On Friday, 5 December 1952, a cold, still fog settled over London and did not lift for four days. There was no wind to carry it away, and a layer of warmer air overhead sat on the city like a lid. Beneath that lid, hundreds of thousands of coal fires in homes, along with power stations, factories and diesel buses, went on pouring smoke into air that had nowhere to go.
By Saturday the fog had turned yellow-black and greasy. People coming in from the street found soot on their collars and a sour, metallic taste at the back of the throat. Drivers abandoned their cars; conductors walked ahead of buses with lamps to find the kerb. Indoors, a concert was stopped because the audience could no longer see the stage.
Hospitals filled with people who could not breathe. In the days that followed, several thousand more Londoners died than would normally have died in that week, most of them from lung and heart failure. The air itself had done it.
Nothing new had been invented that week. The same fuels had been burnt in the same city for a century. What changed was that the weather stopped the air from moving, and the substances people had been quietly adding to it were allowed to build up. Four years later, Britain passed its first Clean Air Act.
This chapter is about those substances — where they come from, what they do, and what can be done about them.
Q1. The gas that makes rain acidic begins as an element locked inside a lump of coal. How does something buried in solid fuel end up dissolved in rain falling many kilometres away? Q2. Polluted air contains far less carbon monoxide than carbon dioxide, and carbon monoxide has no colour, no smell and no taste. Why, then, is it the more dangerous of the two? Q3. Ozone high above the earth is described as a protective shield, but ozone near the ground is listed as a pollutant. How can one gas be both? Q4. Without greenhouse gases the earth would be frozen and lifeless. Why, then, is an increase in those same gases treated as a serious danger? Q5. A factory returns only hot water to a river — no chemicals, no waste of any kind. How can perfectly clean water still kill the fish living downstream?
8.1 Introduction
The environment is the whole physical and biological world in which we live — the air above us, the water around us and the land beneath us, together with every living thing that depends on them. It is not the same everywhere. The air of a coastal town, the water of a hill stream and the soil of a desert are all products of local conditions, and each supports a different set of living things.
For most of human history, whatever people put into the air, water and soil was small enough for nature to deal with. Industry and modern transport changed that. Factories, power stations, vehicles and mines add substances that are harmful and, in many cases, poisonous, and they are now the principal cause of pollution.
Nature is not helpless against these substances. Rain washes gases out of the air, rivers dilute what is thrown into them, and soil bacteria break many wastes down into harmless products. This is why a single smoky fire in a village does no lasting damage.
The trouble begins when harmful substances arrive faster than nature can process them. Then they collect — in the air, in a lake, in the soil — and the surroundings themselves begin to deteriorate. That was exactly what happened over London in 1952, when the weather stopped the air from moving for four days.
8.1.1 Pollution And Pollutants
Environmental pollution is the effect of undesirable changes in our surroundings that harm plants, animals and human beings. Notice that the word describes an effect, not a substance. The substances that cause it have their own name.
A pollutant is a toxic or otherwise harmful substance that has an undesirable impact on the different components of the environment and on living things. Smoke, sewage, waste acid and lead from a battery are all pollutants; the poor health and damaged crops that follow are the pollution.
Environmental Pollution The effect of undesirable changes in our surroundings that have a harmful effect on plants, animals and human beings.
Pollutant A toxic or otherwise harmful substance that has an undesirable impact on the different components of the environment and on life forms.
Where The Word Comes From ‘Pollution’ comes from the Latin verb polluere, meaning to soil or to defile — literally, to make dirty. The word carried a sense of spoiling something that had been clean long before chemists could measure what was doing the spoiling.
Pollutant And Pollution The pollutant is the substance — sulphur dioxide, mercury, soot. The pollution is the harm it does once it is in the surroundings. One is a thing; the other is an effect.
8.1.2 Primary And Secondary Pollutants
Some pollutants reach the air already harmful, straight from whatever released them. Primary pollutants are released directly into the atmosphere from their source and stay in the same form. Dust, ash, smoke, fumes, hydrocarbons and the oxides of sulphur and nitrogen all belong here.
Others are not released at all — they are made in the air itself. Secondary pollutants are formed when primary pollutants react chemically with one another, or with other substances already present in the atmosphere.
Sunlight often supplies the energy for these reactions. When oxides of nitrogen and unburnt hydrocarbons from vehicle exhaust meet in bright sunshine, they react to give peroxyacyl nitrate, usually written as PAN, and ozone. Neither of these came out of an exhaust pipe; both were assembled in the air over the city.
The two kinds are set side by side in Table 8.1.
Table 8.1 — Primary Pollutants Compared With Secondary Pollutants | Basis | Primary pollutant | Secondary pollutant | |—|—|—| | How it reaches the air | Released directly from its source | Formed in the air itself | | Change in form | Remains in the same form as when released | Produced by chemical reaction between primary pollutants and other substances | | Role of sunlight | Not needed | Often needed to drive the reaction | | Examples | Dust, ash, smoke, fumes, hydrocarbons, oxides of sulphur and nitrogen | Peroxyacyl nitrate (PAN), ozone near the ground |
8.1.3 Natural Sources Of Air Pollutants
Pollutants are grouped in a second way as well: by where they originate. On this basis there are two kinds, natural and man-made. Both are shown together in Fig. 8.1-1.
Nature has always polluted the air, and sometimes on a scale no factory can match.
- Volcanoes throw out enormous quantities of gas along with lava and ash. The gases include carbon monoxide, sulphur dioxide, hydrogen sulphide, chlorine, hydrogen chloride and hydrocarbons, together with particulates — fine solid particles and liquid droplets suspended in air.
- Decaying vegetation in the soil is worked on by microbes. This microbial action on organic matter releases nitrous oxide, \(\mathrm{N_2O}\), into the air.
- Forest fires burn wood with too little oxygen reaching the flames, and release the poisonous gas carbon monoxide.
- Winds and dust storms lift particulate matter such as sand and dust and carry it for hundreds of kilometres.
A Gas You Will Meet Again Nitrous oxide from soil microbes is counted here as a natural pollutant. In section 8.4 the same gas appears in a different role — as one of the greenhouse gases that warm the earth.
8.1.4 Man-Made Sources Of Air Pollutants
Man-made sources differ from natural ones in one important way: they are concentrated. A volcano erupts once in a decade, but a city’s traffic runs every day, in the same streets, over the same people.
- Automobiles burn diesel or petrol. The burning is never complete, and the incomplete combustion releases carbon monoxide, sulphur dioxide, hydrocarbons, oxides of nitrogen and particulates such as lead.
- Factories release carbon dioxide, sulphur dioxide, nitrogen monoxide and particulates.
- Industrial processes release different pollutants depending on what is being made. Coal power plants give out carbon monoxide, sulphur dioxide, ash and smoke. Fertiliser industries release oxides of nitrogen and ammonia.
- Decay of crop residue in rural areas is the main source of carbon monoxide and methane.
- Burning of plastics releases carbon monoxide along with other harmful gases.
Look through both lists and one pattern stands out. The same few substances — carbon monoxide, sulphur dioxide, oxides of nitrogen, hydrocarbons and particulates — appear again and again, from a volcano and from a bus alike. These are the substances the rest of the chapter follows.
Smear a thin film of petroleum jelly on two squares of white paper. Tape one inside a room and one outside a window, both out of the rain. After a week, compare them against a fresh square. What has settled on each, and which sits closer to a road?
