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SSC Chemistry Environmental Chemistry Environmental Pollution

Environmental Pollution

Category : SSC




  • The pollutants may be inorganic, biological or radiological in nature.

(i) Bio-degradable pollutants are domestic wastes which are rapidly decomposed by micro-organisms.

(ii) Non-biodegradable pollutants include chemicals, mercuric salts, lead compounds, pesticides, etc.

(iii) Natural pollution is caused by radioactive substances, volcanic eruptions, forests and mines fires floods, etc.

(iv) Artificial pollution is caused by industries, thermal plants, automobile, exhausts, sewage, etc.




  • Environment: The conditions existing around animal or human life.

Atmosphere: The gaseous envelop surrounding the earth. It has been classified into following regions:-

(i) Stratosphere: The layer of the earth's atmosphere above the troposphere and below the mesosphere.

(ii) Troposphere: The lowest region of the atmosphere extending from earth's surface to the lower boundary of the stratosphere. In this region, human beings along with other organisms live. It contains water vapour and is greatly affected by air pollution.

Note: The other two layers are Thermosphere and Mesosphere.


Air pollution


  • Air pollution: The major air pollutants are

(i) Carbon monoxide (CO): It is produced by incomplete combustion of gasoline in motor vehicles, wood, coal, inceneration and forest fires. It is treacherous and deadly poisonous gas. It induces headache, visual difficulty coma and death. It blocks the normal transport of oxygen from the lungs to other parts of the body.

(ii) Sulphur dioxide\[(S{{O}_{2}})\]: It is produced by petrol combustion, coal combustion, and petrol refining and smelting operations. It hinders the movement of air in and out of lungs. It is particularly poisonous to trees causing chlorosis and dwarfing. In presence of air it is oxidised to \[S{{O}_{3}}\] which is also irritant.

            \[2S{{O}_{2}}+{{O}_{2}}(air)\to 2S{{O}_{3}}\]

In presence of moisture \[S{{O}_{3}}\] is converted into highly corrosive sulphuric acid.

            \[S{{O}_{3}}+{{H}_{2}}O(moisture)\to {{H}_{2}}S{{O}_{4}}\]

It attacks marble, limestone, vegetation, paper and textiles and injurious to human beings.

(iii) Oxides of nitrogen: \[N{{O}_{2}}\] and NO, Source - combustion of coal, gasoline, natural gas, petroleum refining, chemical plants, manufacturing explosives and fertilizers, tobacco smoke. Breathing \[N{{O}_{2}}\] causes chlorosis to plants and chronic lung conditions leading to death. \[N{{O}_{2}}\] reacts with moisture to form acids.

            \[2N{{O}_{2}}+{{H}_{2}}O\to HN{{O}_{2}}+HN{{O}_{3}}\],

            \[3HN{{O}_{2}}\to 2NO+HN{{O}_{3}}+{{H}_{2}}O\]

(iv) Smoke, dust:

Sources: cement works, iron and steel works, gas works, power generating stations.

Smog: It is a mixture of smoke and fog in suspended droplet form. It is of two types:

  • London smog or classical smog: It is coal smoke plus fog. The fog part is mainly \[S{{O}_{2}}\] and \[S{{O}_{3}}\].It has sulphuric acid aerosol. It causes bronchial irritation and acid rain. It is reducing in nature.
  • Photochemical smog or Los Angeles smog: The oxidised hydrocarbons and ozone in presence of humidity cause photochemical smog. Hydrocarbons \[+{{O}_{2}}\], \[N{{O}_{2}}\] NO, O, \[{{O}_{3}}\] \[\to \] Peroxides, formaldehyde, peroxyacetylnitrate (PAN), acrolein etc. It is oxidising in nature and causes irritation to eyes, lungs, nose, and asthamatic attack and damage plants.

Acid rain: The oxides of C, N and S present in the atmosphere, dissolve in water and produce acids and lower the pH of water below 5.5.

\[{{H}_{2}}O+C{{O}_{2}}\to {{H}_{2}}C{{O}_{3}}2{{H}^{+}}+CO_{3}^{2-}\]

\[2{{H}_{2}}O+2C{{O}_{2}}+{{O}_{2}}\to 2{{H}_{2}}S{{O}_{4}}2{{H}^{+}}+SO_{4}^{2-}\]

\[2{{H}_{2}}O+4N{{O}_{2}}+{{O}_{2}}\to 4HN{{O}_{3}}{{H}^{+}}+NO_{3}^{-}\]

The acids are toxic to vegetation, react with marble and damage buildings.

\[CaC{{O}_{3}}+{{H}_{2}}S{{O}_{4}}\to CaS{{O}_{4}}+{{H}_{2}}O+C{{O}_{2}}\]

Acids corrode water pipes and produce salts with heavymetals ions viz Cu, Pb, Hg and Al toxic in nature.

(v) Green House effect: The retention of heat by the earth and atmosphere from the sun and its prevention to escape into theater space is known as green-house effect. Global warming is average increase in the temperature of earth due to increase in concentration of green-house gases \[(C{{O}_{2}},{{O}_{3}},N{{O}_{x}})\]etc. Consequences of global warming:

(i) Global warming would result in rise in sea level due to increased rate of melting of glaciers and floods.

