GATE — Environmental Science and Engineering (ES) · India · IITs and IISc on rotation (2026: IIT Guwahati)
Original practice questions written to the published syllabus · not past papersThe chemistry of water, soil and air that pollution, treatment and climate subjects assume: equilibria, the carbonate system, redox, gas solubility, ozone and smog chemistry. GATE ES Section 2 in full.
In the exams: GATE ES Section 2; UGC NET Environmental Sciences Unit 2; ICAR AIEEA-PG Physical Science Unit II (chemistry)
GATE — Environmental Science and Engineering, Section 2 (environmental chemistry)
Numerical problems on concentration units and CaCO3 equivalents, ionic strength and activity, equilibrium constants and ΔG°, first-order kinetics, pH, buffers, alkalinity forms and hardness, carbonate speciation, Henry's law, Ksp, redox and oxidation states, BOD/COD, and atmospheric unit conversions; concepts of ozone and smog chemistry.
How our questions for GATE spread across the units of Environmental Chemistry. Difficulty: 13 easy, 59 medium, 22 hard.
| Speciation of inorganic and organic contaminants | 9 | |
| Water quality parameters and their measurement | 9 | |
| Chemical equilibrium, thermodynamics and reaction kinetics | 8 | |
| Complexation, precipitation and solubility product | 8 | |
| Redox reactions, pe-pH diagrams and redox zonation | 8 | |
| Radioactivity and radionuclides in the environment | 8 | |
| Chemical bonding, concentration units, activity and ionic strength | 7 | |
| Acid-base equilibria, buffers and alkalinity | 7 | |
| Solubility of gases in water: Henry's law | 7 | |
| Tropospheric ozone and photochemical smog | 7 | |
| Stratospheric ozone chemistry and ozone-depleting substances | 6 | |
| The carbonate system and ocean acidification | 5 | |
| Composition and structure of the atmosphere | 5 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
What is the ionic strength of a 0.01 M CaCl2 solution?
A. 0.03 M
I = ½(0.01 × 2² + 0.02 × 1²) = ½(0.04 + 0.02) = 0.03 M. 0.01 is the salt concentration; 0.02 ignores charge squared for Ca; 0.05 omits the ½.
Further reading: See the manual sources: Sawyer, McCarty & Parkin; Manahan; Stumm & Morgan; Seinfeld & Pandis; IS 10500; WMO/UNEP ozone assessment
An acetate buffer contains 0.2 M acetate and 0.1 M acetic acid (pKa 4.76). What is its pH?
B. 5.06
pH = pKa + log([A⁻]/[HA]) = 4.76 + log 2 = 5.06. 4.76 assumes equal amounts; 4.46 inverts the ratio; 5.76 adds one full unit.
Further reading: See the manual sources: Sawyer, McCarty & Parkin; Manahan; Stumm & Morgan; Seinfeld & Pandis; IS 10500; WMO/UNEP ozone assessment
How does the saturation concentration of dissolved oxygen in water change as temperature rises?
D. It decreases
Gas solubility falls with temperature: about 14.6 mg/L at 0 °C, 9.1 at 20 °C and 7.6 at 30 °C.
Further reading: See the manual sources: Sawyer, McCarty & Parkin; Manahan; Stumm & Morgan; Seinfeld & Pandis; IS 10500; WMO/UNEP ozone assessment
Why do amphoteric metal hydroxides such as Al(OH)3 redissolve at very high pH?
D. They form soluble hydroxo-anions such as aluminate
Amphoteric hydroxides dissolve in strong base as aluminate and similar anions, so each metal has a pH of minimum solubility.
Further reading: See the manual sources: Sawyer, McCarty & Parkin; Manahan; Stumm & Morgan; Seinfeld & Pandis; IS 10500; WMO/UNEP ozone assessment
What is the oxidation state of sulphur in sulphate (SO4²⁻)?
B. +6
S + 4(−2) = −2, so S = +6. Sulphide is −2; sulphite +4.
Further reading: See the manual sources: Sawyer, McCarty & Parkin; Manahan; Stumm & Morgan; Seinfeld & Pandis; IS 10500; WMO/UNEP ozone assessment
Which parameter measures the oxygen required to chemically oxidise organic matter using potassium dichromate?
A. Chemical oxygen demand (COD)
COD uses dichromate in acid; BOD is microbial oxygen use over 5 days; DO is dissolved oxygen; TOC measures carbon.
Further reading: See the manual sources: Sawyer, McCarty & Parkin; Manahan; Stumm & Morgan; Seinfeld & Pandis; IS 10500; WMO/UNEP ozone assessment
Which radical is called the 'detergent of the atmosphere' because it initiates oxidation of most trace gases?
A. The hydroxyl radical (OH)
OH, formed from ozone photolysis and water vapour, oxidises methane, CO, VOCs, SO2 and NO2.
Further reading: See the manual sources: Sawyer, McCarty & Parkin; Manahan; Stumm & Morgan; Seinfeld & Pandis; IS 10500; WMO/UNEP ozone assessment
Why does the Antarctic ozone hole form mainly in spring?
D. Spring sun frees cloud-activated chlorine
Winter PSCs convert reservoirs to Cl2 and HOCl; spring sunlight releases Cl atoms that destroy ozone rapidly.
Further reading: See the manual sources: Sawyer, McCarty & Parkin; Manahan; Stumm & Morgan; Seinfeld & Pandis; IS 10500; WMO/UNEP ozone assessment
Which type of radiation is stopped by a sheet of paper but is most hazardous when inhaled or ingested?
A. Alpha particles
Alpha particles have low penetration but high ionisation; inside the body they damage tissue intensely.
Further reading: See the manual sources: Sawyer, McCarty & Parkin; Manahan; Stumm & Morgan; Seinfeld & Pandis; IS 10500; WMO/UNEP ozone assessment
Which radionuclide from nuclear accidents concentrates in the thyroid gland?
D. Iodine-131
I-131 (t½ ≈ 8 days) accumulates in the thyroid; Cs-137 behaves like potassium and Sr-90 like calcium.
Further reading: See the manual sources: Sawyer, McCarty & Parkin; Manahan; Stumm & Morgan; Seinfeld & Pandis; IS 10500; WMO/UNEP ozone assessment
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