8.2 Air Pollution
Air is a mixture, and a remarkably steady one. Wherever you sample clean air — over a field, over the sea, high on a mountain — you find the same gases in almost the same proportions. That steadiness is what pollution disturbs.
Air pollution means the degradation of air quality caused by a build-up of harmful contaminants that affects human, plant and animal life. Once the composition of air is altered in this way, the atmosphere is said to be polluted.
Air Pollution The degradation of air quality due to the concentration of harmful contaminants that affects human, plant and animal lives.
Air pollution is caused by two broad groups of substances. The first are gaseous pollutants — oxides of sulphur, oxides of nitrogen, oxides of carbon and hydrocarbons. The second are particulate pollutants — dust, smoke, mist, spray and fume, all of them solids or liquids fine enough to float in air.
8.2.1 The Composition Of Ordinary Dry Air
Before we can say what a polluted sample contains, we need to know what an unpolluted one contains. Table 8.2 gives the gaseous components of ordinary dry air, and Fig. 8.2-1 shows the same figures as a chart.
Two units are used side by side in the table. The percentage by volume is convenient for the major gases, but useless for a gas present in traces. For those, chemists use parts per million, written ppm.
Parts Per Million (ppm) 1 ppm means one part of the gas in a million parts of air, by volume. It is used for gases too dilute for percentages: \(1\ \text{ppm} = 0.0001\ \%\). Argon at \(0.93\ \%\) is therefore \(9300\ \text{ppm}\).
Table 8.2 — Gaseous Components Of ‘Ordinary’ Dry Air (Non-Polluted)
| Nitrogen |
78.08 |
780,900 |
| Oxygen |
20.94 |
209,400 |
| Inert gases |
|
|
| Argon |
0.93 |
9300 |
| Neon |
|
18 |
| Helium |
|
5 |
| Krypton |
|
1 |
| Xenon |
|
1 |
| Carbon dioxide |
0.03 |
315 |
| Methane |
|
1 |
| Hydrogen |
|
0.5 |
| Natural pollutants |
|
|
| Oxides of nitrogen |
|
0.52 |
| Ozone |
|
0.02 |
Read the last two rows carefully. Even clean air carries oxides of nitrogen and ozone, put there by lightning and by sunlight. A pollutant is not something foreign to air; it is something present in a quantity that does harm.
8.2.2 Gaseous Pollutants And Their Effects
Each pollutant gas attacks something specific — a crop, a lung, a set of teeth — and the effects below are worth reading as a list of symptoms rather than as a list of names.
- Sulphur dioxide lowers the yield of crops and damages the lungs.
- Hydrogen sulphide slows the growth of crops and irritates human eyes.
- Fluorides destroy vegetation and affect teeth and bones.
- Oxides of nitrogen kill many plants and are suspected of causing cancer.
- Carbon monoxide prevents haemoglobin from carrying oxygen to different parts of the body.
- Tobacco smoke causes lung cancer.
- Lead, released into the 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 small group of pollutants accounts for the bulk of the problem. Together, the following contribute more than 90% of global air pollution:
- oxides of nitrogen — \(\mathrm{N_2O}\), \(\mathrm{NO}\) and \(\mathrm{NO_2}\)
- hydrocarbons, mainly methane, \(\mathrm{CH_4}\)
- oxides of sulphur — \(\mathrm{SO_2}\) and \(\mathrm{SO_3}\)
- hydrogen sulphide, \(\mathrm{H_2S}\)
- carbon monoxide, \(\mathrm{CO}\)
- particulates — small solid particles and liquid droplets
8.2.3 Smog
When several of these pollutants gather over a city at once, they do not stay separate. Oxides of nitrogen and sulphur, together with partially oxidised hydrocarbons and their derivatives from industry and traffic, combine with fog to give a dark, thick fog loaded with dust and soot. This mixture is called smog — a word built from smoke and fog.
Smog is noxious and irritating. It cuts visibility so far that traffic stops, it brings on respiratory trouble, and in severe cases it can cause death by suffocation.
Smog A pollutant formed from a combination of oxides of nitrogen and sulphur and of partially oxidised hydrocarbons and their derivatives, produced by industries and automobiles, appearing as a dark, thick fog laden with dust and soot.
Fog And Smog Fog is water — droplets condensed in cool, damp air, and harmless. Smog is fog that has taken up pollutants from industry and traffic, which is what makes it dark, irritating and dangerous.
8.2.4 Oxides Of Nitrogen As Air Pollutants
Nitrogen makes up more than three-quarters of the air and normally does nothing at all. Its molecule is held together too tightly to react at ordinary temperatures. Give it enough energy, however, and it will combine with the oxygen standing right beside it — and that is how nitric oxide, \(\mathrm{NO}\), and nitrogen dioxide, \(\mathrm{NO_2}\), get into the atmosphere.
8.2.4.1 How Oxides Of Nitrogen Enter The Atmosphere
- When fuels burn in furnaces, the temperature rises steeply. At such temperatures the nitrogen and oxygen of the air combine to form oxides of nitrogen.
- When fuel burns in an internal combustion engine, oxides of nitrogen are formed in the cylinder and leave with the exhaust gases of the vehicle.
- Nitric oxide also forms naturally when atmospheric nitrogen and oxygen react in the presence of an electric discharge. This is what happens in a thunderstorm, along the path of the lightning.
nitrogen + oxygen → nitric oxide
\[\mathrm{N_2} + \mathrm{O_2} \xrightarrow{\ \text{lightning}\ } 2\,\mathrm{NO}\]
Nitric oxide does not remain nitric oxide for long. In the air it meets oxygen, and it also meets ozone, and with either of them it is oxidised to nitrogen dioxide.
\[2\,\mathrm{NO} + \mathrm{O_2}\text{(g)} \rightarrow 2\,\mathrm{NO_2}\text{(g)}\]
\[\mathrm{NO}\text{(g)} + \mathrm{O_3}\text{(g)} \rightarrow \mathrm{NO_2}\text{(g)} + \mathrm{O_2}\text{(g)}\]
The route from air to pollutant is traced in FC 8.1.
8.2.4.2 Harmful Effects Of Oxides Of Nitrogen
Nitrogen dioxide is very harmful to plants and animals alike.
- It irritates the mucous membranes — the moist linings of the nose, throat and airways.
- Large concentrations of \(\mathrm{NO_2}\) may cause serious lung diseases.
- It injures vegetation severely, damaging the leaves of plants.
- In sunlight, nitrogen dioxide oxidises hydrocarbons to form photochemical smog, which causes eye irritation, asthma attacks and infections of the nose and throat.
The word photochemical is worth pausing on. It marks a reaction driven by light, which is why this kind of smog builds up over a sunny city in the afternoon rather than on a dark, still night.
8.2.5 Compounds Of Sulphur As Pollutants
Three compounds of sulphur are important pollutants: sulphur dioxide, \(\mathrm{SO_2}\), sulphur trioxide, \(\mathrm{SO_3}\), and hydrogen sulphide, \(\mathrm{H_2S}\).
8.2.5.1 Hydrogen Sulphide And Its Harmful Effects
Hydrogen sulphide is produced by decaying organic matter such as rotting vegetables, by sewage, and by certain industrial operations. It is the gas responsible for the smell of a blocked drain.
It causes nausea and irritates the eyes and the throat. Being acidic in nature, it also destroys vegetable matter.
8.2.5.2 Oxides Of Sulphur And Their Harmful Effects
Sulphur dioxide and sulphur trioxide are produced when sulphur-containing fuels such as coal and oil are burnt. They are also produced by metallurgical processes that treat sulphide ores, since roasting such an ore drives its sulphur off as sulphur dioxide.
