ICSE Class 9 Chemistry Chapter 08 Atmospheric Pollution

Atmospheric Pollution

Syllabus

Acid rain - composition, cause and its impact. Sulphur in fossil fuels giving oxides of sulphur when burnt. High temperatures in furnaces and internal combustion engines produce oxides of nitrogen. (Equations to be included). Acid rain affects soil chemistry and water bodies.

Global warming

Greenhouse gases - their sources and ways of reducing their presence in the atmosphere. (Water vapour, carbon dioxide, methane and oxides of nitrogen).

Ozone depletion

Formation of ozone - relevant equations

Function in the atmosphere.

Destruction of the ozone layer - chemicals responsible for this to be named but reactions not required.

Introduction

The environment comprises the physical and biological world in which we live, that is, air, water and land. It changes from region to region, since it results from a combination of different factors.

Industrial activities and faster modern modes of transport add harmful substances to the environment. These substances are harmful and potentially toxic. They are considered to be the principle cause of pollution.

Environmental pollution is the effect of undesirable changes in our surroundings that have a harmful effect on plants, animals and human beings.

The word 'pollution' is derived from the Latin word 'pollutes', which means "made dirty". Pollution is created by harmful substances generally produced by human beings. At times, nature also pollute the environment. The substances (pollutants) that contaminate and degrade earth's environment have an adverse impact not only on human life but also on plants and animals. Nature certainly has the capacity to accommodate some of these pollutants by way of recycling or storage in harmless form. However, when these pollutants are added to the environment at a rate much faster than nature, it leads to the deterioration of environment.

Toxic and otherwise harmful substances that have an undesirable impact on different components of the environment and life forms, are known as pollutants.

On the basis of their origin (sources), pollutants are of two types: natural and man-made.

Natural Sources Of Air Pollutants

Volcanoes. Volcanoes release large amounts of air pollutants such as carbon monoxide, sulphur dioxide, hydrogen sulphide, chlorine, hydrogen chloride, hydrocarbons and particulates.

Decaying vegetation. Microbial action on organic matter in soil releases pollutant i.e., nitrous oxide.

Forest fires release poisonous gas carbon monoxide.

Winds and dust storms carry particulate matter like sand and dust.

Man-Made Sources Of Air Pollutants

Automobiles use diesel or petrol as fuel. Incomplete combustion of these fuels releases carbon monoxide, sulphur dioxide, hydrocarbons, nitrogen oxides and particulates like lead.

Factories release carbon dioxide, sulphur dioxide, nitrogen monoxide and particulates.

Industrial processes release different types of air pollutants, depending upon the type of process involved.

Examples: Coal power plants release carbon monoxide, sulphur dioxide, ash and smoke. Fertilizer industries release nitrogen oxides and ammonia.

Decay of crop residue in rural areas is the main source of carbon monoxide and methane.

Teacher's Note

Air pollution from vehicles and factories is a major concern in cities, affecting the air quality that people breathe every day. Understanding natural and man-made pollution sources helps us see how both nature and human activities shape our environment.

Air Pollution

Air pollution means degradation of air quality due to concentration of harmful contaminants that affects human, plant and animal lives.

Gaseous Components Of 'Ordinary' Dry Air (Non-Polluted)

If composition of air is altered, it affects not only human life but also animal and plant life, and the atmosphere is then said to be polluted.

Air pollution is caused due to the presence of gaseous pollutants like oxides of sulphur, nitrogen, carbon, hydrocarbons, and particulate pollutants like dust, smoke, mist, spray and fume.

Gaseous Pollutants And Their Effects

Sulphur dioxide affects yield of crops and causes damage to lungs.

Hydrogen sulphide gas reduces growth of crops and causes irritation to human eyes.

Fluorides cause destruction of vegetation and affects teeth and bones.

Nitrogen oxides cause death of many plants and are suspected to cause cancer.

Carbon monoxide prevents haemoglobin from carrying oxygen to different parts of the body.

Tobacco smoke causes lung cancer.

Lead, which enters air from motor vehicles using tetraethyl lead, impairs the body's metabolic activities.

Cotton dust produces lung fibrosis and smoke particles cause asthma and other lung diseases.

A pollutant, which is a combination of oxides of nitrogen and sulphur and of partially-oxidized hydrocarbons and their derivatives produced by industries and automobiles forms a dark, thick, dust and soot laden fog and is known as smog.

Smog is noxious and irritating. It reduces visibility, induces respiratory troubles, and can cause death by suffocation.

