Answer

(a) The gases which absorb sunlight near the earth’s surface and then its radiated back to the earth are called green house gases.

Carbon dioxide, water vapour, methane, ozone, oxides of nitrogen, chlorofluoro carbons etc; are green house gases. CO is not a green house gas.

• $\mathrm{O}_3$ (Ozone) is a greenhouse gas because it absorbs and emits infrared radiation, contributing to the greenhouse effect.
• $\mathrm{CH}_4$ (Methane) is a greenhouse gas because it has a high global warming potential and traps heat in the atmosphere.
• $\mathrm{H}_2\mathrm{O}$ vapour (Water vapour) is a greenhouse gas because it is the most abundant greenhouse gas and significantly contributes to the greenhouse effect by trapping heat in the atmosphere.

Answer

(c) The smog which is formed in presence of sunlight is called photochemical smog. This occurs in the months of summer when $\mathrm{NO} _{2}$ and hydrocarbons are present in large amounts in atmosphere.

Concentration of $\mathrm{O} _{3}$, PAN, aldehydes and ketones builds up in the atmosphere.

$\mathrm{SO} _{2}$ is not responsible for photochemical smog.

• (a) $\mathrm{NO}_2$: Nitrogen dioxide ($\mathrm{NO}_2$) is a key component in the formation of photochemical smog. It reacts with sunlight to produce ozone ($\mathrm{O}_3$) and other pollutants.

• (b) $\mathrm{O}_3$: Ozone ($\mathrm{O}_3$) is a major component of photochemical smog. It is formed when nitrogen oxides ($\mathrm{NO}_x$) and volatile organic compounds (VOCs) react in the presence of sunlight.

• (d) Unsaturated hydrocarbon: Unsaturated hydrocarbons, such as alkenes, are involved in the formation of photochemical smog. They react with nitrogen oxides and other pollutants under sunlight to produce secondary pollutants like ozone and peroxyacetyl nitrate (PAN).

Answer

(a) Smog are of two types-classical smog and photochemical smog.

Classical smog occurs in cold humid climate. It is a mixture of smoke, fog and sulphur dioxide. Chemically it is a reducing mixture thats why, it is also called as reducing smog.

Note Gases released by automobiles and factories are not responsible for classical smog.

• (b) This statement is actually true. Classical smog is indeed produced in cold and humid climates.
• (c) This statement is also true. Classical smog contains compounds of reducing nature.
• (d) This statement is true as well. Classical smog contains smoke, fog, and sulfur dioxide.

Answer

(a) The total amount of oxygen consumed by microorganisms (bacteria) in decomposing organic matter present in certain volume of a sample of water is called Biochemical Oxygen Demand (BOD) of the water.

Water considered to be pure if it has BOD less than 5 ppm whereas highly polluted water has BOD more than 17ppm.

Thus, water having BOD less than 5 ppm is rich in dissolved oxygen.

• (b) poor in dissolved oxygen: This option is incorrect because a BOD value less than 5 ppm indicates that there is a low amount of organic matter in the water, meaning that microorganisms are consuming less oxygen. Therefore, the water is actually rich in dissolved oxygen, not poor.

• (c) highly polluted: This option is incorrect because highly polluted water typically has a high BOD value, often greater than 17 ppm. A BOD value less than 5 ppm indicates that the water is relatively clean and not highly polluted.

• (d) not suitable for aquatic life: This option is incorrect because water with a BOD value less than 5 ppm is considered to be pure and rich in dissolved oxygen, making it suitable for aquatic life.

Answer

(a) $\mathrm{O} _{3}$ is responsible for greenhouse effect. Its contribution is about $8 $%.

Note The warming of earth due to re-emission of sun’s energy absorbed by the earth followed by its absorption by $\mathrm{CO} _{2}$ and $\mathrm{H} _{2} \mathrm{O}$ vapour present near the earth surface and then its radiation back to earth is called green house effect.

• (b) Ozone can oxidize sulfur dioxide present in the atmosphere to sulfur trioxide. This statement is correct, so it is not a reason for being incorrect.

• (c) Ozone hole is thinning of the ozone layer present in the stratosphere. This statement is correct, so it is not a reason for being incorrect.

• (d) Ozone is produced in the upper stratosphere by the action of UV rays on oxygen. This statement is correct, so it is not a reason for being incorrect.

Answer

(c) Dissolved oxygen is essential for aquatic life. Organic waste is oxidised by microorganisms using dissolved oxygen. Hence, oxygen from water decreases. It is harmful for aquatic life.

• (a) Large number of mosquitoes: Mosquitoes do not directly cause the death of fish. They may breed in stagnant water, but their presence does not significantly impact the dissolved oxygen levels or directly harm fish.

• (b) Increase in the amount of dissolved oxygen: An increase in dissolved oxygen would actually benefit aquatic life, including fish, as they rely on oxygen for survival. Therefore, this option is incorrect.