The harmful effects of these oxides run from the human body to the buildings around it.
- They cause headache, vomiting and even death from respiratory failure.
- They destroy vegetation and weaken building materials and constructions.
- They mix with smoke and fog to form smog, which is very harmful.
- Sulphur dioxide is oxidised by atmospheric oxygen to sulphur trioxide, and sulphur trioxide combines with water to give sulphuric acid. This is the reason sulphuric acid appears in rain.
sulphur dioxide + oxygen → sulphur trioxide
\[2\,\mathrm{SO_2}\text{(g)} + \mathrm{O_2}\text{(g)} \rightarrow 2\,\mathrm{SO_3}\text{(g)}\]
sulphur trioxide + water → sulphuric acid
\[\mathrm{SO_3}\text{(g)} + \mathrm{H_2O}\text{(l)} \rightarrow \mathrm{H_2SO_4}\text{(aq)}\]
Where that acid ends up, and what it does when it gets there, is the subject of section 8.3.
8.2.6 Carbon Monoxide As An Air Pollutant
Carbon monoxide is formed by the incomplete combustion of fuels in homes, in factories and in automobiles. When a fuel burns with plenty of air, its carbon ends up as carbon dioxide; when the air supply is short, some of it stops one oxygen atom earlier, at carbon monoxide. It is a highly poisonous gas.
8.2.6.1 Harmful Effects Of Carbon Monoxide
Inhaled carbon monoxide passes straight through the lungs into the blood stream. There it meets haemoglobin, the substance in red blood cells that carries oxygen to the tissues.
Haemoglobin binds carbon monoxide about 200 times more strongly than it binds oxygen. Because of this, even a very low concentration of the gas in air has a magnified effect on the body: the haemoglobin is taken out of service faster than its share of the air would suggest.
haemoglobin + carbon monoxide → carboxyhaemoglobin
The product, carboxyhaemoglobin, cannot carry oxygen. So the oxygen-carrying capacity of the blood falls by an amount equal to the quantity of haemoglobin converted into carboxyhaemoglobin. The change is shown in Fig. 8.2-2.
The heart and the brain are the two tissues most sensitive to a shortage of oxygen, so they show the most serious effects of exposure. At high concentrations, carbon monoxide can kill by paralysing normal brain action.
Q2. Polluted air contains far less carbon monoxide than carbon dioxide, and carbon monoxide has no colour, no smell and no taste. Why, then, is it the more dangerous of the two?
A. Because the danger does not depend on how much of the gas is in the air, but on how tightly it holds haemoglobin — about 200 times more tightly than oxygen does. A few parts per million are therefore enough to lock up a large share of the blood’s haemoglobin as carboxyhaemoglobin, and with no smell or colour there is no warning that it is happening.
8.2.6.2 Control Of Carbon Monoxide Pollution
Carbon monoxide pollution can be controlled in the following ways.
By switching over from internal combustion engines to electrically powered cars. This transfers the source of pollution to the power companies, but pollution produced at a few large power stations is far easier to control than pollution produced by millions of vehicles.
By fitting pollution control devices, which most cars now carry. Most of these devices work by making the fuel burn completely, and complete combustion of gasoline gives only carbon dioxide and water vapour.
gasoline (octane) + oxygen → carbon dioxide + water
\[2\,\mathrm{C_8H_{18}} + 25\,\mathrm{O_2} \rightarrow 16\,\mathrm{CO_2} + 18\,\mathrm{H_2O}\]
By using substitute fuels for gasoline. Natural gas, in both compressed form (CNG) and liquefied form (LNG), is increasingly used as a vehicle fuel, and alcohols are another feasible substitute.
By using catalytic converters, fitted into the exhaust system of a vehicle. A catalyst speeds a reaction up without being used up itself, so the same finely divided metal keeps working for the life of the converter. Fig. 8.2-3 shows how one is built.
In a catalytic converter, nitrogen oxide is reduced back to nitrogen and oxygen over finely divided platinum or palladium.
\[2\,\mathrm{NO} \xrightarrow{\ \mathrm{Pt}\ } \mathrm{N_2} + \mathrm{O_2}\]
\[2\,\mathrm{NO_2} \xrightarrow{\ \mathrm{Pt}\ } \mathrm{N_2} + 2\,\mathrm{O_2}\]
Carbon monoxide, in the same converter, is oxidised to carbon dioxide over finely divided platinum.
\[2\,\mathrm{CO} + \mathrm{O_2} \xrightarrow{\ \mathrm{Pt}\ } 2\,\mathrm{CO_2}\]
Why The Honeycomb, And Why The Fuel Matters A catalyst can only work on the surface it offers, so the platinum inside a converter is spread as a thin coating over a honeycomb block with an enormous internal surface area. This is also why leaded petrol had to go: lead settles on that surface and puts the catalyst permanently out of action. Unleaded fuel was introduced across India for exactly this reason, and the same change removed lead from vehicle exhaust as well.
8.2.7 Major Air Pollutants, Their Origin And Health Impact
Table 8.3 collects the main air pollutants in one place, with the source of each and the harm it does. Read down the middle column and one point becomes hard to miss: burning fuel — petrol, diesel, coal, wood — appears in almost every row.
Table 8.3 — Major Air Pollutants: Their Origin And Health Impact
| Carbon monoxide |
Incomplete burning of petrol, diesel, wood and tobacco |
Reduces oxygen in the blood, causing retardation and dizziness |
| Carbon dioxide (\(\mathrm{CO_2}\)) |
Burning of coal, oil and natural gases |
Reduces oxygen levels |
| Chlorofluorocarbons (CFCs) |
Released by refrigerators and air-conditioning systems |
Reduce the ozone layer that protects us from the harmful ultraviolet rays of the sun |
| Lead |
Present in petrol, diesel, paints and batteries |
Damages the nervous and digestive systems and can cause cancer |
| Ozone in the troposphere |
Emissions from vehicles and industries that result in ozone layer depletion |
Burning, itching, watery eyes and lowered resistance to respiratory diseases |
| Oxides of nitrogen |
Burning of petrol, diesel and coal |
Smog, a combination of fog and smoke, produces acid rain that makes children more susceptible to respiratory diseases |
| Sulphur dioxide (\(\mathrm{SO_2}\)) |
Burning of coal, petrol and diesel |
Smog and acid rain containing sulphuric acid make children more susceptible to respiratory diseases |
| Suspended particulate matter (SPM) |
Solids suspended in smoke, dust and vapour |
Small particles lodge in the lungs and gradually damage their functioning |
A sample of city air taken beside a busy crossing is found to contain 30 ppm of carbon monoxide, while Table 8.2 gives carbon dioxide in clean air as 315 ppm. Name the process at the crossing that produces the carbon monoxide, and explain why the smaller figure describes the more dangerous situation.
8.3 Acid Rain
Rain has never been quite neutral. As a drop falls, it dissolves a little of the carbon dioxide it passes through, and the two combine to give a weak acid called carbonic acid.
carbon dioxide + water → carbonic acid
\[\mathrm{CO_2}\text{(g)} + \mathrm{H_2O}\text{(l)} \rightarrow \mathrm{H_2CO_3}\text{(aq)}\]
That small amount of carbonic acid gives normal rain a pH of about 5.6 — slightly acidic, and harmless. The term acid rain is used for all precipitation that is more acidic than this. It covers not only rain but snow, fog and dew as well, since all of them can carry dissolved acid down to the ground.
