UGC NET Environmental Science Unit-II: Environmental Chemistry Syllabus

How do chlorofluorocarbons (CFCs) catalytically destroy stratospheric ozone?

UV cleaves CFCs releasing Cl atoms: Cl + O₃ → ClO + O₂; ClO + O → Cl + O₂. The Cl is regenerated, so one Cl atom destroys thousands of O₃ molecules in a catalytic chain.

What is photochemical smog and what are its key ingredients and conditions?

Photochemical smog is an oxidizing haze formed when NOx and volatile organic compounds (VOCs) react under sunlight, producing ozone, PAN, aldehydes, and fine particulates. It needs sunlight, vehicle emissions, and stagnant warm air.

Why is water called a universal solvent, and which property is responsible?

Water dissolves a wide range of ionic and polar substances because of its polarity and high dielectric constant, plus its ability to form hydrogen bonds and hydrate ions, stabilizing dissolved species.

Define Dissolved Oxygen (DO) and state its significance in water quality.

DO is the amount of free molecular oxygen dissolved in water (mg/L). It is essential for aquatic life; high DO indicates good water quality, while low DO signals pollution and oxygen-demanding wastes.

Define BOD (Biochemical Oxygen Demand) and the standard test conditions.

BOD is the amount of dissolved oxygen consumed by microorganisms to biologically oxidize organic matter in water. The standard test (BOD₅) is measured over 5 days at 20°C, expressed in mg/L.

Define COD (Chemical Oxygen Demand) and how it compares with BOD.

COD is the oxygen equivalent required to chemically oxidize organic matter (using strong oxidants like K₂Cr₂O₇). COD ≥ BOD always, since COD oxidizes both biodegradable and non-biodegradable matter; a high COD/BOD ratio indicates poor biodegradability.

Differentiate sedimentation, coagulation, and flocculation in water treatment.

Sedimentation is gravity settling of suspended particles. Coagulation is destabilization of colloids by adding coagulants (e.g. alum) that neutralize surface charges. Flocculation is gentle mixing that aggregates the destabilized particles into larger settleable flocs.

What is the role of filtration in water treatment, and name a common filter type.

Filtration removes remaining suspended solids, flocs, and microorganisms by passing water through a porous medium. Common types: rapid sand filters and slow sand filters; sand/gravel beds are most widely used.

Define pH and redox potential (Eh) and what each indicates about an aquatic system.

pH = −log[H⁺], indicating acidity/alkalinity. Eh (redox potential, in volts) measures the tendency of a system to gain or lose electrons (oxidizing vs reducing conditions). Together (pe–pH/Eh–pH diagrams) they control speciation of elements.

Outline the key steps of the nitrogen biogeochemical cycle.

Nitrogen fixation (N₂ → NH₃/NH₄⁺), nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻), assimilation (uptake into organisms), ammonification (organic N → NH₄⁺), and denitrification (NO₃⁻ → N₂ returned to atmosphere).

Describe the main reservoirs and fluxes of the carbon cycle.

Carbon moves among the atmosphere (CO₂), oceans (dissolved CO₂/carbonates), biosphere (organic matter), and lithosphere (fossil fuels, carbonate rocks). Photosynthesis fixes CO₂; respiration, decomposition, and combustion release it.

What is distinctive about the phosphorus cycle compared with the nitrogen and carbon cycles?

The phosphorus cycle is a sedimentary cycle with no significant gaseous (atmospheric) phase. P is released by weathering of rocks (phosphate), cycled through organisms, and lost to ocean sediments; it is often the limiting nutrient in freshwater.

Outline the sulphur biogeochemical cycle.

Sulphur cycles among rocks/sediments, water, atmosphere, and organisms. Key transformations: mineralization of organic S to H₂S/SO₄²⁻, oxidation to sulphate, reduction (sulphate-reducing bacteria → H₂S), and atmospheric SO₂/sulphate from volcanism and fossil fuel burning.

How are pesticides classified based on chemical nature? Give examples.

Major classes: organochlorines (e.g. DDT, endosulfan), organophosphates (e.g. malathion, parathion), carbamates (e.g. carbaryl), and pyrethroids (e.g. permethrin). They are also classed by target as insecticides, herbicides, fungicides, rodenticides.

Why are organochlorine pesticides like DDT especially harmful in the environment?

They are persistent (resist degradation), lipophilic, and bioaccumulate and biomagnify up food chains, causing toxic effects (e.g. eggshell thinning in birds) and long-term ecosystem contamination.

What is the biochemical toxic effect of mercury (Hg), and what is its most toxic form?

Mercury binds to sulfhydryl (−SH) groups of proteins/enzymes, damaging the nervous system and kidneys. Methylmercury (CH₃Hg⁺) is the most toxic, highly bioaccumulative form, causing Minamata disease.

Describe the toxic effects of lead (Pb) and cadmium (Cd) on humans.

Lead inhibits heme synthesis enzymes (e.g. ALA dehydratase), causing anemia and neurotoxicity (especially in children). Cadmium damages kidneys and bones, causing Itai-itai disease, and disrupts Zn/Ca-dependent enzymes.