The other main pollutants together contribute to more than 90% of global air pollution, and they are as follows:

Nitrogen oxides (N2O, NO and NO2)

Hydrocarbons (mainly methane, CH4)

Sulphur oxides (SO2 and SO3)

Hydrogen sulphide (H2S)

Carbon monoxide (CO)

Particulates (small solid particles and liquid droplets)

Oxides Of Nitrogen As Air Pollutants

Nitric oxide (NO) and nitrogen dioxide (NO2) enter the atmosphere in the following ways:

On burning of fuels in furnaces, temperature increases. At high temperature, nitrogen and oxygen present in air combine to form oxides of nitrogen.

When fuel burns in an internal combustion engine, oxides of nitrogen are produced, and they enter the atmosphere as exhaust gases from automobile engines.

Nitric acid is formed by the reaction between atmospheric nitrogen and oxygen in the presence of electric discharge, which happens during thunder storms, when there is lightning.

Nitric oxide further reacts with atmospheric oxygen (O2) and ozone (O3) to form nitrogen dioxide.

\[2NO + O_2(g) \rightarrow 2NO_2(g)\]

\[NO(g) + O_3(g) \rightarrow NO_2(g) + O_2(g)\]

Harmful Effects Of The Oxide Of Nitrogen

Nitrogen dioxide (NO2) is very harmful to plants and animals.

It causes irritation in mucous membrane.

Large concentrations of NO2 may cause serious lung diseases.

Nitrogen dioxide causes serious injury to vegetation; it damages plant leaves.

In sunlight, nitrogen dioxide oxidizes hydrocarbons to form photochemical smog. Photochemical smog causes eye irritation, asthma attacks and nasal and throat infections.

Compounds Of Sulphur As Pollutants

Compounds of sulphur like sulphur dioxide (SO2), sulphur trioxide (SO3) and hydrogen sulphide (H2S) are pollutants.

Hydrogen sulphide (H2S) is produced by decaying organic matter, such as rotten vegetables, by sewage and certain industrial operations.

Harmful Effects Of Hydrogen Sulphide

It causes nausea and irritates the eye and the throat. Being acidic in nature, H2S destroys vegetable matter.

Sulphur dioxide (SO2) and sulphur trioxide (SO3) are produced by combustion of sulphur containing fuels like coal and oil. It is also produced by metallurgical processes involving sulphide ores.

Harmful Effects Of Oxides Of Sulphur

It causes headache, vomiting and even death due to respiratory failure.

It destroys vegetation and weakens building materials/constructions.

It mixes with smoke and fog to form smog, which is very harmful.

It is oxidized by atmospheric oxygen into sulphur trioxide (SO3), which combines with water to form sulphuric acid (H2SO4). Sulphuric acid is the cause of acid rain.

\[2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)\]

\[SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq)\]

Carbon Monoxide (CO) As Air Pollutant

Carbon monoxide is formed by incomplete combustion of fuels in homes, factories and automobiles. Carbon monoxide is a highly poisonous gas.

If inhaled, it passes through the lungs directly into the blood stream. There it combines with haemoglobin, the substance that carries oxygen to body tissues. Because haemoglobin binds with carbon monoxide more than 200 times more strongly than does oxygen, even low concentrations of carbon monoxide in air have magnified effects on the body. It reduces the oxygen-carrying capacity of blood by an amount equivalent to the amount of haemoglobin converted into carboxy haemoglobin.

\[Haemoglobin + CO \rightarrow Carboxy\ haemoglobin\]

Since heart and brain are the two tissues most sensitive to oxygen depletion, they show the most serious effects of carbon monoxide exposure. In high concentration, carbon monoxide may thus kill by paralyzing normal brain action.

Control Of Carbon Monoxide Pollution

Carbon monoxide pollution can be controlled in the following ways:

(a) By switching over from internal combustion engines to electrically powered cars.

Although the latter would transfer source of pollution to power companies, it is easy to control the pollution created by power companies.

(b) Many pollution control devices are now installed in cars. Most of these devices help reduce pollution by burning gasoline completely. Complete combustion of gasoline produces only carbon dioxide and water vapour.

\[2C_8H_{18} + 25O_2 \rightarrow 16CO_2 + 18H_2O\]

octane

(c) By using substitute fuels for gasoline: Natural gas in both compressed (CNG) and liquefied forms (LNG) is now increasingly being used as fuel. Alcohols are other feasible substitutes.

(d) By using catalytic converters

Nitrogen oxide is reduced to nitrogen and oxygen in the presence of finely divided platinum or palladium as catalyst.

\[2NO \xrightarrow{Pt} N_2 + O_2\]

\[2NO_2 \xrightarrow{Pt} N_2 + 2O_2\]

Carbon monoxide changes into carbon dioxide in the presence of finely divided platinum as catalyst.

\[CO \xrightarrow[O]{Pt} CO_2 + H_2O\]

Teacher's Note

Every time we drive a car or use electricity, we are contributing to air pollution through carbon monoxide and nitrogen oxides. Learning about catalytic converters and cleaner fuels shows how technology can help reduce the pollution we create daily.

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