NEET Environmental Chemistry Previous Year Questions with Solutions

Eutrophication is defined as the process in which a water body becomes overly enriched with nutrients (primarily phosphates and nitrates from agricultural runoff, fertilizers, and sewage):

Statement I asserts that "nutrient deficient water bodies lead to eutrophication." This directly contradicts the canonical definition, as oligotrophic (nutrient-deficient) waters support low biological productivity and do NOT undergo eutrophication. Statement I is INCORRECT:

Nutrient enrichment stimulates explosive growth of algae (algal bloom). When the dense algal mass dies, aerobic bacteria consume enormous quantities of dissolved oxygen to decompose the dead organic matter. Consequently, the concentration of dissolved oxygen (DO) plummets below critical levels, killing fish and other aquatic organisms. Statement II is TRUE:

Sulfur dioxide ($SO_2$) emitted from burning fossil fuels is oxidized in the troposphere to sulfur trioxide ($SO_3$). In the absence of catalysts this is slow, but in polluted air with particulate matter and ozone it occurs rapidly, contributing directly to gaseous tropospheric pollution: $$2\text{SO}_2(g) + \text{O}_2(g) \longrightarrow 2\text{SO}_3(g) \implies \mathbf{\text{(a)-(iv)}}$$

In the Antarctic winter, polar stratospheric clouds form and convert inactive chlorine species ($ClONO_2$ and $HCl$) into hypochlorous acid ($HOCl$). In spring, when sunlight returns, $HOCl$ undergoes photolysis, releasing highly reactive chlorine free radicals ($\dot{C}l$) that initiate catalytic chain reactions destroying stratospheric ozone: $$\text{HOCl}(g) \xrightarrow{h\nu} \dot{\text{O}}\text{H} + \dot{\text{C}}\text{l} \implies \mathbf{\text{(b)-(iii)}}$$

Acid rain contains sulfuric and nitric acids. When sulfuric acid falls on marble monuments ($CaCO_3$, such as the Taj Mahal), it attacks the calcium carbonate, producing calcium sulfate, water, and carbon dioxide. This phenomenon is known as "stone cancer" caused by acid rain: $$\text{CaCO}_3 + \text{H}_2\text{SO}_4 \longrightarrow \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2 \implies \mathbf{\text{(c)-(i)}}$$

Nitrogen dioxide ($NO_2$) absorbs sunlight and photolyzes into nitric oxide ($NO$) and nascent oxygen ($O$). The reactive oxygen atom rapidly combines with atmospheric $O_2$ to form ozone ($O_3$), which subsequently oxidizes unburnt hydrocarbons to acrolein, formaldehyde, and peroxyacetyl nitrate (PAN), forming photochemical smog: $$\text{NO}_2(g) \xrightarrow{h\nu} \text{NO}(g) + \text{O}(g) \implies \mathbf{\text{(d)-(ii)}}$$

Carbon monoxide ($$\text{CO}$$) is a dangerous gaseous air pollutant produced by incomplete combustion of carbonaceous fuels (coal, firewood, petrol). When inhaled, it crosses the alveolar membrane and binds to the ferrous iron ($$\text{Fe}^{2+}$$) of blood haemoglobin to form carboxyhaemoglobin.

NCERT states verbatim in Environmental Chemistry: "Carboxyhaemoglobin is about 300 times more stable than the oxygen-haemoglobin complex." Because CO binds with 300-fold higher affinity, it displaces oxygen and prevents reversible oxygen binding.

When concentration of carboxyhaemoglobin reaches 3-4%, the oxygen-carrying capacity of blood is drastically reduced, leading to headache, impaired vision, cardiovascular stress, and fatal asphyxiation.

Statement (3) asserts that carboxyhaemoglobin is "less stable than oxyhaemoglobin." This directly contradicts biochemistry, making Statement (3) the correct answer to the question.