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Practice each question, then open the answer or worked solution.
Which of the following gases is not a greenhouse gas?
(i) CO
(ii) \[{{\mathbf{O}}_{\mathbf{3}}}\]
(iii) \[\mathbf{C}{{\mathbf{H}}_{\mathbf{4}}}\]
(iv) \[{{\mathbf{H}}_{\mathbf{2}}}\mathbf{O}\] vapour
Ans: Option (i) CO is correct since it does not absorb sunlight near the earth’s surface and hence it is not radiated back to the earth. Greenhouse gases have this kind of property.
Photochemical smog occurs in warm, dry, and sunny climate. One of the following is not amongst the components of photochemical smog, identify it.
(i) \[\mathbf{N}{{\mathbf{O}}_{\mathbf{2}}}\]
(ii) \[{{\mathbf{O}}_{\mathbf{3}}}\]
(iii) \[\mathbf{S}{{\mathbf{O}}_{\mathbf{2}}}\]
(iv) Unsaturated hydrocarbon
Ans: Option (iii) \[\text{S}{{\text{O}}_{\text{2}}}\] is correct since photochemical fog is formed in the presence of sunlight in summers when NO and hydrocarbons are present in large amounts in the atmosphere. \[\text{S}{{\text{O}}_{\text{2}}}\] is not responsible for photochemical fog.
Which of the following statements is not true about classical smog?
(i) Its main components are produced by the action of sunlight on emissions of automobiles and factories.
(ii) Produced in cold and humid climates.
(iii) It contains compounds of reducing nature.
(iv) It contains smoke, fog, and Sulphur dioxide.
Ans: Option (i) i.e., Its main components are produced by the action of sunlight on emissions of automobiles and factories is the answer since classical smog is a mixture of smoke, fog and sulphur dioxide and occurs in cold humid climates. While the gases released by automobiles and factories are not responsible for classical fog.
Biochemical Oxygen Demand (BOD) is a measure of organic material present in water. BOD value less than 5 ppm indicates a water sample to be __________.
(i) rich in dissolved oxygen.
(ii) poor in dissolved oxygen.
(iii) highly polluted.
(iv) not suitable for aquatic life.
Ans: Option (i) i.e., rich in dissolved oxygen is the answer since the total amount of oxygen consumed by microorganisms in decomposing organic matter present in a certain volume of water is known as Biochemical Oxygen Demand (BOD) of water. If the BOD value is less than 5 ppm, then the sample of water is considered to be pure and is rich in dissolved oxygen.
Which of the following statements is wrong?
(i) Ozone is not responsible for the greenhouse effect.
(ii) Ozone can oxidize Sulphur dioxide present in the atmosphere to Sulphur trioxide.
(iii) Ozone hole is thinning of the ozone layer present in the stratosphere.
(iv) Ozone is produced in the upper stratosphere by the action of UV rays on oxygen.
Ans: Option (i) i.e., Ozone is not responsible for greenhouse effect is the answer since \[{{\text{O}}_{\text{3}}}\] is responsible for the greenhouse effect, its contribution is about 8% to 10%. About 75% of the solar energy reaching the earth is absorbed by the earth’s surface, which increases its temperature. The rest of the heat radiates back to the atmosphere while some of the heat is trapped by gases such as carbon dioxide, methane, ozone, chlorofluorocarbon compounds (CFCs) and water vapour in the atmosphere. Thus, they add to the heating of the atmosphere. This is the reason behind global warming.
Sewage containing organic waste should not be disposed of in water bodies because it causes major water pollution. Fishes in such a polluted water die because of
(i) Large number of mosquitoes.
(ii) Increase in the amount of dissolved oxygen.
(iii) Decrease in the amount of dissolved oxygen in water.
(iv) Clogging of gills by mud.
Ans: Option (iii) i.e., Decrease in the amount of dissolved oxygen in water is the answer since organic waste is oxidized by microorganisms in the presence of dissolved oxygen. Hence, oxygen decreases in water as a result it is harmful for aquatic life.
Which of the following statements about photochemical smog is wrong?
(i) It has a high concentration of oxidizing agents.
(ii) It has a low concentration of oxidizing agents.
(iii) It can be controlled by controlling the release of NO2 , hydrocarbons, ozone etc.
(iv) Plantation of some plants like Pinus helps in controlling photochemical smog.
Ans: Option (ii) i.e., It has low concentration of oxidizing agents is the answer since the common components of photochemical smog are ozone, nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate (PAN). Photochemical smog causes serious health problems and both ozone and PAN act as powerful eye irritants. Photochemical smog contains high concentrations of oxidants.
The gaseous envelope around the earth is known as the atmosphere. The lowest layer of this is extended up to 10 km from sea level, this layer is _________.
(i) Stratosphere
(ii) Troposphere
(iii) Mesosphere
(iv) Hydrosphere
Ans: Option (ii) Troposphere is correct since the lowest region of atmosphere in which the human beings along with other organisms live is called troposphere. It extends up to the height of approx. 10 km from sea level.
Dinitrogen and dioxygen are main constituents of air, but these do not react with each other to form oxides of nitrogen because _________.
(i) the reaction is endothermic and requires very high temperatures.
(ii) the reaction can be initiated only in presence of a catalyst.
(iii) oxides of nitrogen are unstable.
(iv) \[{{\mathbf{N}}_{\mathbf{2}}}\] and \[{{\mathbf{O}}_{\mathbf{2}}}\] are unreactive.
Ans: Option (i) the reaction is endothermic and requires very high temperature is correct since dinitrogen and dioxygen are the main constituents of air. These gases do not react with each other at a normal temperature. The dissociation energy of \[{{\text{N}}_{\text{2}}}\]is very high due to the presence of triple bond and it is very stable.
The pollutants which come directly in the air from sources are called primary pollutants. Primary pollutants are sometimes converted into secondary pollutants. Which of the following belongs to secondary air pollutants?
(i) CO
(ii) Hydrocarbon
(iii) Peroxyacetyl nitrate
(iv) NO
Ans: Option (iii) Peroxyacetyl nitrate is correct since,
\[\text{N}{{\text{O}}_{\text{2}}}\text{(g)}\xrightarrow{\text{hv}}\text{NO(g)+ }\!\![\!\!\text{ O }\!\!]\!\!\text{ }\] (i)
Oxygen atoms are very reactive and combine with the \[{{\text{O}}_{\text{2}}}\] in air to produce ozone.
\[\text{ }\!\![\!\!\text{ O }\!\!]\!\!\text{ +}{{\text{O}}_{\text{2}}}\text{(g)}\to {{\text{O}}_{\text{3}}}\text{(g)}\] (ii)
The ozone formed in the above reaction (ii) reacts rapidly with the NO(g) formed in the reaction (i) to regenerate \[\text{N}{{\text{O}}_{\text{2}}}\]. \[\text{N}{{\text{O}}_{\text{2}}}\]is a brown gas and at sufficiently high levels can contribute to haze.
\[\text{NO(g)+}{{\text{O}}_{\text{3}}}\text{(g)}\to \text{N}{{\text{O}}_{\text{2}}}\text{(g)+}{{\text{O}}_{\text{2}}}\text{(g)}\]
Ozone is a toxic gas and both \[\text{N}{{\text{O}}_{\text{2}}}\] and \[{{\text{O}}_{\text{3}}}\] are strong oxidizing agents and can react with the unburnt hydrocarbons in the polluted air to produce chemicals such as formaldehyde, Acrolein and peroxyacetyl nitrate (PAN) etc.