Questions & explanations
1. In which layer of the atmosphere does temperature increase with altitude?
- Troposphere
- Mesosphere
- Thermosphere
- Stratosphere
Answer: Thermosphere
In the thermosphere, temperature increases with altitude due to absorption of high-energy solar radiation (X-rays and UV) by gases like oxygen and nitrogen. This layer extends from about 80 km upward, and temperatures can reach over 1000°C. The stratosphere also shows an increase, but the thermosphere is the most prominent and highest layer with this property.
2. Why are N₂ and O₂ not greenhouse gases while CO₂ and H₂O are?
- N₂ and O₂ have no vibrational mode that changes dipole moment.
- N₂ and O₂ are nonpolar, so they cannot absorb any radiation.
- N₂ and O₂ are lighter than air, so they escape the atmosphere.
- N₂ and O₂ do not absorb infrared because they are diatomic.
Answer: N₂ and O₂ have no vibrational mode that changes dipole moment.
For a molecule to absorb infrared radiation, its vibration must change its dipole moment. Homonuclear diatomic molecules like N₂ and O₂ have no dipole moment change during vibration, making them IR-inactive. CO₂ and H₂O have vibrational modes that change dipole moment, so they are greenhouse gases.
3. A river water sample has DO = 3 ppm, BOD = 22 ppm, NO3- = 80 ppm, Pb = 0.08 ppm. Which statement is correct?
- Aquatic life is stressed; BOD indicates high pollution; NO3- and Pb exceed BIS limits causing blue-baby syndrome and neurotoxicity.
- Aquatic life is viable; BOD indicates moderate pollution; only NO3- exceeds BIS limit causing blue-baby syndrome.
- Aquatic life is viable; BOD indicates clean water; only Pb exceeds BIS limit causing neurotoxicity.
- Aquatic life is stressed; BOD indicates moderate pollution; NO3- and Pb are within BIS limits.
Answer: Aquatic life is stressed; BOD indicates high pollution; NO3- and Pb exceed BIS limits causing blue-baby syndrome and neurotoxicity.
DO = 3 ppm is below the 5-6 ppm needed for healthy aquatic life, so aquatic life is stressed. BOD = 22 ppm exceeds 17 ppm, indicating high pollution. NO3- = 80 ppm exceeds the BIS limit of 50 ppm, causing blue-baby syndrome. Pb = 0.08 ppm exceeds the BIS limit of 0.05 ppm, causing neurotoxicity.
4. Which of the following best describes the human perturbation of the carbon cycle?
- Deforestation adds ~10 Gt C/yr, oceans absorb ~10 Gt C/yr, keeping CO₂ constant.
- Fossil fuel combustion adds ~10 Gt C/yr, natural sinks absorb ~5 Gt C/yr, leading to net atmospheric CO₂ increase of ~2 ppm/yr.
- Photosynthesis removes ~10 Gt C/yr, respiration adds ~5 Gt C/yr, leading to net decrease of ~5 Gt C/yr.
- Volcanic eruptions add ~10 Gt C/yr, weathering removes ~10 Gt C/yr, maintaining steady state.
Answer: Fossil fuel combustion adds ~10 Gt C/yr, natural sinks absorb ~5 Gt C/yr, leading to net atmospheric CO₂ increase of ~2 ppm/yr.
Human activities, mainly fossil fuel combustion, add about 10 Gt C per year to the atmosphere. Natural sinks (oceans, forests) absorb only about 5 Gt C per year, resulting in a net increase of about 5 Gt C per year, which corresponds to an atmospheric CO₂ rise of about 2 ppm per year.
5. A coal plant burns 1000 kg/h of coal with 2% sulfur. A limestone scrubber removes 95% of SO2. What mass of CaCO3 (in kg/h) is required?
- 31.3
- 59.4
- 62.5
- 118.8
Answer: 59.4
Using stoichiometry: S + O2 → SO2, then CaCO3 + SO2 → CaSO3 + CO2. Sulfur mass = 20 kg/h, moles S = 0.625 kmol/h, SO2 produced = 0.625 kmol/h. Scrubber removes 95%: SO2 removed = 0.59375 kmol/h. Moles CaCO3 needed = 0.59375 kmol/h, mass = 0.59375 × 100 = 59.375 ≈ 59.4 kg/h.