• (d) Clogging of gills by mud: While clogging of gills can harm fish, it is not the primary reason for fish deaths in water polluted by organic waste. The main issue is the depletion of dissolved oxygen due to the oxidation of organic waste by microorganisms.

Answer

(b) Photochemical smog or Los Angles smog was first observed in Los Angles in 1950. It is formed due to photochemical reactions taking place when air contain $\mathrm{NO} _{2}$ and hydrocarbons.

The concentrations of $O_3,$ PAN, RCHO and $R_2 CO$ builds up in the atmosphere. These compounds produce irritation in the eyes. Photochemical smog has high concentration of oxidants such as $\mathrm{O} _{3}$, organic oxidant etc.

• Option (a): This statement is correct, not incorrect. Photochemical smog indeed has a high concentration of oxidizing agents such as ozone ($\mathrm{O}_3$) and other organic oxidants.

• Option (c): This statement is correct, not incorrect. Controlling the release of $\mathrm{NO}_2$, hydrocarbons, and ozone can help in controlling photochemical smog because these are the primary precursors involved in its formation.

• Option (d): This statement is correct, not incorrect. Plantation of certain plants like pinus can help in controlling photochemical smog as these plants can absorb some of the pollutants and reduce the overall concentration of smog-forming chemicals in the atmosphere.

Answer

(b) Troposphere is the lowest region of the atmosphere. It extends upto the height of $\sim 10 \mathrm{~km}$ from sea level. Troposphere is a turbulent, dusty zone containing air, much water vapour and clouds.

Note The atmosphere is divided into four major regions

(i) Troposphere

(ii) Stratosphere

(iii) Mesosphere

(iv) Thermosphere

• Stratosphere: The stratosphere is the second layer of the atmosphere, situated above the troposphere. It extends from about 10 km to 50 km above sea level. It is characterized by a gradual increase in temperature with altitude and contains the ozone layer, which absorbs and scatters ultraviolet solar radiation.

• Mesosphere: The mesosphere is the third layer of the atmosphere, located above the stratosphere. It extends from about 50 km to 85 km above sea level. In this layer, the temperature decreases with altitude, and it is where most meteorites burn up upon entering the Earth’s atmosphere.

• Hydrosphere: The hydrosphere refers to all the water present on Earth, including oceans, seas, rivers, lakes, and even underground water. It is not a layer of the atmosphere but rather encompasses all forms of water in liquid, solid, and gaseous states on the planet.

Answer

(a) Major components of atmosphere are dinitrogen, dioxygen and water vapour $\mathrm{N} _{2}=78.08 $%, $\mathrm{O} _{2}=20.95 $%.

Both do not react with each other as nitrogen is an inactive gas. The triple bond in $\mathrm{N} _{2}$ is very stable and its dissociation energy is very high. Both react with each other at very high temperature.

$$ \mathrm{N} _{2}(g)+\mathrm{O} _{2}(g) \xrightarrow{3000^{\circ} } 2 \mathrm{NO}(g) $$

• (b) The reaction can be initiated without a catalyst, as it primarily requires high temperature to overcome the activation energy barrier.
• (c) Oxides of nitrogen are not inherently unstable; they can exist under certain conditions, such as high temperatures.
• (d) While $\mathrm{N} _{2}$ is relatively unreactive due to its triple bond, $\mathrm{O} _{2}$ is quite reactive. The main reason they do not react under normal conditions is the high activation energy required, not the unreactivity of both gases.

Answer

(c) Hydrocarbons present in atmosphere combine with oxygen atom produced by the photolysis of $\mathrm{NO} _{2}$ to form highly reactive intermediate called free radical. Free radical initiates a series of reaction.

Peroxyacetyl nitrates are formed, which can be said as secondary pollutants.

$$ \begin{aligned} \text { Hydrocarbon }+\mathrm{O} & \rightarrow \mathrm{RCO}^{\bullet} \text { (free radicals) } \\ \mathrm{RCO}^{\circ}+\mathrm{O} _{2} & \rightarrow \mathrm{RCO} _{3}^{\cdot} \\ \mathrm{RCO} _{3}^{\cdot}+\mathrm{NO} _{2} & \rightarrow \mathrm{RCO} _{3} \cdot \mathrm{NO} _{2} \end{aligned} $$

• (a) CO: Carbon monoxide (CO) is a primary pollutant because it is directly emitted into the atmosphere from sources such as vehicle exhaust, industrial processes, and combustion of fossil fuels.

• (b) Hydrocarbon: Hydrocarbons are primary pollutants as they are directly released into the atmosphere from sources like vehicle emissions, industrial activities, and the burning of organic matter.

• (d) NO: Nitric oxide (NO) is a primary pollutant because it is directly emitted from sources such as vehicle exhaust, power plants, and other combustion processes.

Answer

(c) Ozone hole is thinning of ozone layer of stratosphere at some place.

Two types of compounds have been found to be the most responsible for depleting the ozone layer.

These are (i) $\mathrm{NO}$ (ii) chlorofluoro carbon

$$ \begin{array}{rl} \mathrm{NO}+\mathrm{O} _{3} & \rightarrow \mathrm{NO} _{2}+\mathrm{O} _{2} \\ \mathrm{CF} _{2} \mathrm{Cl} _{2} \xrightarrow{hv} ^{\bullet}\mathrm{CF} _{2} \mathrm{Cl}+\mathrm{Cl}^{\bullet} \\ \mathrm{Cl}^{\cdot}+\mathrm{O} _{3} & \rightarrow \mathrm{ClO}^{\bullet}+\mathrm{O} _{2} \\ \mathrm{ClO}^{*}+\mathrm{O} & \rightarrow \mathrm{Cl}^{\bullet}+\mathrm{O} _{2} \end{array} $$

These reactions occur in stratosphere.

• (a) Ozone hole is a hole formed in stratosphere from which ozone oozes out:

  • Incorrect because the term “ozone hole” does not refer to an actual hole or gap from which ozone escapes. Instead, it refers to a significant reduction in the concentration of ozone in the stratosphere over certain regions.

• (b) Ozone hole is a hole formed in troposphere from which ozone oozes out:

  • Incorrect because the ozone hole phenomenon occurs in the stratosphere, not the troposphere. The troposphere is the lowest layer of Earth’s atmosphere, whereas the stratosphere is the second layer where the ozone layer is located.

• (d) Ozone hole means vanishing of ozone layer around the earth completely:

  • Incorrect because the ozone hole does not imply the complete disappearance of the ozone layer around the entire Earth. It refers to a significant thinning of the ozone layer in specific regions, particularly over the polar areas, but not a total vanishing.

Answer

(d) Using plastic cans for neatly storing substances will not come under green chemistry. Water in lakes and rivers have been polluted by the use of plastic materials. The plastic materials are non-biodegradable.

• (a) Making use of soap made of vegetable oils instead of using synthetic detergents is considered green chemistry because vegetable oil-based soaps are biodegradable and less harmful to the environment compared to synthetic detergents.

• (b) Using $\mathrm{H} _{2} \mathrm{O} _{2}$ for bleaching purposes instead of using chlorine-based bleaching agents is considered green chemistry because hydrogen peroxide breaks down into water and oxygen, making it an environmentally friendly alternative to chlorine-based agents, which can produce harmful by-products.

• (c) Using a bicycle for traveling small distances instead of using petrol/diesel-based vehicles is considered green chemistry because it reduces the emission of greenhouse gases and pollutants, thereby minimizing environmental impact.

Answer

$(c, d)$

Polluted water may contain nutrients for the growth of algae, which covers the water surface and reduces the oxygen concentration in water. This leads to anaerobic condition, accumulation of abnoxious decay and animal death. This is process of eutrophication.

The amount of oxygen required by becteria to break down the organic matter present in a certain volume of sample of water, is called Biochemical Oxygen Demand. Clean water would have BOD value of $5 \mathrm{ppm}$ whereas highly polluted could have BOD value of 17ppm or more.

Normally rain water has $\mathrm{pH}$ of 6 due to $\mathrm{H}^{+}$ion formed by reaction of rain water with carbon dioxide in the atmosphere. When the $\mathrm{pH}$ of the rain water drops below 5.6, it is called acid rain.

• (a) The pH of rainwater is 5.6: Normally, rainwater has a pH of around 6 due to the formation of $\mathrm{H}^{+}$ ions from the reaction of rainwater with carbon dioxide in the atmosphere. A pH of 5.6 is slightly acidic but not indicative of a polluted environment. Acid rain, which indicates pollution, has a pH below 5.6.

• (b) The amount of carbon dioxide in the atmosphere is 0.03%: This is the normal concentration of carbon dioxide in the atmosphere and does not indicate pollution. Elevated levels of carbon dioxide could indicate pollution, but 0.03% is within the natural range.

Answer

$(a, b)$

Fertilisers contain phosphates as additives. The addition of phosphates in water enhances, algae growth. Such profuse growth of algae covers the water surface and reduces the oxygen concentration in water.

This leads to anaerobic condition, commonly with accumulation of abnoxious decay and animal death. Thus, bloom infested water inhibits the growth of other living organisms in the water body.

This process in which nutrient enriched water bodies support a dense plant population which kills animal life by depriving it of oxygen and results in subsequent loss of biodiversity is known as eutrophication.

• (c) Deposition of calcium phosphate: Phosphate-containing fertilizers primarily cause eutrophication, which leads to enhanced growth of algae and decreased oxygen levels in water. Deposition of calcium phosphate is not a direct consequence of phosphate pollution in water bodies.

• (d) Increase in fish population: The addition of phosphates in water bodies leads to eutrophication, which results in decreased oxygen levels and can cause the death of aquatic animals, including fish. Therefore, it does not lead to an increase in fish population; rather, it can significantly reduce it.

Answer

$(b, c, d)$

$\mathrm{CO} _{2}$ is slightly soluble in water forming carbonic acid.

$$ \mathrm{CO} _{2}+\mathrm{H} _{2} \mathrm{O} \rightarrow \mathrm{H} _{2} \mathrm{CO} _{3} $$

The oxides of nitrogen undergo oxidation reaction followed by reaction with water vapours to form nitric acid.

$$ \begin{aligned} \mathrm{NO}+\mathrm{O} _{2} & \rightarrow 2 \mathrm{NO} _{2} \\ 2 \mathrm{NO} _{2}+\mathrm{H} _{2} \mathrm{O} & \rightarrow \mathrm{HNO} _{3}+\mathrm{HNO} _{2} \end{aligned} $$

The oxidation of $\mathrm{SO} _{2}$ into $\mathrm{SO} _{3}$ in presence of dust particles or metal ions. $\mathrm{SO} _{3}$, then react with water vapours to form $\mathrm{H} _{2} \mathrm{SO} _{4}$.

$$ \begin{array}{ll} 2 \mathrm{SO} _{2}+\mathrm{O} _{2} \xrightarrow[\text { Dust particles }]{\text { or metal ions }} 2SO_3 \begin{array}{l} \\ \end{array} \\ \mathrm{SO} _{3}+\mathrm{H} _{2} \mathrm{O} \rightarrow \mathrm{H} _{2} \mathrm{SO} _{4}\\ \end{array} $$

• (a) Peroxyacetylnitrate: Peroxyacetylnitrate (PAN) is not an acid. It is a component of photochemical smog and acts as a pollutant, but it does not contribute to the acidity of rain. PAN is an oxidant and a lachrymator, but it does not dissolve in water to form an acid.

Answer

$(a, b)$

If the rate at which solar radiation are arrivng the earth remain constant but the amount of $\mathrm{CO} _{2}$ in the air increases. The heat radiated back to the earth will increase consequently, the temperature of the earth surface will increase.

This increase in temperature will disturb the thermal balance on the earth and could cause glaciers and ice caps to melt.

• (c) increased biochemical oxygen demand: This option is incorrect because increased biochemical oxygen demand (BOD) is typically a result of organic pollution in water bodies, not directly a consequence of global warming. While global warming can exacerbate water pollution issues, BOD is more directly related to the presence of organic matter and microbial activity in water.

• (d) eutrophication: This option is incorrect because eutrophication is primarily caused by the excessive input of nutrients, such as nitrogen and phosphorus, into water bodies, leading to overgrowth of algae and depletion of oxygen. While global warming can influence the rate of eutrophication by affecting water temperatures and weather patterns, it is not the direct cause of eutrophication.

Answer

The various gases which bring green house effect responsible for global warming are the following with relative contributions

Answer

Acid rain contains $\mathrm{H} _{2} \mathrm{CO} _{3}, \mathrm{HNO} _{3}$ and $\mathrm{H} _{2} \mathrm{SO} _{4} \cdot \mathrm{H} _{2} \mathrm{CO} _{3}$ is formed by the dissolution of $\mathrm{CO} _{2}$ of the air in which the water vapour present.

$$ \mathrm{CO} _{2}+\mathrm{H} _{2} \mathrm{O} \rightarrow \mathrm{H} _{2} \mathrm{CO} _{3} $$

Forest fire and lightning are the natural source of nitric oxide (NO). Nitrogen oxides are also produced by combustion engines, aircraft, furnaces, incinerators, industrial plants.

Nitric oxide slowly reacts with atmospheric air and produce $\mathrm{NO} _{2} . \mathrm{NO} _{2}$ dissolves in water to form $\mathrm{HNO} _{3}$.

$$ 3 \mathrm{NO} _{2}+\mathrm{H} _{2} \mathrm{O} \rightleftharpoons 2 \mathrm{HNO} _{3}+\mathrm{NO} $$

Sulphur oxides are produced by the burning of fossil fuels and in extraction of metals from their sulphide ores etc. Sulphur dioxide also produces sulphuric acid in the similar way.

$$ \mathrm{SO} _{2}+\mathrm{O} _{2}+\mathrm{H} _{2} \mathrm{O} \quad \underset{\text { Metal oxide }}{\text { Soot particles }} \rightarrow \mathrm{H} _{2} \mathrm{SO} _{4}+[\mathrm{O}] $$

Answer

The ozone layer in the stratosphere is a natural feature of the earth’s environment. The ozone layer exists between 20 to $35 \mathrm{~km}$ above the earth surface layer. This layer protects the earth from the harmful effects of the ultraviolet radiation of the sun.

A depletion of ozone layer is considered as a serious threat to all forms of life on the earth. A $5 $% decrease in ozone concentration could increases the incidence of skin cancer by $20 $%. Ultraviolet radiation is also the factor for disease of eye, including cataract formation.

It can cause genetic mutations and destroy crops and other forms of vegetation. Aquatic animals and aquatic plants are generally affected by UV-radiation.

Answer

The process which are responsible for the reduction of dissolved oxygen in water are-use of phosphatic and nitrate fertilisers, detergents, the discharge of human sewage and organic waste from food, paper and pulp industries.

The microorganisms which oxidise organic matter also utilise oxygen dissolved in water. Moreover during night, photosynthesis stops but the aquatic plants continue to respire, reduction of dissolved oxygen

Answer

Chlorofluorocarbons are introduced into the atmosphere from aerosol sprays in which they function as propellants and from refrigerating equipments in which they act as coolants. It is also used as solvents.

It has very long life and stay in atmosphere for years and ultimately reach the upper layer of atmosphere (stratosphere) where it decompose in presence of UV-radiation of the sun. On account of decomposition, it forms chlorine atom or free radicals.

$$ \mathrm{CF} _{2} \mathrm{Cl} _{2} \xrightarrow{hv} ^{\bullet} \mathrm{CF} _{2} \mathrm{Cl}+\mathrm{Cl}^{\bullet} $$

The active chlorine atoms then destroy the ozone layer.

$$ \begin{aligned} \mathrm{Cl}^{\bullet}+\mathrm{O} _{3} & \rightarrow \mathrm{ClO}^{\bullet}+\mathrm{O} _{2} \\ \mathrm{ClO}^{\circ}+\mathrm{O} _{3} & \rightarrow \mathrm{Cl}^{\bullet}+\mathrm{O} _{2} \end{aligned} $$

It has been found that one molecule of chlorofluorocarbon can destroy one thousand ozone molecules in the stratosphere.

Answer

All the solid wastes either domestic or industrial are of two types

(i) biodegradable and

(ii) non-biodegradable

If the disposal of these wastes is not properly done, these wastes may find their way into sewers and some may be eaten up by the cattle. The non-biodegradable waste like polythene bags, if swallowed by cattle, can result into their death.

Thinking Process

This phenomenon show the process of eutrophication. Eutrophication is the process in which the waste materials are enters into water. As a result, formation of algae is accelerated and concentration of dissolved oxygen decreases.

Answer

The process of eutrophication is responsible for this condition of lake. The domestic waste and organic compounds such as detergents can provide plant nutrients which can enhance the growth of algae and aquatic plants.

These are decomposed by the bacterial population giving disagreeable odour and spoiling the beauty of the lake.

Answer

Biodegradable pollutants are those which are decomposed by bacteria e.g., sewage, cow dung, fruit, vegetable etc.

Non-biodegradable pollutants are those which cannot be decomposed by bacteria

e.g., mercurv. aluminium. lead. copper. DDT etc.

Answer

The following are the sources of dissolved oxygen in water

(i) Photosynthesis by aquatic plants

(ii) Due to direct contact of the water surface with air i.e., natural aeration

(iii) Mechanical aeration

Answer

Biological Oxygen Demand (BOD) is the measure of level of pollution caused by organic biodegradable material. These biodegradable materials are decomposed by microorganism (bacteria) consuming dissolved oxygen.

Low value of BOD indicates that water contains less biodegradable material.

Answer

Presence of excessive algae growth shows that water contains a lot of phosphate due to inflow of fertilizers, etc., from the surroundings.

The decomposition of algae growth produces bad smell and unattractive appearance making it unfit for recreational use like swimming, boating etc. Further, decrease in dissolved oxygen may be harmful for aquatic species like fishes etc.

Answer

The symptoms observed in a village indicate that nitrogen oxide and sulphur oxide are released from the chimney of the factory. These are produced by the burning of fossil fuels such as gasoline, coal, natural gas etc. In an automobile engine, at high temperature when fossil fuel is burnt, dinitrogen and dioxygen combine to yield $\mathrm{NO}$ is i.e., nitric oxide.

$$ \begin{aligned} & \mathrm{N} _{2}+\mathrm{O} _{2} \xrightarrow{1200-1500^{\circ} \mathrm{C}} 2 \mathrm{NO} \\ & 2 \mathrm{NO}+\mathrm{O} _{2} \xrightarrow{1100^{\circ} \mathrm{C}} 2 \mathrm{NO} _{2} \end{aligned} $$

$\mathrm{SO} _{2}$ is produced by burning of sulphur containing fossil fuel or by roasting of sulphide ores such a iron pyrites, copper pyrites etc.

$$ \mathrm{Cu} _{2} \mathrm{~S}+\mathrm{O} _{2} \rightarrow 2 \mathrm{Cu}+\mathrm{SO} _{2} $$

Answer

The oxidation of sulphur dioxide into sulphur trioxide can occur both photochemically or non-photochemically. In the near ultraviolet region, the $\mathrm{SO} _{2}$ molecules react with ozone photochemically.

$$ \begin{aligned} \mathrm{SO} _{2}+\mathrm{O} _{3} \xrightarrow{hv} \mathrm{SO} _{3}+\mathrm{O} _{2} \\ 2 \mathrm{SO} _{2}+\mathrm{O} _{2} \xrightarrow{hv} 2 \mathrm{SO} _{3} \end{aligned} $$

Non-photochemically, $\mathrm{SO} _{2}$ may be oxidised by molecular oxygen in presence of dust and soot particles.

$$ 2 \mathrm{SO} _{2}+\mathrm{O} _{2} \xrightarrow{\text { Particulates }} 2 \mathrm{SO} _{3} $$

Answer

Sunlight cause photochemical decomposition of $\mathrm{NO} _{2}$ into $\mathrm{NO}$ and $\mathrm{O}$.

$$ \mathrm{NO} _{2} \xrightarrow{hv} \mathrm{NO}+[\mathrm{O}] $$

Atomic oxygen is a highly reactive species. It combines with diatomic oxygen and forms ozone.

$$ \mathrm{O} _{2}+\mathrm{O}+\mathrm{M} \rightarrow \mathrm{O} _{3}+\mathrm{M} $$

where, $M$ is inert gas such as nitrogen. This, $\mathrm{O} _{3}$ is formed during the formation of smog.

Answer

Ozone in stratosphere is a product of action of UV-radiations on dioxygen $\left(\mathrm{O} _{2}\right)$ molecules. The UV radiation split apart molecular oxygen into free oxygen atoms. These oxygen atoms combine with the molecular oxygen to form ozone.

$$ \begin{aligned} & \mathrm{O} _{2}(g) \xrightarrow[\text { UV -adiation }]{hv} \mathrm{O}+\mathrm{O} \\ & \mathrm{O} _{2}(g)+\mathrm{O}(g) \underset{\text { radiations }}{\stackrel{U V}{\rightleftharpoons}} \mathrm{O} _{3} \end{aligned} $$

Note In stratosphere, a photon with a wavelength between $180 \mathrm{~nm}$ and $240 \mathrm{~nm}$ breaks the $\mathrm{O} _{2}$ molecule into atomic oxygen.

Answer

In stratosphere, the formation of $\mathrm{O} _{3}$ goes on continuously but $\mathrm{O} _{3}$ is also decomposed by UV- radiation between $240 \mathrm{~nm}$ to $360 \mathrm{~nm}$.

$$ \mathrm{O} _{3}+\mathrm{H} _{2} \stackrel{(240-360 \mathrm{~nm})}{\rightarrow} \mathrm{O} _{2}+\mathrm{O} $$

The $\mathrm{O}$-atom reacts will sand $\mathrm{O} _{3}$ molecule

$$ \begin{aligned} \mathrm{O} _{3}+\mathrm{O} & \rightarrow 2 \mathrm{O} _{2} \\ \end{aligned} $$

$$ \begin{array}{lll} \text { Net reaction } & 2 \mathrm{O} _{3} & \rightarrow 3 \mathrm{O} _{2} \end{array} $$

Thus, the reaction form a delicate balance in which the rate of $\mathrm{O} _{3}$ decomposition match the rate of $\mathrm{O} _{3}$ formation is a dynamic equilibrium exists and maintains a constant concentration of $\mathrm{O} _{3}$.

Answer

In summer season, nitrogen dioxide and methane react with chlorine monoxide and chlorine atoms forming chlorine sinks, preventing much ozone depletion, whereas in winter, special type of clouds called polar stratospheric clouds are formed over Antarctica.

These polar stratospheric clouds provide surface on which chlorine nitrate gets hydrolysed to form hypochlorous acid. It also reacts with hydrogen chloride to give molecular chlorine.

$$ \mathrm{ClO}^{*}(g)+\mathrm{NO} _{2}(g) \rightarrow \underset{\text { Chlorine nitrate }}{\mathrm{ClONO} _{2}(g)} $$

$$ \mathrm{Cl}^{*}(g)+\mathrm{CH} _{4}(g) \rightarrow{ }^{\bullet} \mathrm{CH} _{3}(g)+\mathrm{HCl}(g) $$

$$ \mathrm{ClONO} _{2}(g)+\mathrm{H} _{2} \mathrm{O}(g) \xrightarrow{{\text {Hydrolysis }}} \mathrm{HOCl}(g)+\mathrm{HNO} _{3}(g) $$

$$ \mathrm{ClONO} _{2}(g)+\mathrm{HCl}(g) \rightarrow \mathrm{Cl} _{2}(g)+\mathrm{HNO} _{3}(g) $$

When sunlight returns to the Antarctica in the spring, the sun’s warmth breaks up the clouds and $\mathrm{HOCl}, \mathrm{Cl} _{2}$ are photolysed by sunlight.

$$ HOCl(g) \xrightarrow{hv} O^* H(g)+Cl^* (g) $$

$$ \mathrm{Cl} _{2}(g) \xrightarrow{hv} 2 \mathrm{Cl}^{*}(g) $$

The chlorine radicals thus formed, initiate the chain reaction for ozone depletion.

Answer

The laxative effect is observed only when the sulphates present in water have concentration greater than $500 \mathrm{ppm}$. Otherwise at moderate levels it is charmless.

Answer

A. $\rightarrow(3,4)$

B. $\rightarrow(4,5)$

C. $\rightarrow$ (2)

D. $\rightarrow(1)$

A. Acid rain is due to oxides of carbon, sulphur (unsaturated hydrocarbon) and nitrogen.

B. Photochemical smog is formed by unburnt fuel (unsaturated hydrocarbon and $\mathrm{SO} _{2}$.

C. Carbon monoxide with haemoglobin is poisonous.

D. Chlorofluorocarbon $\left(\mathrm{CHCl} _{2}-\mathrm{CHF} _{2}\right)$ cause ozone depletion.

Answer

A. $\rightarrow$ (4)

B. $\rightarrow(5)$

C. $\rightarrow$ (1)

D. $\rightarrow(3)$

E. $\rightarrow$ (2)

A. Low concentration of sulphur dioxide causes respiratory disease e.g., asthma, bronchitis etc.

B. The irritant red haze in the traffic and congested place is due to oxides of nitrogen.

C. The increased amount of $\mathrm{CO} _{2}$ in air is mainly responsible for global warming.

D. Excess nitrate in drinking water cause methemoglobinemia (blue baby syndrome).

E. Lead can damage kidney, liver, reproductive system etc.

Answer

A. $\rightarrow$ (5)

B. $\rightarrow$ (4)

C. $\rightarrow(1)$

D. $\rightarrow$ (2)

E. $\rightarrow$ (3)

A. Classical smog, acid rain, water pollution, induce breathing problems, damage to buildings, corrosion of metals.

B. May cause nerve disease in human.

C. Water pollution.

D. Photochemical smog, damage to plant life, corrosion to building material, induce breathing problems, water pollution.

E. Damaging ozone layer.

Answer

A. Phosphate fertilisers increase growth of algae increasing BOD level, causing eutrophication.

B. Methane oxidises to $\mathrm{CO} _{2}$ which causes, global warming.

C. Synthetic detergents increases BOD level.

D. Nitrogen oxide mix with water forming nitric acid.

Assertion and Reason

In the following questions a statement of Assertion (A) followed by a statement of Reason ( $\mathrm{R}$ ) is given. Choose the corrcet option out of the choices given below in each question.

Answer

(c) Both assertion and reason are not correct.

In cold countries, sunlight required to grow plants is less. Hence, plants are kept in a house made of glass, placed in such a manner, so that sunlight enters the green house, heat up the soil and plants.

The warm soil and plants emit infrared radiations. Since, glass is opaque to infrared radiations, it partly reflects and partly absorbs these radiations.

Answer

(b) Both assertion and reason are correct but reason is not the correct explanation of assertion.

Normally rain water has a pH of 5.6 due to the presence of $\mathrm{H}^{+}$ions formed by the reaction of rain water with carbon dioxide present in the atmosphere.

$$ \begin{aligned} \mathrm{H} _{2} \mathrm{O}(l)+\mathrm{CO} _{2}(\mathrm{~g}) & \rightarrow \mathrm{H} _{2} \mathrm{CO} _{3}(\mathrm{aq}) \\ \mathrm{H} _{2} \mathrm{CO} _{3}(\mathrm{aq}) & \rightleftharpoons \mathrm{H}^{+}(\mathrm{aq})+\mathrm{HCO} _{3}^{-}(\mathrm{aq}) \end{aligned} $$

When the $\mathrm{pH}$ of rain water drops below 5.6 it is called acid rain.

Answer

(a) Both assertion and reason are correct and reason is the correct explanation of assertion.

When fossil fuel are burnt, a variety of pollutants are emitted into the earth’s troposphere. Two of the pollutants that are emitted hydrocarbons (unburnt fuels) and nitric oxide (NO).

When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sun light in which $\mathrm{NO}$ oxidises to $\mathrm{NO} _{2}$. This $\mathrm{NO} _{2}$ in turns absorbs energy from sunlight and breaks up into nitric oxide and free oxygen atom.

$$ \mathrm{NO} _{2}(g) \quad h \nu \rightarrow \mathrm{NO}(g)+\mathrm{O}(g) $$

Oxygen atoms are very reactive and combine with $\mathrm{O} _{2}$ in air to produce ozone.

$$ \mathrm{O}(\mathrm{g})+\mathrm{O} _{2}(\mathrm{~g}) \rightleftharpoons \mathrm{O} _{3}(\mathrm{~g}) $$

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(b) Both assertion and reason are correct but reason is not the correct explanation of assertion.

Carbon dioxide is one of the important green house gases. It is largely produced by the combustion of fossil fuels like coal, natural gas, petroleum, etc. It is also produced by respiratory function of animals and plants.

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(d) Assertion is not correct but reason is correct.

Ozone layer is found in the stratosphere. The depletion of ozone layer (creation of ozone hole) is taking place due to reaction with $\mathrm{NO}$ (produced from natural sources or human activity or in the exhaust gases of engines of supersonic planes) or by reaction with chlorofluorocarbons (CFC’s) called freons (produced from aerosol sprays used in propellents or from refrigerators where they are used as coolants).

The ozone hole allows the UV radiations to pass through and reach us, increasing chances of skin cancer.

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(a) Both assertion and reason are correct but reason is the correct explanation of assertion. Insecticides, pesticides and herbicides cause soil and water pollution. They are non-biodegradable.

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(c) Both assertion and reason are not correct.

Amount of oxygen required by bacteria to breakdown the organic matter present in a certain volume of a sample of water is called Biochemical Oxygen Demand (BOD).

If $\mathrm{BOD}$ level is less than $5 \mathrm{ppm}$, water is almost pure. High biological oxygen demand means high activity of bacteria in water.

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(a) The most straight forward way to reduce or prevent the formation of photochemical smog is to minimise the release of oxides of nitrogen and hydrocarbons to the atmosphere.

The following methods can be applied to minimise the oxides of nitrogen and hydrocarbons.

(i) By fitting efficient catalytic converters in automobiles, the harmful gases are converted catalytically into harmless gases.

(ii) By spraying certain compound into the atmosphere which generate free radicals that readily combine with the free radicals that initiate the reactions forming toxic compounds of the photochemical smog.

The compound diethyl hydroxylamine has been found to posses smog inhibiting property.

(iii) Certain plants such as pinus, juniparus, pyrus, irtis etc., can metabolise oxides of nitrogen.

(b) Solvents used to dryclean clothes are usually chlorinated compounds which are carcinogen. Suitable detergents which work in liquid carbon dioxide have been discovered to replace the chlorinated compounds.

For bleaching of clothes in laundry, $H_{2} O_{2}$ and not $Cl_{2}$ is used which gives better results and is not harmful. Earlier, $Cl_{2}$ gas was used for bleaching paper. Chlorine is highly toxic in nature. It’s use has been replaced by $H_{2} O_{2}$ in presence of a suitable catalyst.

(c) To reduce the use of synthetic detergents as cleaning agent, emphasis should be made on the use of soaps prepared of vegetable oils. The vegetable oils are biodegradable while detergents do not undergo biodegradation.

(d) Instead of petrol and diesel, the use of CNG (Condensed Natural Gas) and LNG (Liquified Natural Gas) has been preferred as they are pollution free fuels.

The other sources such as hydrogen, ethyl alcohol, etc., can be tried in place of petrol and diesel.

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Carbon dioxide is a natural constituent of atmosphere and is vital for all forms of plant life. It forms about ${0 . 0 3 3 }$ % by volume of atmosphere. It helps to maintain the temperature of the earth required for living organisms.

A balance of $\mathrm{CO} _{2}$ is maintained in air because $\mathrm{CO} _{2}$ is produced from respiration, burning of fossil fuels and decomposition of lime stone but at the same time, it is consumed in photosynthesis by plants.

However, human activities have disturbed this balance and $\mathrm{CO} _{2}$ level in atmosphere is in increasing order. This has happened due to deforestation, burning of more fossil fuel and industrialisation. It has been estimated that $\mathrm{CO} _{2}$ concentration has risen about 25 % in the past century.

During the past nearly 120 years, the average temperature of the planet has increased by somewhere between $0.4^{\circ} C$ to $0.8^{\circ} C$. Current estimated are that doubling the $CO_{2}$ concentration will result in a temperature increase of between $1.0^{\circ} C$ and $3.5^{\circ} C$. In green house effect, contribution of $CO_2$ is 50 % and of other trace gases is also about 50 %.

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Visible light from the sun reaches the earth and heats it up. However, when the earth cools, the energy is re-emitted from the earth’s surface in the form of infrared radiation which have longer wavelength and heating effect.

These infrared radiation can be absorbed by $\mathrm{CO} _{2}$ and $\mathrm{H} _{2} \mathrm{O}$ vapours. The heat thus absorbed is radiated back to the surface of the earth. In this way, warming of the earth occurs.

If the rate at which solar energy is arriving the earth remains constant but the amount of $\mathrm{CO}_{2}$ in the air increases, the heat radiated back to the earth will increase. Consequently, the temperature of the earth’s surface will increase.

Hence, global warming depends on the concentration of those gases which are responsible for green house effect.

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Pesticides from soil are transferred into the crops and from the crops these are transferred into rearing fish food. Pesticides entered[ into water through rearing fish food and finally entered into the bodies of the fishes.

Therefore, pesticides are transferred from lower trophic level to higher trophic level through food chain. Over the time, the concentration of pesticides in fishes reach a level which causes serious metabolic and physiological disorders.

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Tetrachloroethane, $\mathrm{Cl} _{2} \mathrm{CH}-\mathrm{CHI} _{2}$ is suspected to be carcinogenic and also contaminates the ground water. The harmful effect will be prevented by using liquified $\mathrm{CO} _{2}$ along with suitable detergent.

Use of liquified $\mathrm{CO}_{2}$ along with detergent will not be completely safe because most of the detergents are non-biodegradable and they cause water pollution. Moreover, liquified $\mathrm{CO} _{2}$ will ultimately enter into the atmosphere and contributed to the green house effect.