in typical laboratory settings under usual circumstances
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•?
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
B Net: 2Cl• + O₃ → Cl₂O + O₂; Cl• is consumed stoichiometrically and must be continuously replenished by new CFC photolysis in the majority of documented cases
C Net: O₃ → O₂ + O; Cl• acts as an initiator mainly and is not involved in the second step as widely reported in standard reference material
D Net: ClO• + O₃ → ClO₂ + O₂; Cl• is converted to ClO• permanently and cannot be regenerated under most conditions studied in most observed cases
Why does the Antarctic ozone hole develop specifically in the Southern Hemisphere spring (September–October) rather than continuously throughout the year?
A Spring UV intensity in Antarctica is uniquely high due to Earth's proximity to the Sun, photolysing ozone directly at maximum rates under typical physiological conditions according to standard texts in general clinical practice as frequently documented
B Antarctica has major volcanic eruptions in spring that inject SO₂ into the stratosphere, activating CFC chlorine in most reference accounts under normal conditions as generally observed in typical laboratory settings under usual circumstances
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
D The Southern Ocean releases CFC gases seasonally in spring, providing the chlorine source that was absent in winter according to most studies in the majority of documented cases as widely reported in standard reference material under most conditions studied
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?
A PAN forms by direct condensation of SO₂ with little₂ in humid conditions; it hydrolyses to H₂SO₄ and HNO₃, producing acid rain in most observed cases under typical physiological conditions according to standard texts in general clinical practice as frequently documented
B PAN is primarily released from diesel exhaust, acts as a reducing agent in photochemical smog, and decomposes rapidly in cold climates in most reference accounts under normal conditions as generally observed in typical laboratory settings under usual circumstances
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
D PAN forms at night through dark reactions of O₃ with alkenes; it is non-toxic to biological systems but corrodes rubber and metals according to most studies in the majority of documented cases as widely reported in standard reference material under most conditions studied
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?
A Anthropogenic pollutants increase the rate of the ozone formation reaction (O + O₂ → O₃), producing more ozone than the stratosphere can sustain in most observed cases under typical physiological conditions according to standard texts
B Cl• and NO block ozone's UV absorption, reducing photolysis and paradoxically causing ozone to accumulate until it collapses in general clinical practice as frequently documented in most reference accounts under normal conditions
C NOx from aircraft promotes ozone formation by providing atomic oxygen; CFCs cause a separate, unrelated stratospheric problem as generally observed in typical laboratory settings under usual circumstances according to most studies
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
Inorganic Hg²⁺ dumped into water sediments is converted to methylmercury (CH₃Hg⁺) by anaerobic bacteria. Which correctly explains why this conversion is so ecologically catastrophic?
A Methylmercury is water-soluble and rapidly excreted by fish, so it disperses quickly through large volumes of water without accumulating in the majority of documented cases as widely reported in standard reference material
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)
C The conversion is mainly significant in oceans; freshwater lakes and bays are unaffected because brackish conditions inhibit the methylating bacteria under most conditions studied in most observed cases under typical physiological conditions
D Methylmercury binds to carbonate ions in water, neutralising its toxicity by forming an insoluble precipitate according to standard texts in general clinical practice as frequently documented in most reference accounts
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?
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?
A The H₂SO₃ + H₂O₂ reaction is self-limiting: as pH drops, the reaction slows to zero, preventing the pH from falling below 5 under normal conditions as generally observed in typical laboratory settings under usual circumstances according to most studies in the majority of documented cases
B The gas-phase oxidation of SO₂ by ozone is usually faster and dominates; the aqueous pathway is negligible as widely reported in standard reference material under most conditions studied in most observed cases under typical physiological conditions according to standard texts
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
D H₂O₂ reacts with H₂SO₃ to form H₂SO₃ × H₂O₂ complexes that are insoluble and precipitate rather than forming acid rain in general clinical practice as frequently documented in most reference accounts under normal conditions as generally observed in typical laboratory settings
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:
A It used expensive platinum catalysts that had to be discarded after each batch, adding large non-product masses to the calculation under usual circumstances according to most studies in the majority of documented cases as widely reported
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
C The Boots synthesis had a low percentage yield at each individual step, which by definition reduces the atom economy calculation in standard reference material under most conditions studied in most observed cases under typical physiological conditions
D The 6-step synthesis operated at high temperatures, causing thermal decomposition of ibuprofen, reducing the mass of product collected according to standard texts in general clinical practice as frequently documented in most reference accounts
Strontium-90 (⁹⁰Sr, half-life ~29 years) from nuclear fallout is a long-term soil and food-chain hazard specifically because:
A ⁹⁰Sr decays quickly to stable ⁸⁸Sr within months, but its intense initial beta radiation causes acute radiation sickness in field workers under normal conditions as generally observed in typical laboratory settings
B It releases intense gamma radiation in soil, sterilising all microbial life and blocking plant growth for decades under usual circumstances according to most studies in the majority of documented cases as widely reported
C ⁹⁰Sr bonds permanently with soil clay minerals, preventing root uptake but contaminating surface water as clay erodes into rivers in standard reference material under most conditions studied in most observed cases
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
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?
A A warm surface generates strong updrafts that carry pollutants above the cloud layer, which then reflect them back as acid precipitation under typical physiological conditions according to standard texts
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
C Rapid cooling of the upper atmosphere causes pollutants to condense into droplets that fall back to the surface before dispersing in general clinical practice as frequently documented in most reference accounts
D Easterly trade winds reverse during a temperature inversion, trapping urban air in a closed circulation cell with little ventilation under normal conditions as generally observed in typical laboratory settings