Environmental Chemistry – NDA Chemistry PYQs

Solve NDA Chemistry previous-year questions on Environmental Chemistry with detailed solutions and explanations.

Chapter-wise PYQs • Concept-based explanations • Exam insights

NDA 2025-I

Q. 1. For acid rain, the pH of rain water should be less than

(a) 7·0

(b) 6·6

(d) 8·0

Answer: (c) 5·6

Explanation: Normal (unpolluted) rain has a pH of about 5.6 due to dissolved CO₂ forming carbonic acid. Acid rain is defined as rain with pH below 5.6 – caused by SO₂ and NOₓ pollutants forming stronger acids (H₂SO₄, HNO₃). Concept Tested: Acid rain definition – pH below 5.6

NDA 2019-I

Q. 2. Which one of the following is an example of Salt-Crystal growth?

(a) Chemical weathering

(b) Physical weathering

(d) Bio-chemical weathering

Answer: (b) Physical weathering

Explanation: Salt crystal growth (salt weathering) is a physical weathering process. Salt crystals grow in rock pores and cracks, exerting pressure that splits the rock. No chemical reaction occurs – only physical stress from crystal growth. Concept Tested: Salt crystal growth – physical weathering

NDA 2018-II

Q. 3. Which one of the following is NOT a process of chemical weathering?

(a) Solution

(b) Carbonation

(d) Exfoliation

Answer: (d) Exfoliation

Explanation: Exfoliation (onion-skin weathering) is a PHYSICAL weathering process – layers of rock peel off due to differential expansion/contraction. Solution (dissolving minerals), carbonation (CO₂ + H₂O + minerals), and oxidation (reaction with O₂) are all chemical weathering processes. Concept Tested: Physical vs chemical weathering – exfoliation is physical

NDA 2018-I

Q. 4. Chemical weathering of rocks is much greater in a place with

(a) cold and dry climate

(b) hot and humid climate

(d) cold and humid climate

Answer: (b) hot and humid climate

Explanation: Chemical weathering (hydrolysis, oxidation, carbonation) is accelerated by heat and moisture. Hot temperatures increase reaction rates. Humidity (water) is essential for hydrolysis and carbonation reactions. Hot and humid climates therefore give the greatest chemical weathering. Concept Tested: Chemical weathering – greatest in hot and humid climate

NDA 2017-II

Q. 5. Which one of the following gases is placed second in respect of abundance in the Earth’s atmosphere?

(a) Oxygen

(b) Hydrogen

(d) Carbon dioxide

Answer: (a) Oxygen

Explanation: Atmospheric composition: Nitrogen ~78%, Oxygen ~21%, Argon ~0.9%, CO₂ ~0.04%. Oxygen is the second most abundant gas in the Earth’s atmosphere. Concept Tested: Second most abundant gas in atmosphere – oxygen (21%)

Q. 6. Which one of the following is a cause of acid rains?

(a) Ozone

(b) Ammonia

(d) Carbon monoxide

Answer: (c) Sulphur dioxide

Explanation: Acid rain is mainly caused by sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) released by burning fossil fuels. SO₂ reacts with water and oxygen in the atmosphere to form sulphuric acid (H₂SO₄), which falls as acid rain. CO and ozone are not primary causes of acid rain. Concept Tested: Acid rain cause – sulphur dioxide (SO₂)

NDA 2016-II

Q. 7. Statement I: Ozone is a tri-atomic molecule of oxygen. Statement II: Ozone is concentrated mainly in stratosphere.

(a) Both the statements are individually true and Statement II is the correct explanation of Statement I

(b) Both the statements are individually true but Statement II is not the correct explanation of Statement I

(d) Statement I is false but Statement II is true

Answer: (b) Both the statements are individually true but Statement II is not the correct explanation of Statement I

Explanation: Statement I is correct: ozone (O₃) is a triatomic molecule of oxygen. Statement II is correct: the ozone layer is concentrated in the stratosphere (15–35 km altitude). However, the fact that ozone is in the stratosphere does not explain or relate to why it is a triatomic molecule. Both are true but II does not explain I. Concept Tested: Ozone – triatomic molecule; located in stratosphere

NDA 2016-I

Q. 8. One of the main causes of air pollution in cities is emissions from vehicles like cars and trucks. Cars emit various pollutants which are bad for human health when inhaled, like 1. Nitrogen oxides (NOₓ) 2. Carbon monoxide (CO) 3. Carbon dioxide (CO₂) 4. Benzene Which of the above pollutants are not tolerated by human beings even at very low levels ?