How does chromium toxicity depend on its oxidation state?

Cr(III) is an essential trace nutrient and relatively non-toxic, whereas Cr(VI) (chromate/dichromate) is highly toxic, mutagenic, and carcinogenic, being readily absorbed and a strong oxidant inside cells.

What are the toxicological concerns of the metalloids arsenic (As) and selenium (Se)?

Arsenic inhibits cellular respiration enzymes and is a carcinogen (causes skin lesions, cancers; arsenicosis from contaminated groundwater). Selenium is essential in trace amounts but toxic in excess (selenosis), with a narrow safe-to-toxic range.

What is PAN, how is it formed, and why is it significant?

PAN (peroxyacetyl nitrate, CH₃COOONO₂) is a secondary pollutant formed in photochemical smog from acetyl radicals + NO₂. It is a powerful eye irritant, phytotoxic to plants, and a reservoir that transports NOx over long distances.

Why are carbon monoxide (CO) and ground-level ozone (O₃) harmful pollutants?

CO binds hemoglobin ~200× more strongly than O₂, forming carboxyhemoglobin and causing tissue hypoxia. Ground-level O₃ is a strong oxidant that irritates the respiratory tract, damages lungs, and harms vegetation.

What are VOCs and POPs in the context of air pollution?

VOCs (volatile organic compounds, e.g. benzene, toluene) are organics that readily evaporate and serve as smog/ozone precursors. POPs (persistent organic pollutants, e.g. PCBs, dioxins, DDT) resist degradation, bioaccumulate, and travel long distances (Stockholm Convention).

Name important carcinogens found in air and a common source of each.

Benzene (vehicle exhaust, petrol), benzo[a]pyrene and other PAHs (incomplete combustion, smoke), formaldehyde (combustion, building materials), asbestos fibers (insulation), and 1,3-butadiene (vehicle exhaust).

What is titrimetry (volumetric analysis) and what defines the equivalence point?

Titrimetry determines an analyte's concentration by reacting it with a standard solution of known concentration until the reaction is complete. The equivalence point is when stoichiometrically equivalent amounts have reacted; the endpoint (indicator change) signals it.

What is gravimetric analysis and on what principle is the result calculated?

Gravimetry quantifies an analyte by converting it to a pure, weighable solid (usually a precipitate) and measuring its mass. The analyte amount is calculated from the mass of product using stoichiometry and the gravimetric factor.

What does a bomb calorimeter measure and under what conditions?

A bomb calorimeter measures the heat of combustion (calorific value) of a sample at constant volume, giving ΔU (internal energy change). Sample is burned in excess O₂ in a sealed bomb immersed in water; heat released raises the water temperature.

State the basic principle of chromatography and name the two phases involved.

Chromatography separates components of a mixture based on their differential distribution (partitioning/adsorption) between a stationary phase and a moving mobile phase; components with greater affinity for the mobile phase move faster.

What is the principle of flame photometry and which elements is it best suited for?

Flame photometry (flame emission spectroscopy) measures the intensity of light emitted by atoms excited in a flame, which is proportional to concentration. It is best for easily excited alkali and alkaline-earth metals: Na, K, Li, Ca, Ba.

State the principle of spectrophotometry and the law it relies on.

Spectrophotometry measures absorption (or transmission) of light by a solution at a specific wavelength. It relies on the Beer–Lambert law: A = εcl, where absorbance is proportional to concentration (c), path length (l), and molar absorptivity (ε).

What is electrophoresis and what governs the separation of species?

Electrophoresis separates charged molecules (ions, proteins, nucleic acids) by their migration in an electric field through a medium (e.g. gel). Separation depends on charge, size, and shape — smaller, more highly charged molecules move faster.

What do XRF and XRD analyze, and how do their purposes differ?

XRF (X-ray fluorescence) identifies and quantifies elemental composition by measuring characteristic X-rays emitted after excitation. XRD (X-ray diffraction) determines crystalline structure and mineral phases using Bragg's law (nλ = 2d sinθ).

What structural information do NMR and FTIR spectroscopy provide?

NMR (nuclear magnetic resonance) reveals the molecular/structural environment of nuclei (e.g. ¹H, ¹³C) — connectivity and number of H/C atoms. FTIR (Fourier-transform infrared) identifies functional groups by their characteristic vibrational absorption bands.

What does GC-MS do, and why is it powerful for environmental analysis?

GC-MS couples gas chromatography (separates volatile/semi-volatile compounds) with mass spectrometry (identifies/quantifies by mass-to-charge ratio). It enables sensitive identification and quantification of trace organic pollutants like pesticides, PAHs, and VOCs.

Compare SEM and TEM in terms of imaging and resolution.

SEM (scanning electron microscope) scans the surface with an electron beam to give 3D-like surface/topographic images at moderate resolution. TEM (transmission electron microscope) passes electrons through ultrathin samples for very high-resolution internal/structural images at near-atomic scale.