IMO Practice Test: Environmental Chemistry | Class 11 Chemistry

Question 1 of 13

One Cl atom can destroy thousands of O₃ molecules because in the depletion cycle Cl is:

consumed permanently

regenerated (acts as a catalyst)

converted to CO₂

fixed into HNO₃

Explanation: ClO• + O → Cl• + O₂ regenerates Cl, so it acts catalytically in a chain.

Question 2 of 13

Both London smog and photochemical smog harm health, but they differ chemically: London smog is reducing while photochemical smog is:

neutral

oxidising

basic

inert

Explanation: Photochemical smog contains oxidants (O₃, PAN); London smog (SO₂/soot) is reducing.

Question 3 of 13

A factory effluent raises a river’s BOD from 4 to 25 ppm. The most likely consequence downstream is:

oxygen depletion and fish kills

increased fish population

higher pH only

ozone formation

Explanation: High BOD means microbes consume oxygen rapidly, lowering DO and killing fish.

Question 4 of 13

If a synthesis gives 90% yield but only 35% atom economy, the best green-chemistry conclusion is that the process:

is fully green

wastes much reactant mass as by-products

produces no waste

has high atom economy

Explanation: High yield with low atom economy means most reactant atoms leave as unwanted by-products.

Question 5 of 13

Which sequence correctly orders the atmosphere from ground upward?

stratosphere then troposphere

troposphere then stratosphere

mesosphere then troposphere

stratosphere then mesosphere then troposphere

Explanation: The troposphere (0–10 km) lies below the stratosphere (10–50 km).

Question 6 of 13

Replacing chlorine bleaching with H₂O₂ reduces pollution mainly by avoiding:

water vapour

oxygen gas

toxic organochlorine by-products

carbon dioxide

Explanation: Chlorine bleaching forms persistent organochlorines; H₂O₂ gives only water and O₂.

Question 7 of 13

Eutrophication and BOD are linked because algal-bloom decay:

adds oxygen to water

lowers BOD

raises BOD and depletes dissolved oxygen

removes nutrients permanently

Explanation: Decaying algae are organic matter; their microbial breakdown raises BOD and uses up DO.

Question 8 of 13

Match the pollutant to its dominant effect: NO₂ contributes to BOTH:

acid rain and photochemical smog

ozone hole and eutrophication

global warming and fluorosis

soil pesticide and algal bloom

Explanation: NO₂ forms HNO₃ (acid rain) and drives photochemical smog with hydrocarbons.

Question 9 of 13

A water sample has DO close to zero and BOD above 20 ppm. The water is best described as:

pristine

ideal for fish

free of organic matter

anoxic and heavily polluted

Explanation: Near-zero DO with very high BOD indicates severe organic pollution and oxygen starvation.

Question 10 of 13

DDT biomagnifies, meaning its concentration is highest in organisms that are:

at the bottom of the food chain

plants only

bacteria only

at the top of the food chain

Explanation: Fat-soluble DDT accumulates and concentrates upward, peaking in top predators.

Question 11 of 13

The greenhouse effect occurs because greenhouse gases:

absorb outgoing infrared radiation

block incoming sunlight

destroy ozone

raise the water table

Explanation: They let sunlight in but trap the infrared (heat) re-emitted by the Earth.

Question 12 of 13

Which is the BEST single statement of a green-chemistry strategy?

dilute the waste in rivers

design the process to avoid making hazardous waste

store waste in landfills

burn waste in the open

Explanation: Green chemistry prevents hazardous waste at the design stage rather than disposing of it later.

Question 13 of 13

Why is supercritical or liquid CO₂ attractive as a solvent in green chemistry?

it is highly toxic

it forms acid rain

it is non-toxic and easily recovered/reused

it depletes ozone

Explanation: Liquid/supercritical CO₂ is non-toxic, leaves no solvent residue and can be recycled.

IMO Practice Test — Environmental Chemistry