(a) 1 and 4 only

(b) 1 and 2 only

(d) 1, 2 and 4 only

Answer: (d) 1, 2 and 4 only

Explanation: NOₓ, CO and benzene are all highly toxic even at very low concentrations. CO binds to haemoglobin and is lethal. Benzene is a carcinogen. NOₓ causes respiratory damage. CO₂, while a pollutant, is tolerated at relatively higher concentrations by the human body. Concept Tested: Toxic vehicular pollutants at low levels – NOₓ, CO, benzene

NDA 2015-II

Q. 9. Which one of the following is not a Green House Gas ?

(a) Water vapours

(b) Methane

(d) Carbon Monoxide

Answer: (d) Carbon Monoxide

Explanation: Water vapour, methane (CH₄) and ozone (O₃) are all greenhouse gases – they absorb infrared radiation. Carbon monoxide (CO) is a pollutant but is NOT considered a significant greenhouse gas (it does not directly absorb IR radiation in the same way). Concept Tested: Non-greenhouse gas – carbon monoxide

Q. 10. Which one of the following statements is not correct?

(a) Application of lime makes the soil acidic

(b) High acidity in soil is typical of humid climate

(d) Arid climate is characterized by alkaline soil

Answer: (a) Application of lime makes the soil acidic

Explanation: Application of lime (CaO or Ca(OH)₂) makes soil alkaline (basic), NOT acidic. Lime is applied precisely to correct soil acidity. Options (b), (c) and (d) are all correct. Concept Tested: Liming of soil – lime increases alkalinity, not acidity

NDA 2015-I

Q. 11. When heat rays are reflected from Earth, gases like Carbon dioxide, Nitrous oxide do not allow them to escape back to the space causing our planet to heat up. These gases are known as

(a) Noble gas

(b) Green-house gas

(d) Blue gas

Answer: (b) Green-house gas

Explanation: Greenhouse gases (CO₂, N₂O, CH₄, water vapour) absorb and re-emit infrared (heat) radiation from the Earth’s surface, trapping heat in the atmosphere. This causes global warming – the greenhouse effect. Concept Tested: Greenhouse gases – CO₂, N₂O trap infrared radiation

NDA 2010-I

Q. 12. Which one of the following gases, present in the air near the surface of the Earth, has maximum concentration?

(a) Oxygen (O₂)

(b) Hydrogen (H₂)

(d) Methane (CH₄)

Answer: (c) Nitrogen (N₂)

Explanation: Nitrogen (N₂) constitutes approximately 78% of the atmosphere near the Earth’s surface – the highest concentration of any gas. Oxygen is ~21%, argon ~0.9%, CO₂ ~0.04%. Hydrogen and methane are present in trace amounts. Concept Tested: Most abundant gas in atmosphere near Earth – nitrogen (78%)

Q. 13. Match List–I with List–II and select the correct answer using the code given below the Lists:

List–I

A. Ozone gas

B. Nitrous oxide

C. Carbon dioxide

D. Carbon monoxide

List–II

1. Combines with haemoglobin

2. Ultraviolet radiation

3. Component of air in small quantity

4. Visible radiation

5. Infrared radiation

Code :

(a) A-1 B-4 C-5 D-3

(b) A-2 B-3 C-5 D-1

(d) A-2 B-5 C-3 D-1

(a) A-1 B-4 C-5 D-3

(b) A-2 B-3 C-5 D-1

(d) A-2 B-5 C-3 D-1

Answer: (b) A-2, B-3, C-5, D-1

Explanation: A. Ozone: absorbs UV radiation (UV = 2). B. Nitrous oxide (N₂O): a trace component of air (3). C. Carbon dioxide: absorbs infrared radiation (greenhouse gas, IR = 5). D. Carbon monoxide: combines with haemoglobin (1). Code (b). Concept Tested: Matching atmospheric gases to their properties

NDA 2009-II

Q. 14. Which one of the following is the correct order in which the gases from the atmosphere disappear as one moves away from the surface of the earth?

(a) Carbon dioxide—Oxygen—Nitrogen

(b) Oxygen—Nitrogen—Carbon dioxide

(d) Nitrogen—Oxygen—Carbon dioxide

Answer: (a) Carbon dioxide—Oxygen—Nitrogen

Explanation: Heavier gases are more concentrated near the Earth’s surface and thin out more rapidly with altitude. Molecular masses: CO₂ = 44, O₂ = 32, N₂ = 28. Heavier gases thin out first as altitude increases: CO₂ disappears first, then O₂, then N₂ (lightest, persists longest). Concept Tested: Atmospheric gases disappearing with altitude – in order of molecular mass

NDA 2009-I

Q. 15. Which among the following elements (metals) pollutes the air of a city having large number of automobiles?

(a) Cadmium

(b) Lead

(d) Nickel

Answer: (b) Lead

Explanation: Lead (Pb) is/was added to petrol as tetraethyl lead (TEL) to prevent engine knocking. Vehicles emit lead compounds into the air, causing lead pollution. This is why unleaded petrol was introduced. Lead is a major vehicle-related air pollutant. Concept Tested: Lead pollution from automobiles – tetraethyl lead (TEL)