Free Environmental Chemistry MCQs with Answers

141. Chlorofluorocarbons (CFCs) are important compounds that are used as refrigerants but these are also responsible for ozone layer depletion. If a chlorofluorocarbon CFCl3 is present in stratosphere, which of it's reaction intermediates are actually responsible for the breakdown of the ozone molecule?

• A. CFCl2* and Cl*
• B. Cl* and ClO*
• C. CFCl2* and ClO*
• D. CFCl2 and CFCl3

Explanation: Ozone breakdown is mediated by free radicals derived from CFCs. Free radicals have unpaired electrons and are highly unstable, thereby breaking down ozone into oxygen. As per the following reaction mechanism, you can see that chlorine radicals are the chief substance involved in ozone depletion, as it catalyses the breakdown of ozone into oxygen and gets regenerated at the end of the reaction via forming a ClO* intermediate, enabling it to break down more ozone molecules.As such, the option, containing Cl* and ClO*, is correct, which is option B.Option A contains a carbon or alkyl containing radical, which is not as reactive as chloride radical due to the presence of a stabilising carbon/ alkyl group, and does not mention ClO*. Option C does not mention the chloride radical.Option D merely mentions examples of CFCs.

142. Aerobic decomposition of organic matter i.e. glucose by bacteria in water sediments produces?

• A. Propene
• B. Ethane
• C. Methane
• D. Butane

Explanation: One natural source of methane is the anaerobic decomposition of organic matter by bacteria in water sediments and in soils.

143. Choose the factor that increases the BOD of water supply:

• A. CO2
• B. O3
• C. H2O
• D. C2H5OH

Explanation: The correct answer is option A: CO2. When the levels of CO2 increase, it can stimulate bacterial growth, leading to higher BOD values in the water supply. Options B, C, and D are incorrect because ozone (Option B) can actually help reduce organic pollutants, water (Option C) does not directly impact BOD levels, and the impact of ethanol (Option D) on BOD levels is variable depending on concentration and water composition.

144. Which oxide is released as a pollutant by vehicles?

• A. CO
• B. NO2
• C. Both Options A and B are correct
• D. None of these options are correct

Explanation: Carbon monoxide, nitrogen oxides, and hydrocarbons are released as pollutants when fuel burns in vehicles.

145. Choose the compound that produces toxic effects:

• A. Carbon dioxide-Hb
• B. Carbon monoxide-Hb
• C. Oxygen-Hb
• D. Carbon suboxide-Hb

Explanation: Carbon monoxide is a poisonous gas that is 300 times more reactive than oxygen. It reacts with hemoglobin and produces a toxic effect.

146. Increased concentration of SO2 and NO2 in the atmosphere causes _.

• A. Acid rain
• B. Global warming
• C. Ozone formation
• D. U.V. rays absorption

Explanation: Sulfur dioxide and nitrogen oxides present in the air reacts with water, oxygen etc. in the atmosphere and cause acid rain by forming acidic pollutant nitric acid, sulphuric acid etc.

147. The ozone layer that absorbs sun's ultraviolet radiation is present in _ layer of atmosphere.

• A. Thermosphere
• B. Mesosphere
• C. Stratosphere
• D. Troposphere

Explanation: The ozone layer that absorbs sun's harmful ultraviolet radiation is present in stratosphere layer of atmosphere. The stratosphere layer is about 50 kilometers far from earth surface.

148. Choose the factor that is responsible for ozone layer depletion:

• A. CFCs
• B. CBrCs
• C. CH2Br2
• D. All are correct

Explanation: Chlorofluorocarbons (CFCs) that are produced by air conditioners, refrigerators, etc. are responsible for ozone layer depletion. It is also known as Freon and its life in the atmosphere is about 20 - 100 years.

149. Besides carbondioxide and nitrous oxide, the other greenhouse gas is:

• A. Methane
• B. Carbon monoxide
• C. Both a and b
• D. None of these

Explanation: Greenhouse gases such as carbon dioxide, nitrous oxide, methane, carbon monoxide, water vapors, and CFCs (chlorofluorocarbons) causes the greenhouse effect on plants.

150. All oxides of N are common pollutant except _.

• A. NO2
• B. N2O5
• C. NO
• D. N2O

Explanation: NO2, NO and N2O gases are common air pollutant except N2O5.