6. Which of the following is the correct definition of environmental chemistry?
- Study of chemical species, their sources, reactions, transport, effects and fates in the environment
- Study of the physical structure of the Earth's crust and its mineral composition
- Study of the biological interactions among living organisms in an ecosystem
- Study of the chemical reactions that occur only in the atmosphere
Answer: Study of chemical species, their sources, reactions, transport, effects and fates in the environment
Environmental chemistry, as per legacy NCERT Class 11 Chemistry Part II Ch 14 §14.1, is defined as the study of chemical species, their sources, reactions, transport, effects, and fates in the environment. Option a correctly captures this definition.
7. Rising atmospheric CO₂ causes ocean acidification. Which statement about its effect on marine life is correct?
- Increased H⁺ reacts with CaCO₃ to form Ca(HCO₃)₂, making shells stronger.
- Increased H⁺ shifts CaCO₃ equilibrium right, increasing CO₃²⁻ for shell formation.
- Increased H⁺ shifts CaCO₃ equilibrium left, reducing CO₃²⁻ for shell formation.
- Increased H⁺ has no effect on CaCO₃ equilibrium because oceans are buffered.
Answer: Increased H⁺ shifts CaCO₃ equilibrium left, reducing CO₃²⁻ for shell formation.
Ocean acidification increases H⁺ concentration. H⁺ combines with CO₃²⁻ to form HCO₃⁻, reducing CO₃²⁻ availability. This shifts the equilibrium Ca²⁺ + CO₃²⁻ ⇌ CaCO₃(s) to the left, making shell formation harder for marine organisms.
8. Why does the Antarctic ozone hole appear in spring rather than in winter?
- Because sunlight is needed to photolyse Cl2 and HOCl into chlorine radicals.
- Because polar stratospheric clouds form only in spring.
- Because CFCs are transported to Antarctica only in spring.
- Because the ozone layer is naturally thinner in spring.
Answer: Because sunlight is needed to photolyse Cl2 and HOCl into chlorine radicals.
In winter, polar stratospheric clouds convert inactive chlorine reservoirs into Cl2 and HOCl. When spring sunlight returns, it photolyses these molecules, releasing chlorine radicals that catalytically destroy ozone.
9. Which heavy metal is associated with the Minamata disease outbreak in Japan?
- Mercury
- Cadmium
- Lead
- Arsenic
Answer: Mercury
Minamata disease was caused by methylmercury (CH3Hg+) discharged from a chemical factory into Minamata Bay. The mercury bioaccumulated in fish and caused severe neurological damage in villagers who consumed the fish.
10. The Ozone Depletion Potential (ODP) of HCFC-22 relative to CFC-12 is approximately:
- 1.0
- 0.5
- 0.05
- 0.005
Answer: 0.05
HCFC-22 has a hydrogen atom, making it reactive with OH radicals in the troposphere, so less chlorine reaches the stratosphere. Its ODP is about 0.05, while CFC-12 has ODP 1.0. Thus the relative ODP is 0.05.
11. A paper mill bleaches pulp with Cl2, producing chloro-organic waste. Which green chemistry principle is violated by the old process?
- Principle 9 (catalysis) because Cl2 is a catalyst; a stoichiometric reagent would be greener.
- Principle 1 (prevention) because it is better to prevent waste than to treat it; the old process generates chloro-organic waste.
- Principle 2 (atom economy) because Cl2 bleaching has low atom economy; H2O2 bleaching has higher atom economy.
- Principle 5 (safer solvents) because Cl2 is toxic; H2O2 is a greener alternative.
Answer: Principle 1 (prevention) because it is better to prevent waste than to treat it; the old process generates chloro-organic waste.
Principle 1 (prevention) is violated because the old process generates hazardous chloro-organic waste, which could be prevented by using a greener alternative like H2O2. Prevention is better than treatment.
12. Aerobic composting of glucose produces CO₂ and H₂O. What are the products of anaerobic digestion of glucose?
- CH₄ and CO₂
- CO₂ and H₂O
- CH₄ and H₂O
- C₂H₅OH and CO₂
Answer: C₂H₅OH and CO₂
Anaerobic digestion of glucose by yeast or bacteria under oxygen-free conditions typically yields ethanol and carbon dioxide via alcoholic fermentation. The balanced equation is C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂.