NCERT Exercises

Question 1.
Define environmental chemistry.
Solution.
The environment around us is made up of chemical species which undergo chemical reactions constantly e.g., photosynthesis in plants, formation of ozone in stratosphere, etc. Environmental chemistry deals with the study of such reactions which take place in the origin, transport and life-cycle of chemical species in the environment.

Question 2.
Explain tropospheric pollution in 100 words.
Solution.
(i) Troposphere is the lowest layer of atmosphere where life exists. The layer gets polluted due to the presence of particulate matter. Such particulate matter may either be:
(a) Solid matter : e.g., dust, mist, fumes, smoke, smog, etc. Smoke particulates consist of solid or mixture of solid and liquid particulates formed during combustion of organic matter. Dust is composed of fine solid particles (over 1 um in diameter), produced during crushing, grinding and attribution of solid materials. Mists are produced by particles of spray liquids and by condensation of vapours in air. Fumes are generally obtained by the condensation of vapours during sublimation, distillation, boiling and several other chemical reactions. The word smog is derived from smoke and fog. There are two types of smog : Classical smog occurs in cool humid climate. It is a mixture of smoke, fog and sulphur dioxide. Photochemical smog occurs in warm, dry and sunny climate. The main components of the photochemical smog result from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides produced by automobiles and factories.
(b) Gaseous matter : e.g., oxides of sulphur and carbon and hydrocarbons, etc. Oxides of sulphur are produced when sulphur containing fossil fuel is burnt. Even a low concentration of sulphur dioxide causes respiratory diseases e.g. asthma, bronchitis, emphysema in human beings. While most of these pollutants are produced by human activities such as mining, burning of fossil fuels, smoke from industries, etc., they may also be produced by natural activities like volcanic eruptions which throw up large quantities of particulate matter or landslides which create an envelope of dust. Hydrocarbons are composed of hydrogen and carbon only and are formed by incomplete combustion of fuel used in automobiles. Hydrocarbons are carcinogens, i.e., they cause cancer. They harm plants by causing ageing, breakdown of tissues and shedding of leaves, flowers and twigs. Carbon monoxide (CO) is one of the most serious air pollutants. It is produced as a result of incomplete combustion of carbon. Carbon dioxide (CO₂) is released into the atmosphere by respiration and burning of fossil fuels for energy. The increased amount of CO₂ in the air is mainly responsible for global warming, thereby, leading to tropospheric pollution.

Question 3.
Carbon monoxide gas is more dangerous than carbon dioxide gas. Why?
Solution.
Carbon monoxide is a colourless, odourless gas which has the tendency to bind to haemoglobin, the oxygen carrying molecule in blood and forms a complex called carboxyhaemoglobin. This complex is 300 times more stable than oxyhaemoglobin complex. When the concentration of carboxyhaemoglobin reaches about 3-4 per cent, the oxygen carrying capacity of blood is greatly reduced. This oxygen deficiency, results into headache, weak eyesight, nervousness and cardiovascular disorder and therefore, if once formed can seriously hamper the body and may cause death. That is why CO is said to be more dangerous than CO₂.

Question 4.
List gases which are responsible for greenhouse effect.
Solution.
Gases responsible for greenhouse effect are :
CO₂, CH₄, N₂O, H₂O_(g), CFCs and O₃. Carbon dioxide molecules trap heat as they are transparent to sunlight but not to the heat radiations. Carbon dioxide is, thus, the major contributor to global warming. Methane is produced naturally when vegetation is burnt, digested or rotted in the absence of oxygen. Large amounts of methane are released in paddy fields, coal mines from rotting garbage dumps and fossil fuels.

Chlorofluorocarbons (CFCs) are man-made industrial chemicals used in air conditioning, etc. CFCs are also damaging the ozone layer. These gases together are responsible for greenhouse effect.

Question 5.
Statues and monuments in India are affected by acid rain. How?
Solution.
Monuments and statues are made of either stone or metal M both of which have a tendency to react with acid and therefore, get affected by acid rain which contains H₂SO₄.

As a result, the monuments are being slowly disfigured.

Question 6.
What is smog? How is classical smog different from photochemical smog?
Solution.
The presence of smoke in fog is called smog. Classical smog occurs in cool and humid climate when smoke and SO₂ are suspended in air. Due to the presence of SO₂ it is found to be reducing in nature. On the other hand photochemical smog occurs in warm, dry and sunny climate. Photochemical smog is produced by the action of light on unsaturated hydrocarbons and NO_x, in air. It is oxdising in nature.

Question 7.
Write down the reactions involved during the formation of photochemical smog.
Solution.
Photochemical smog is produced by action of sunlight on hydrocarbons and nitrogen oxides. NO is converted to NO₂ by the effect of sunlight. This NO₂ further absorbs sunlight and disintegrates into NO and O₂. Thus, a chain reaction begins.

These oxygen atoms react with O₂ to form ozone as :
O_2(g) +O_(g) ⇌ O_3(g)
O_3(g) + NO_(g) ⇌ NO_2(g) + O_2(g)
These two gases formed, react with hydrocarbons to produce other chemicals.

Question 8.
What are the harmful effects of photochemical smog and how can they be controlled?
Solution.
The components of photochemical smog are : O₃, NO, Acrolein, formaldehyde and PAN. Such smog causes damage to human health and property.
(i) Effect on organisms :

•  O₃ and PAN are eye-irritants.
•  O₃ + NO irritate the nose and throat, cause headache, chest pain, cough and breathing problems.
•  Damage to plant life

(ii) Damage to property : These chemicals cause

•  cracking of rubber
•  corrosion of metals, stones, rubber and painted surfaces. If the primary precursors of photochemical smog such as NO₂ and hydrocarbons and the secondary precursors such as ozone and PAN can be controlled, then photochemical smog will be automatically reduced. Catalytic converters are used in automobiles, which prevent the release of nitrogen oxide and hydrocarbons in the atmosphere. Certain plants e.g., Pinus, Juniparus, Quercus, Pyrus and Vitis can metabolise nitrogen oxide and therefore, their plantation could help in reducing photochemical smog.

Question 9.
What are the reactions involved for ozone layer depletion in the stratosphere?
Solution.

1.  The main reason for ozone layer depletion besides other gases are chlorofluorocarbons (CFCs).
2.  Once CFCs are released into the atmosphere, they reach the stratosphere and release free-radicals by the action of sunlight.
   
3.  These radicals further react with ozone and bring about its depletion.
   
4.  Such regeneration of Cl˙ radicals, causes continuous eating away of the O₃ layer creating ozone holes.

Question 10.
What do you mean by ozone hole? What are its consequences?
Solution.
(i) The depletion of ozone layer is termed as ozone hole. It was first reported by a group of scientists working in Antarctica in 1980s.
(ii) Ozone layer is responsible for prevention of infiltration of UV rays which has the potential to cause serious damage to plants, animals and human life. Due to depletion of this O₃ layer, these harmful rays will find an easy route into the earth’s atmosphere and create problems such as mutation of cells leading to cancer of the skin or increased transpiration in plants and reduced water level in soil. Increase in UV radiations damages paints and fibres causing them to fade faster.

Question 11.
What are the major causes of water pollution? Explain.
Solution.
The major causes of water pollution may be enlisted as:
(i) Pathogens : Pathogens include bacteria and other organisms that enter water from domestic sewage and animal excreta. Human excreta contains bacteria such as Escherichia coli and Streptococcus faecalis which cause gastrointestinal diseases.
(ii) Organic wastes : The other major water pollutant is organic matter such as leaves, grass, trash, etc. They pollute water as a consequence of run off. Excessive phytoplankton growth within water is also a cause of water pollution. These wastes are biodegradable.
(iii) Chemical pollutants : Water soluble inorganic chemicals that include heavy metals such as cadmium, mercury, nickel, etc. constitute an important class of pollutants. These metals then can damage kidneys, central nervous system, liver, etc. Acids (like sulphuric acid) from mine drainage and salts from many different sources including raw salt used to melt snow and ice in the colder climates (sodium and calcium chloride) are water soluble chemical pollutants.

Question 12.
Have you ever observed any water pollution in your area? What measures would you suggest to control it?
Solution.
Yes. The water pollution can be controlled by :

1. Treatment of sewage :
   1.  by removing impurities,
   2.  by passing chlorine,
   3.  by treatment with alum.
2. Treatment of industrial waste :
   1.  by precipitating impurities,
   2.  by photocatalysis,
   3.  by using ion exchangers.

Question 13.
What do you mean by Biochemical Oxygen Demand (BOD)?
Solution.
Organic matter in water is biodegradable, i.e., it can be decomposed by the action of bacteria. Now, these bacteria need oxygen to decompose the organic waste. Thus, the amount of oxygen required by bacteria to breakdown the organic matter in a certain volume of a sample of water is called Biological Oxygen Demand (BOD) for that sample.

Question 14.
Do you observe any soil pollution in your neighbourhood? What efforts will you make for controlling the soil pollution?
Solution.
Soil pollution can be controlled by :

1.  using manures
2.  using bio-fertilizers
3.  using proper sewerage system
4.  salvage and recycling waste products.

Question 15.
What are pesticides and herbicides? Explain giving examples.
Solution.
(a) Pesticides : They are chemicals that are used to kill pests. They act by blocking the neurotransmission in the pest. e.g., DDT, BHC, etc.
(b) Herbicides : They are chemicals such as NaClO₃, Na₃AsO₃ which are used to control the population of weeds in the fields.

Question 16.
What do you mean by green chemistry? How will it help decrease environmental pollution?
Solution.
(a) Green chemistry is a production process that would bring about minimum pollution or deterioration to the environment.
Green chemistry aims at :
(a) Use of environment friendly medium for the reaction; and
(b) Use of methods that completely convert the reactants to products such that there are no harmful side-products formed. This is arrived at by working out optimum conditions for synthesis such that there are no harmful side-products formed. Thus, green chemistry produces products which have no adverse impact on the environment.

Question 17.
What would have happened if the greenhouse gases were totally missing in the earth’s atmosphere? Discuss.
Solution.
Greenhouse gases are responsible for trapping heat and thereby warming up earth’s atmosphere to a temperature which is conducive to the formation of molecules of life such as water, amino acids, etc. Had there been no greenhouse gases, the earth would have remained as cold as Mars and life would not have originated.

Question 18.
A large number of fishes are suddenly found floating dead on a lake. There is no evidence of toxic dumping but you find an abundance of phytoplankton. Suggest a reason for the fish kill.
Solution.
Like other organisms, fishes need oxygen for survival. They derive this from the oxygen dissolved in water. But, when large amounts of phytoplankton grow in a pond/lake, they tend to take up all the O₂ dissolved in water. As a result, the amount of O₂ available for fish goes down dramatically and it dies. This is why the fishes in the lake die when large number of phytoplanktons grow therein. Such a lake is called a dead lake.

Question 19.
How can domestic waste be used as manure?
Solution.
Domestic waste consists mainly of two kinds of wastes – Biodegradable and non-biodegradable.
If the biodegradable part is separated and collected from homes it can be dumped in land fills and converted to compost by the action of bacteria present in soil.

Question 20.
For your agricultural field or garden you have developed a compost producing pit. Discuss the process in the light of bad odour, flies and recycling of wastes for a good produce.
Solution.
While a compost pit can be a blessing for the agricultural field since it produces manure, it can also be source of trouble in the form of flies, foul odour and home for growth of bacteria, however, the negative effects can be done away with by making the pit at a place away from the residential area preferably somewhere in the field itself. Equally important is covering the pit with a suitable, lid to prevent foul odour. In such a way one can derive the benefits of a compost pit while negating the disadvantages.