(ii) Increase in infectious diseases like malaria, dengue, etc.

(vi) Ozone layer and its depletion: The ozone layer, existing between 20 to 35 km above the earth's surface, shield the earth from the harmful U. V. radiations from the sun. The U. V. radiations cause skin cancer, cataract of eye, and harm to vegetation.

Depletion of ozone is caused by oxides of nitrogen

            \[\underset{reactive\,\,nitric\,\,oxide}{\mathop{{{N}_{2}}O+hv\to NO+N}}\,\]

            \[NO+{{O}_{3}}\to N{{O}_{2}}+{{O}_{2}}\],

            \[{{O}_{3}}+hv\to {{O}_{2}}+O\],

            \[N{{O}_{2}}+O\to NO+{{O}_{2}}\]

            \[2{{O}_{3}}+hv\to 3{{O}_{2}}\](net reaction)

The presence of chlorofluorocarbons also increase the decomposition of \[{{O}_{3}}\].

            \[C{{F}_{2}}C{{l}_{2}}+hv\to C{{F}_{2}}Cl+Cl\],

\[CFC{{l}_{3}}+hv\to CFC{{l}_{2}}+Cl\],

      \[Cl+{{O}_{3}}\to ClO+O\],

     \[ClO+O\to Cl+{{O}_{2}}\],

       \[{{O}_{3}}+O\to 2{{O}_{2}}(Netreaction)\]

(vii) Control of air pollution: It can be controlled by

  1. Dissolving HCl, HF, \[Si{{F}_{4}}\] in water and \[S{{O}_{2}}\], \[C{{l}_{2}}\],\[{{H}_{2}}S\] in alkaline solution.
  2. Adsorbing gas and liquid molecules over activated charcoal and silica gel.
  3. Chemical reactions.
  4. Use of precipitators to settle charge particles.
  5. Use of settling Chambers under the action of gravity
  6. Use of natural gas in place of diesel, petrol, etc.


Water pollution


  • Water pollution: The contamination of water by foreign substances which would constitute a health hazard and make it harmful for all purposes (domestic, industrial or agriculture etc.) is known as water pollution. The polluted water may have offensive odour, bad taste, unpleasant colour, murky oily, etc.

(i) Sources of water pollution

(a) Domestic sewage: Discharges from kitchens, baths, lavatories, etc.

(b) Industrial waters: Wastes from manufacturing processes which includes acids, alkalines, pesticides, insecticides, metals like copper. Zinc, lead, mercury, fungicides, etc.

(c) Oil: From oil spills or washings of automobiles.

(d) Atomic explosion and processing of radioactive materials.

(e) Suspended particles (organic or inorganic) viruses, bacterias, algae, protozoa, etc.

(f) Wastes from fertilizer plants such as phosphates, nitrates ammonia, etc.

(g) Clay: Ores, minerals, fine particles of soil.

(ii) Aerobic and anaerobic oxidation: The oxidation of organic compounds present in sewage in presence of good amount of dissolved or free oxygen (approx. 8.5 ml/l) by aerobic bacterials is called aerobic oxidation. When dissolved or free oxygen is below a certain value the sewage is called stale. Anaerobic bacterias bring out putrefaction producing \[{{H}_{2}}S,N{{H}_{3}},C{{H}_{4}},{{(N{{H}_{4}})}_{2}}S,\] etc. This type of oxidation is called anaerobic oxidation.

(iii) Biological Oxygen Demand (BOD): It is defined as the amount of free oxygen required for biological oxidation of the organic matter by aerobic conditions at 20°C for a period of five days. Its unit is mg/l or ppm. An average sewage has BOD of 100 to 150 mg/l.

(iv) Chemical Oxygen Demand (COD): It is a measure of all types of oxidiable impurities present in the sewage. COD values are higher than BOD values.


Soil pollution


  • Soil pollution: The addition of substances in an indefinite proportion changing the productivity of the soil is known as soil or land pollution.
  • Sources of soil pollution:

(i)  Agricultural pollutants: Chemicals like pesticides, fertilizers, bacteriacides, fumigants, insecticides, herbicides, fungicides.

(ii) Domestic refuge and industrial wastes.

(iii) Radioactive wastes from research centres, and hospitals.

(iv) Soil conditioners containing toxic metals like Hg, Pb, As, Cd, etc.

(v) Farm wastes from poultries, dairies and piggery farms.

(vi) Improper disposal of human and animal extreta.

(vii) Pollutants present in air from chemical works.




  • Pesticides: The chemical substances used to kill or stop the growth of unwanted organisms are called pesticides.

They are further classified as

(i) Insecticides: They are used to kill insects. The most common insecticides are

(i) D.D.T                        (ii) BHC, 666, game xene            (iii) Baygon                    (iv) Sevin Carbaryl

(v) Parathion                  (vi) Methoxychlor                        (vii) Aldrin

(ii) Herbicides: They are used to kill weeds

(i)  2, 4-dichlorophenoxyacid

(ii) Triazines

(iii) \[\underset{Sodium\,\,chloride}{\mathop{NaCl{{O}_{3}}}}\,\]


The (iii) and (iv) are not used now a-days. Addition of phosphate fertilizers to water leads to nutrient enrichment (eutrophication).

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