The pH of acid rain usually lies between 5.6 and 3.5, and at times it falls as low as 2. Remember that each step down the pH scale means ten times more acid, so rain at pH 3.5 is not slightly worse than normal rain — it is over a hundred times more acidic.
The pH Scale pH measures how acidic a solution is, on a scale from 0 to 14. Below 7 is acidic, 7 is neutral, above 7 is alkaline. Each whole number down means ten times more acidic: pH 4 rain is ten times more acidic than pH 5 rain.
Weak And Dilute Carbonic acid is a weak acid — it only partly splits into ions, however concentrated it is. A dilute acid is simply one with plenty of water added. Weak describes the acid itself; dilute describes the solution.
8.3.1 Composition Of Acid Rain
Acid rain owes its acidity to two mineral acids that are far stronger than carbonic acid: nitric acid and sulphuric acid, both already present in polluted air.
These acids are not released by any chimney. They are formed in the atmosphere, when oxides of nitrogen and oxides of sulphur come into contact with rain water.
8.3.2 Causes Of Acid Rain
Fossil fuels are not pure carbon. Coal and oil were formed from the remains of living things, and living things contain nitrogen and sulphur, so compounds of both elements are locked into the fuel alongside the carbon.
When such a fuel is burnt, every element in it burns. Large amounts of sulphur dioxide, oxides of nitrogen and carbon dioxide are discharged into the atmosphere, where they dissolve in rain water to form mineral acids.
The main cause of acid rain, then, is the formation of mineral acids — carbonic acid, nitric acid and sulphuric acid — in the rain itself. The whole route is set out in FC 8.2, and Fig. 8.3-1 shows it happening over a landscape.
8.3.3 Impact Of Acid Rain
Acid rain affects soil chemistry. It removes calcium and potassium, both of them basic ingredients of soil, so the soil loses its fertility and forests are damaged in the long run. Acid rain also causes a loss of nutrients from plants, which damages their leaves.
Acid rain has serious ecological effects on water bodies. The water of lakes and rivers is gradually turning acidic, and this affects the aquatic life living in it.
Acid rain causes extensive damage to building and sculptural materials such as marble, limestone, slate and mortar. All of these contain calcium carbonate, which reacts with the acids in the rain. The surface becomes pitted, and the material is weakened mechanically.
calcium carbonate + sulphuric acid → calcium sulphate + carbon dioxide + water
\[\mathrm{CaCO_3} + \mathrm{H_2SO_4} \rightarrow \mathrm{CaSO_4} + \mathrm{CO_2} + \mathrm{H_2O}\]
calcium carbonate + nitric acid → calcium nitrate + carbon dioxide + water
\[\mathrm{CaCO_3} + 2\,\mathrm{HNO_3} \rightarrow \mathrm{Ca(NO_3)_2} + \mathrm{CO_2} + \mathrm{H_2O}\]
The Taj Mahal is facing this serious problem. Its marble is calcium carbonate, and every acidic shower takes a little of the polished surface away as soluble salt and gas.
Acid rain increases the corrosion of metals. The formation of rust on structures such as iron gates and iron frames is speeded up by acid rain.
Acid rain affects human beings too. In sufficiently high concentrations it can affect a person’s breathing. Sulphur dioxide irritates the upper respiratory tract, the part of the body that expels soot particles and dust from inhaled air. At lower concentrations it injures the tissue of the lungs.
8.3.4 Reducing The Impact Of Acid Rain
The sensible place to attack acid rain is at its root cause — the emission of oxides of sulphur and nitrogen. One direct way is to use coal or oil with a low sulphur content, so that less sulphur dioxide is produced in the first place.
Emissions can also be cut by using a scrubber, a device that absorbs gaseous pollutants before they leave the chimney. A scrubber used to remove sulphur dioxide from a smoke stack works with a fine spray of water. The gas rising from the stack is passed through the scrubber, the water absorbs the sulphur dioxide, and this constituent of acid rain is never released. Fig. 8.3-2 shows the arrangement.
Taken together, the remedial measures are these.
- Using scrubbers, electrostatic precipitators and similar devices in factories.
- Reducing the emissions of oxides of sulphur and nitrogen from industries.
- Using lime to neutralise the acidity of soil.
- Raising public awareness.
Measure (iii) treats the damage rather than the cause. The others stop the acid from being made at all, which is why they matter more.
Lime Quicklime and slaked lime are basic. Spread on soil soured by acid rain, they neutralise the acid and restore the pH a crop needs. They treat the symptom, though — the acid keeps arriving until the emissions stop.
8.4 Global Warming
About three-fourths of the sun’s energy that reaches the earth is absorbed by the earth’s surface, and this is what raises the temperature of the surface. Some of the heat is also held back in the atmosphere itself.
The gases that hold it back are carbon dioxide, methane, nitrous oxide, ozone, the chloro-fluoro-carbon compounds usually called CFCs, and water vapour. Their trapping of heat is what leads to global warming.
8.4.1 The Greenhouse Effect
The name comes from gardening. In the colder parts of the world, plants are grown inside glass structures called green houses. Sunlight passes freely through the glass and warms the soil and plants inside, but the heat radiated back from them cannot get out through the glass. Heat therefore builds up, and the inside of the green house stays warm even when the air outside is freezing.
The atmosphere does the same thing on a planetary scale, with certain gases playing the part of the glass. This is called the greenhouse effect, and Fig. 8.4-1 sets the two situations side by side.
Greenhouse Effect The heating of the earth and its environment due to solar radiation trapped by greenhouse gases such as carbon dioxide and water vapour in the atmosphere.
Greenhouse Effect And Global Warming The greenhouse effect is natural and permanent — it keeps the earth warm enough for life. Global warming is what happens when human activity strengthens that effect beyond its natural level.
8.4.2 Greenhouse Gases And Their Sources
The gases that contribute to the greenhouse effect are carbon dioxide, water vapour, oxides of nitrogen, methane, ozone and the chloro-fluoro-carbons. They are known together as greenhouse gases.
They do not contribute equally. Table 8.4 gives the share of each in the total greenhouse effect.
Table 8.4 — Relative Contribution Of Greenhouse Gases
| % contribution |
50 |
19 |
17 |
8 |
4 |
2 |
Carbon Dioxide
- Burning of fossil fuels such as coal, natural gas and petroleum.
- Industrial processes such as the manufacture of lime, and the processes carried out in fermentation units.
- Biological decay of plants.
- Respiration by animals, human beings and plants.
Note that the last two are entirely natural, and have been going on for as long as there have been living things. It is the first two that have been added in the last two centuries.
If the level of carbon dioxide continues to rise at the present rate, the earth is expected to become warmer by 1°C to 3°C by 2050 A.D.
Water Vapour
The main source of water vapour is the burning of hydrocarbons. Every hydrocarbon contains hydrogen, and that hydrogen burns to water, so the reaction gives two greenhouse gases at once.
hydrocarbon + oxygen → carbon dioxide + water vapour
Oxides Of Nitrogen
The combustion of fossil fuels such as coal, oil, natural gas and gasoline in automobiles and power plants produces very high temperatures. At those temperatures the nitrogen and oxygen of the air combine to form nitric oxide and nitrogen dioxide. Some chemical industries also give out oxides of nitrogen as by-products.
Methane
Methane is emitted in large quantities during the anaerobic decomposition of organic matter in soil, water and sediments. Anaerobic simply means without air: where oxygen cannot reach, decay produces methane instead of carbon dioxide. The incomplete combustion of fossil fuels also produces methane and other hydrocarbons.
Turning A Greenhouse Gas Into A Fuel A biogas plant is a sealed tank that deliberately does what a marsh does by accident. Cattle dung and crop waste are digested inside it without air, and the methane produced is piped away and burnt for cooking instead of leaking into the atmosphere. The same gas that would have added to the greenhouse effect cooks a meal, and the digested slurry left behind goes back to the field as manure.
8.4.3 Mechanism Of The Greenhouse Effect
The sunlight that reaches the earth is not one kind of radiation but three: ultraviolet (UV) radiation, visible radiation and infra-red (IR) radiation.
As sunlight passes through the atmosphere, most of the UV radiation is absorbed by ozone. Of the infra-red radiation, 30% reaches the earth’s surface, where it heats the surface up.
Now the heated earth behaves like any warm object and radiates energy of its own. What it sends back out carries less energy than the radiation that came in, and therefore has a longer wavelength. This difference is the heart of the matter: the atmosphere is fairly transparent to the incoming radiation but not to the outgoing kind.
Some of the emitted infra-red radiation escapes from the earth’s surface into space, and some is absorbed by carbon dioxide and stays on the earth. This trapped radiation warms both the earth’s surface and the lower parts of the atmosphere. The whole sequence is drawn in Fig. 8.4-2.
8.4.4 Advantages Of The Greenhouse Effect
Before treating the greenhouse effect as a problem, it is worth seeing what it does for us.
- It has played an important part in the evolution of life on earth. Without greenhouse gases, all the heat arriving from the sun would simply escape again, and the earth would be as cold and barren as the moon.
- It is necessary for the evaporation of water and the formation of clouds, and therefore for rain.
Greenhouse gases are thus essential for life — but only within an optimum range. If their proportion in the atmosphere increases, the earth’s surface temperature increases with it. A rise in the average temperature of the earth’s surface is called global warming, and even a slight increase, of the order of 1°C to 3°C, can have far-reaching consequences for the earth’s environment.
Global Warming The long-term increase in the earth’s average surface temperature caused by human activities that release greenhouse gases into the atmosphere; these gases trap heat energy from the sun that would otherwise escape into space, enhancing the natural greenhouse effect and warming the planet.
Q4. Without greenhouse gases the earth would be frozen and lifeless. Why, then, is an increase in those same gases treated as a serious danger?
A. Because the effect is useful only in the right amount. The natural level of these gases holds back just enough heat to keep the planet liveable and to keep water evaporating. Add more of them, and more heat is held back than before, so the average surface temperature climbs. It is not the greenhouse effect that is harmful, but the strengthening of it.
8.4.5 Effects Of Global Warming
- Rise in sea level. Glaciers and polar ice caps have started to melt, and this may gradually raise the level of the sea. Coastal areas in countries such as India, Bangladesh, the Netherlands and the Maldives would be flooded.
- More water vapour. Warming causes more water to evaporate from water bodies. Since water vapour is itself a greenhouse gas, the warming increases further — an effect that feeds on itself, as FC 8.3 shows.
- Shift in crop zones. A change in the pattern of rainfall shifts the regions where a crop can be grown. Wheat-producing zones, for instance, would move away from Russia and Canada towards the less fertile polar regions.
- Loss of forests and wildlife. The same change in rainfall affects the trees and plants of forests, which are the natural habitats of wild animals. As forests are destroyed, many species of wildlife begin to die out.
- Heatwaves. These cause illness and death, particularly among vulnerable groups such as the elderly, children and low income populations.
- Shrinking mountain glaciers. Most mountain glaciers are shrinking as global temperatures rise. This significantly decreases the flow of water in summer and disrupts irrigation in areas that depend on glacial meltwater.
- Spread of disease. Warmer temperatures create favourable conditions for mosquitoes and other vectors, increasing the spread of diseases such as malaria and dengue fever.
- Heavy storms. Increased precipitation brings heavy storms, which result in flooding and severe damage to infrastructure and ecosystems.
- Forest fires and droughts. Both have become more frequent and more intense, with serious effects on ecosystems and on communities worldwide.
Using Table 8.4, name the two greenhouse gases whose contributions together exceed two-thirds of the total effect, and state one human activity that releases each of them.
8.4.6 Ways Of Reducing Global Warming
- Minimise the use of automobiles. Depending on the situation, use a bicycle, public transport or a car pool.
- Pass industrial gases through water before releasing them into the air. Water-soluble pollutants are absorbed and are prevented from polluting the atmosphere.
- Use renewable energy sources such as wind and solar energy, which reduce greenhouse gas emissions.
- Use wet scrubbers, of the kind described in section 8.3.4, to trap harmful gases such as sulphur dioxide and ammonia along with other industrial fumes.
- Plant more trees to increase the green cover.
- Avoid burning dry leaves, wood and similar waste.
- Avoid smoking. It is illegal to smoke in public places and work places, because the smoke harms not only the smoker but also those sitting nearby.
- Help people to understand global warming, since most people are unaware of it.
Put a thermometer in each of two identical glass jars and stand both in direct sunlight. Cover the mouth of one with a sheet of clear plastic and leave the other open. Read both thermometers after fifteen minutes. Which jar is warmer, and which part of Fig. 8.4-1 does your result illustrate?
8.5 Ozone In The Atmosphere
Ozone is a light bluish gas whose molecule, \(\mathrm{O_3}\), holds three atoms of oxygen instead of the usual two. It is found in the upper layer of the atmosphere, the stratosphere, and it is made there by the action of ultraviolet rays from the sun on ordinary oxygen molecules.
The ozone layer acts as a blanket about 16 km above the earth’s surface. The average concentration of ozone is above 10 ppm within this layer, and only 0.3 ppm in the atmosphere as a whole — which tells you how sharply the gas is concentrated in that one band. It is this thin layer that shields the earth from the sun’s harmful ultraviolet rays. Fig. 8.5-1 shows where it sits.
8.5.2 Function Of Ozone In The Atmosphere
Look again at both steps of the cycle and you will see that ultraviolet radiation is consumed in each of them. That is the whole point of the layer: the radiation spends its energy tearing molecules apart instead of reaching the ground.
Ozone therefore absorbs the harmful ultraviolet rays coming from the sun and prevents them from reaching the earth. This protects life on earth from the effects of ultraviolet rays, which can:
- cause skin cancer;
- damage ecosystems and destroy many organisms necessary for life in general.
8.5.3 Depletion Of The Ozone Layer
Under normal conditions an equilibrium is set up between the generation and the destruction of ozone, so that its concentration remains constant. Pollutants upset that balance.
Oxides of nitrogen and chlorofluorocarbons, usually shortened to CFCs, disturb the equilibrium. Molecules of ozone react with these pollutants and are destroyed, and the quantity of ozone in the upper layers of the atmosphere falls. This is called ozone depletion.
Ozone Depletion The gradual thinning of the ozone layer in the upper atmosphere, caused by the release of chemicals such as chlorine and chlorofluorocarbons.
Where depletion is extensive, it leads to the formation of an ozone hole, through which more ultraviolet rays reach the earth. Scientists detected such a hole above Antarctica in the 1980s; it is mapped in Fig. 8.5-2.
Ozone depletion is a major environmental problem, because it increases the amount of ultraviolet radiation reaching the earth’s surface. This leads to:
- higher rates of skin cancer, eye cataracts and immune system damage;
- harm to agriculture and to aquatic ecosystems.
Ozone Depletion And Global Warming They are different problems. Depletion is a thinning of the ozone layer, which lets more ultraviolet radiation in. Global warming is extra heat held back near the ground. CFCs happen to contribute to both, which is why the two are often confused.
8.5.4 Chemicals Responsible For The Destruction Of The Ozone Layer
Excessive Use Of Chlorofluorocarbons
Chlorofluorocarbons, known commercially as freons, enter the atmosphere because they are used so widely — as solvents, in aerosol sprays, as propellants, as refrigerants, and as blowing agents for plastic foams.
High in the atmosphere, ultraviolet rays decompose these compounds and set chlorine free in its atomic form, which is highly reactive. A single atom left with an unpaired electron in this way is called a free radical, and it is written in square brackets or with the atom standing alone.
\[\mathrm{CF_2Cl_2}\text{(g)} \xrightarrow{\ \text{ultraviolet rays}\ } \mathrm{CF_2Cl}\text{(g)} + \mathrm{Cl}\text{(g)}\]
The chlorine free radical attacks ozone, and chlorine monoxide is formed.
\[\mathrm{Cl}\text{(g)} + \mathrm{O_3}\text{(g)} \rightarrow \mathrm{ClO}\text{(g)} + \mathrm{O_2}\text{(g)}\]
This is what causes the depletion of ozone. But the story does not end there. The chlorine monoxide reacts further with an atom of oxygen, and gives the chlorine radical back again.
\[\mathrm{ClO}\text{(g)} + \mathrm{O}\text{(g)} \rightarrow \mathrm{Cl}\text{(g)} + \mathrm{O_2}\text{(g)}\]
That returned radical destroys another ozone molecule, and the process repeats. This is why a small quantity of CFC does such large-scale damage: the chlorine is not used up, so one atom of it can go round the cycle again and again. FC 8.4 follows the loop.
Q3. Ozone high above the earth is described as a protective shield, but ozone near the ground is listed as a pollutant. How can one gas be both?
A. Because what matters is where it is. In the stratosphere, ozone absorbs the sun’s ultraviolet rays before they can reach living things, and there it protects us. Near the ground there is no ultraviolet radiation left for it to absorb, and what remains is a reactive gas that irritates the eyes and airways and lowers resistance to respiratory disease — which is why Table 8.3 lists ozone in the troposphere among the pollutants.
Fuel Of Planes
When the fuel of an aircraft burns, a large quantity of nitric oxide and other gases is released into the atmosphere. Nitric oxide reacts with ozone to give nitrogen dioxide, by the reaction already met in section 8.2.4.1, and the nitrogen dioxide attacks ozone in its turn to form nitrogen trioxide.
\[\mathrm{NO_2}\text{(g)} + \mathrm{O_3}\text{(g)} \rightarrow \mathrm{NO_3}\text{(g)} + \mathrm{O_2}\text{(g)}\]
This too causes depletion of ozone.
Industrial Chemicals
The chemicals used in industry such as methyl chloride, \(\mathrm{CH_3Cl}\), methyl bromide, \(\mathrm{CH_3Br}\), and carbon tetrachloride, \(\mathrm{CCl_4}\), deplete the ozone layer as well, and so do the gases responsible for global warming. Ultraviolet rays decompose all of them to give highly reactive species.
In the stratosphere, some of these reactive species — chlorine atoms in particular — get locked up in stable compounds and can no longer attack ozone. The locking up of chlorine monoxide and of free radicals of chlorine is called SCAVENGING. Nitrogen dioxide scavenges chlorine monoxide, and methane scavenges chlorine atoms.
\[\mathrm{ClO}\text{(g)} + \mathrm{NO_2}\text{(g)} \rightarrow \mathrm{ClONO_2}\text{(g)}\]
\[\mathrm{Cl}\text{(g)} + \mathrm{CH_4}\text{(g)} \rightarrow \mathrm{CH_3}\text{(g)} + \mathrm{HCl}\text{(g)}\]
Scavenging removes some of the radicals, but not all of them. The rest continue to break ozone molecules, and the depletion of ozone goes on.
The Refrigerant That Had To Be Replaced CFCs were once thought ideal for refrigerators: unreactive, non-flammable and safe to handle. Their very unreactivity turned out to be the problem, because it let them drift intact all the way to the stratosphere before ultraviolet light broke them apart. Under an international agreement signed in 1987 at Montreal, countries including India phased them out, and refrigerators and air conditioners were redesigned around replacement refrigerants that carry no chlorine.
8.5.5 Effects Of Ozone Depletion
Effects On Human Health
- Ultraviolet radiation damages skin cells and the DNA inside them. This raises the risk of mutations, and so of skin cancers, including melanoma.
- Prolonged exposure to ultraviolet radiation accelerates the ageing of skin.
- Ultraviolet rays cause cataracts and may lead to blindness.
Effects On Plants
- Increased ultraviolet radiation weakens plants and makes them more susceptible to disease.
- Crops exposed to high levels of ultraviolet radiation may develop smaller leaves and show stunted growth.
Effects On The Aquatic Ecosystem
- Ultraviolet radiation reduces the motility and the survival rate of phytoplankton, the microscopic marine algae that form a key part of the marine food chain.
- A fall in phytoplankton populations affects the larger organisms that feed on them, and so damages aquatic biodiversity and food webs.
A refrigerator leaks a chlorofluorocarbon that reaches the stratosphere. Write the equations for the three steps by which one chlorine atom from it destroys two molecules of ozone, and state what happens to that chlorine atom at the end.
8.6 Water Pollution
Water pollution is the contamination of water bodies such as lakes, rivers and oceans by harmful substances, which makes the water toxic to human beings and to the environment. It is one of the most serious environmental problems we face.
It occurs when water is contaminated by substances such as human and animal wastes, toxic chemicals, metals and oils. Several large cities release huge volumes of untreated sewage into harbours or coastal waters, and discharges from sewage outlets and factory drains are often poorly controlled. Runoff from construction sites and farmland adds to the load, carrying sediment — that is, soil particles — and nutrients into streams and lakes.
Water Pollution The contamination of water bodies such as lakes, rivers and oceans by harmful substances, which makes the water toxic to humans and to the environment.
Pollution does not stop at the surface. It affects rainwater, rivers, lakes and oceans, and also the water lying beneath the earth’s surface, known as groundwater, which feeds our springs and wells.
Polluted water may look perfectly clean or obviously dirty — appearance settles nothing. It can carry germs, chemicals or other materials that cause illness, inconvenience or even death. Such impurities must be removed before the water can be safely used for drinking, cooking, washing or laundering, and even industries must treat water before using it in manufacturing.
8.6.1 Sources Of Water Pollution
The chief sources of water pollution are industrial wastes, sewage, agricultural chemicals and wastes, thermal pollution, oil spills and plastic pollution. Fig. 8.6-1 shows several of them reaching the same stretch of river.
8.6.1.1 Industrial Wastes
In most countries, industries discharge large quantities of pollutants into wastewater that contains toxic chemicals, and much of this waste is released directly into water bodies. Industrial effluents may contain heavy metals, chlorides, carbonates, nitrates and radioactive substances.
Thermometer and dry cell industries use mercury in large quantities, and the waste from these industries reaches rivers and canals. Mercury is also used in redox reactions for reducing organic compounds. During such processes some mercury escapes into the air, while industrial waste containing mercury is discharged into waterways.
What makes mercury so dangerous is that it does not pass through. It stays in the body of whatever swallows it, and builds up along the food chain. The most famous case gave its name to a disease.
Minamata disease is caused by mercury pollution of water. It was first identified in Minamata city, Japan, where mercury-contaminated industrial waste entered the water bodies. Fish and shellfish accumulated the mercury, and the people who ate them suffered mercury poisoning. Its symptoms include numbness of the hands and feet, muscle weakness, impaired hearing and difficulty in speaking.
8.6.1.2 Sewage
Sewage consists of human waste, rubbish, and the water used for bathing and laundering. Much of it is treated in treatment plants that remove solids and dissolved substances such as nitrogen and phosphorus.
Some sewage passes through septic tanks and leaching fields before filtering into the ground. A significant amount, however, remains untreated and is discharged directly into rivers and oceans.
8.6.1.3 Agricultural Chemicals And Wastes
Rainwater, or melted snow, flows off farmland into streams and carries with it the fertilisers and pesticides used by farmers. Animal wastes from feedlots holding large numbers of animals add to this. Cattle, pigs, sheep and poultry kept in feedlots produce large quantities of waste, which often flows into nearby water bodies.
Water used for irrigation may itself become polluted by salts, pesticides and toxic chemicals lying on the soil surface, before it seeps back into the ground.
8.6.1.4 Thermal Pollution
Some industries pollute water without adding a single chemical to it. They discharge hot water that has been used to cool machinery. Power stations use water to condense the steam that drives their turbines, and when this heated water is released into rivers or lakes it causes thermal pollution, which harms aquatic plants and animals.
Thermal pollution reduces the amount of dissolved oxygen in water, because a gas is always less soluble in warm water than in cold. High water temperatures can kill fish and aquatic plants outright, and cold-water species such as salmon and trout may die if the temperature rises above a certain limit.
Q5. A factory returns only hot water to a river — no chemicals, no waste of any kind. How can perfectly clean water still kill the fish living downstream?
A. Because fish do not breathe water, they breathe the oxygen dissolved in it — and warm water holds less dissolved gas than cold water. Hot water returning from a cooling system therefore lowers the oxygen supply of the whole stretch downstream, and the species that need the most oxygen suffocate first. The water is chemically clean, but it can no longer support life.
Fill two bottles to the brim with cold tap water and cap them. Stand one in sunlight and keep the other in a cool, dark place. After an hour, look at the inside walls of each. Where have small bubbles collected, and what does that tell you about warm water and dissolved gases?
8.6.1.5 Oil Spills
Oil spills contaminate marine and freshwater ecosystems by forming toxic slicks that coat wildlife, destroy habitats and enter the food chain.
Oil spilled from ships or from offshore wells floats on the water surface and can kill aquatic plants, shellfish and wildlife. Fish may die from the oil itself, from polychlorinated biphenyls, or from a lack of dissolved oxygen.
8.6.1.6 Plastic Pollution
Plastic waste such as bottles, bags, straws, fishing nets, lines and the microbeads used in cosmetics enters waterways through littering and through storm drains. Plastics accumulate in water bodies and cause immense environmental damage.
Plastic debris harms aquatic organisms through ingestion and entanglement, destroys their habitats, and poses risks to marine life, to freshwater ecosystems and to human health.
8.6.2 Substances In Water That Life Depends On
Not everything dissolved in water is a pollutant. Natural water carries substances that living things cannot do without, and pollution is best understood as an interference with these.
The first is dissolved oxygen. Fish and other aquatic animals take in oxygen dissolved in the water through their gills, so the quantity of dissolved oxygen decides how much life a river can support.
The second group is the essential minerals. Water flowing over rock dissolves small quantities of salts of calcium, magnesium and other elements. These are drawn up by plants as nutrients, and are taken in by animals and human beings as part of a normal diet.
8.6.3 Effects Of Polluted Water
Water polluted with human and animal waste spreads diseases such as typhoid, cholera and dysentery. Toxic chemicals and metals — polychlorinated biphenyls, arsenic, lead, mercury and pesticides — have been found in drinking water, and even small amounts of these consumed over long periods can be harmful.
Polluted water used for irrigation damages crops, reduces the fertility of the soil, and destroys the beneficial microorganisms living in it.
What Boiling Does And Does Not Do Boiling water before drinking is sound practice, and it kills the bacteria and other germs that cause typhoid and cholera. It does nothing at all to dissolved lead, arsenic, mercury, nitrates or pesticides — those are chemicals, not living things, and boiling only concentrates them a little as steam escapes. Water suspected of chemical contamination needs filtering or proper treatment, not a longer boil.
8.6.3.1 Dissolved Oxygen And Biochemical Oxygen Demand
Water pollution disrupts natural processes that depend on dissolved oxygen. Aerobic bacteria — bacteria that need oxygen — break organic waste down into simpler substances in a process called mineralisation. Some of the breakdown products, such as phosphates and nitrates, act as nutrients for plants.
Useful as this is, it comes at a cost. Aerobic bacteria consume oxygen while decomposing organic matter, and the more waste there is, the more oxygen they take. The amount of oxygen required for this process is measured as the Biochemical Oxygen Demand, or BOD.
A high BOD therefore indicates heavy pollution: it means there is a great deal of organic matter for the bacteria to work on. If too much oxygen is consumed, fish such as salmon and sturgeon cannot survive.
Biochemical Oxygen Demand (BOD) The amount of oxygen required by aerobic bacteria to decompose the organic matter present in a sample of water; a high value indicates heavy pollution.
Dissolved Oxygen And BOD Dissolved oxygen is the oxygen actually present in the water. BOD is the oxygen that bacteria will demand from it. A clean river has high dissolved oxygen and low BOD; a polluted one is the other way round.
8.6.3.2 Eutrophication And Biomagnification
The nutrients released by mineralisation, together with the fertilisers washed in from farmland, feed the plant life of a lake. In excess, they feed it far too well.
Excess nutrients cause eutrophication, the excessive growth of algae over a water body. The algae and plants respire, and later they die and decay. Both processes consume oxygen, so the oxygen available to aquatic life falls sharply. FC 8.5 traces the sequence.
A second effect works through the food chain. Chemicals from industrial effluents are taken in by small organisms, and each animal that eats them keeps what it swallows. Through this biomagnification, the concentration climbs at every level of the food chain, until the animals at the top — human beings among them — receive doses large enough to cause nervous disorders and even death. It was biomagnification that turned a mercury discharge into Minamata disease, and Fig. 8.6-2 shows why.
8.6.4 Major Water Pollutants And Their Health Effects
Table 8.5 brings the categories together with the harm each one does.
Table 8.5 — Categories Of Water Pollutants And Their Major Health Effects
| Heavy metals from industrial waste |
Mercury, lead, cadmium, nickel, copper |
Damage to the nervous system and brain, kidney and liver disorders, developmental problems in children, respiratory diseases and increased risk of cancer |
| Agricultural and chemical pollutants |
Pesticides, detergents, polychlorinated biphenyls (PCBs), fertilizers |
Skin disorders, hormonal imbalance, reduced fertility, long-term toxic effects on organs and increased cancer risk |
| Biological contaminants in sewage |
Bacteria, viruses, protozoa and intestinal worms |
Water-borne diseases such as cholera, typhoid, dysentery, jaundice and diarrhoea |
| Disease spreading vectors |
Mosquitoes and snails breeding in polluted water |
Malaria, dengue fever, filariasis and other vector-borne diseases |
| Oil pollution |
Crude oil and petroleum products |
Presence of toxic hydrocarbons that may be carcinogenic and can cause skin and respiratory problems |
| Plastics and microplastics |
Plastic waste and synthetic fibres |
Entry into the food chain leading to hormonal disruption, digestive problems, cardiovascular risks and possible cancer |
8.6.5 Control Of Water Pollution
The control of water pollution requires strict treatment of industrial and municipal wastewater, proper agricultural management, responsible disposal of waste, reduced consumption of plastics, and the enforcement of environmental laws.
Read that list against the sources in section 8.6.1 and it matches them one for one. Each measure closes off a route by which pollutants reach the water — which is the only place a pollutant can be dealt with cheaply, before it is in the river rather than after.
Two samples are drawn from the same river, one upstream of a town and one just below its sewage outfall. The second sample has a much higher BOD. State what this tells you about each sample, and predict which stretch of river would support more fish, giving your reason.
8.7 Summary
The environment is the physical and biological world around us — air, water and land. Environmental pollution is the effect of undesirable changes in these surroundings that harm plants, animals and human beings.
A pollutant is a toxic or otherwise harmful substance that has an undesirable impact on the components of the environment and on life forms.
Primary pollutants enter the air directly from their source and stay in the same form. Secondary pollutants, such as peroxyacyl nitrate (PAN) and ozone, are formed in the air by reactions between primary pollutants and other substances.
Natural sources of air pollutants include volcanoes, decaying vegetation, forest fires and dust storms. Man-made sources include automobiles, factories, industrial processes, decaying crop residue and the burning of plastics.
Air pollution is the degradation of air quality by harmful contaminants. It is caused by gaseous pollutants such as oxides of sulphur, nitrogen and carbon and by hydrocarbons, and by particulate pollutants such as dust, smoke, mist, spray and fume.
Ordinary dry air contains about 78.08% nitrogen and 20.94% oxygen by volume, with argon at 0.93% and carbon dioxide at 0.03%. Even clean air carries traces of oxides of nitrogen and ozone.
Each gaseous pollutant does specific damage: sulphur dioxide lowers crop yields and damages lungs, hydrogen sulphide irritates the eyes, fluorides affect teeth and bones, oxides of nitrogen kill plants, lead impairs metabolic activity, and cotton dust and smoke particles cause lung disease.
Smog is a dark, thick fog laden with dust and soot, formed from oxides of nitrogen and sulphur together with partially oxidised hydrocarbons. It reduces visibility, causes respiratory trouble and can cause death by suffocation.
Oxides of nitrogen form wherever nitrogen and oxygen are given enough energy — in furnaces, in internal combustion engines and in lightning.
\[\mathrm{N_2} + \mathrm{O_2} \xrightarrow{\ \text{lightning}\ } 2\,\mathrm{NO} \qquad 2\,\mathrm{NO} + \mathrm{O_2} \rightarrow 2\,\mathrm{NO_2}\]
In sunlight, nitrogen dioxide oxidises hydrocarbons to photochemical smog, which causes eye irritation, asthma attacks and infections of the nose and throat.
Sulphur present in fuels burns to sulphur dioxide, which is oxidised to sulphur trioxide; this dissolves in water to give sulphuric acid.
\[2\,\mathrm{SO_2} + \mathrm{O_2} \rightarrow 2\,\mathrm{SO_3} \qquad \mathrm{SO_3} + \mathrm{H_2O} \rightarrow \mathrm{H_2SO_4}\]
Carbon monoxide is formed by the incomplete combustion of fuels. It binds haemoglobin about 200 times more strongly than oxygen does, forming carboxyhaemoglobin, so the oxygen-carrying capacity of the blood falls and the heart and brain suffer first.
Carbon monoxide pollution is controlled by electrically powered vehicles, by devices that burn the fuel completely, by substitute fuels such as CNG and LNG, and by catalytic converters, in which platinum or palladium reduces oxides of nitrogen and oxidises carbon monoxide.
Normal rain has a pH of about 5.6, because dissolved carbon dioxide forms weak carbonic acid. Acid rain is any precipitation more acidic than this, with pH usually between 5.6 and 3.5 and at times as low as 2.
Acid rain results from nitric acid and sulphuric acid formed when oxides of nitrogen and sulphur, released by burning fossil fuels, come into contact with rain water.
\[\mathrm{N_2} + \mathrm{O_2} \xrightarrow{\ 3000^\circ\text{C}\ } 2\,\mathrm{NO} \qquad 2\,\mathrm{NO_2} + \mathrm{H_2O} \rightarrow \mathrm{HNO_2} + \mathrm{HNO_3}\]
\[\mathrm{S} + \mathrm{O_2} \rightarrow \mathrm{SO_2} \qquad 2\,\mathrm{SO_2} + \mathrm{O_2} + 2\,\mathrm{H_2O} \rightarrow 2\,\mathrm{H_2SO_4}\]
- Acid rain removes calcium and potassium from soil and lowers its fertility, turns the water of lakes and rivers acidic, attacks marble and limestone structures such as the Taj Mahal, speeds up the corrosion of metals, and injures the human respiratory tract.
\[\mathrm{CaCO_3} + \mathrm{H_2SO_4} \rightarrow \mathrm{CaSO_4} + \mathrm{CO_2} + \mathrm{H_2O}\]
Acid rain is controlled by using low-sulphur fuels, by fitting scrubbers and electrostatic precipitators, by reducing industrial emissions of oxides of sulphur and nitrogen, by liming acidic soil, and by raising public awareness.
The greenhouse effect is the heating of the earth and its environment by solar radiation trapped by greenhouse gases such as carbon dioxide and water vapour. It is named after the glass green houses used to grow plants in cold regions, which let sunlight in but do not let radiated heat out.
The greenhouse gases are carbon dioxide, water vapour, oxides of nitrogen, methane, ozone and the chlorofluorocarbons. Their contributions to the effect are 50%, 2%, 4%, 19%, 8% and 17% respectively.
Sunlight reaches the earth as ultraviolet, visible and infra-red radiation. Most ultraviolet radiation is absorbed by ozone, and 30% of the infra-red radiation reaches and heats the surface. The warm earth re-emits radiation of longer wavelength, part of which is absorbed by carbon dioxide and warms the surface and the lower atmosphere.
Without greenhouse gases the earth would be as cold and barren as the moon, and there would be no evaporation, clouds or rain. Beyond the optimum level, however, the same gases raise the surface temperature, and this rise is called global warming.
Global warming raises sea levels, shifts crop zones, shrinks mountain glaciers, brings heatwaves, storms, droughts and forest fires, and spreads vector-borne diseases such as malaria and dengue.
Global warming is reduced by using bicycles, public transport and car pools, by passing industrial gases through water, by using renewable energy, by fitting wet scrubbers, by planting trees, by not burning dry leaves and wood, and by spreading awareness.
Ozone is a light bluish gas concentrated in a layer about 16 km above the earth’s surface, at above 10 ppm, against 0.3 ppm in the atmosphere as a whole. Ultraviolet radiation makes it from oxygen and also breaks it down again, and this continuous cycle keeps its concentration steady.
\[\mathrm{O_2} + \text{UV} \rightarrow \mathrm{O} + \mathrm{O} \qquad \mathrm{O} + \mathrm{O_2} \rightarrow \mathrm{O_3} \qquad 3\,\mathrm{O_2}\text{(g)} + \text{UV} \rightarrow 2\,\mathrm{O_3}\text{(g)}\]
Ozone absorbs the sun’s harmful ultraviolet rays and so protects living things from skin cancer and from damage to ecosystems.
Ozone depletion is the gradual thinning of the ozone layer caused by chemicals such as chlorine and the chlorofluorocarbons, and extensive depletion forms an ozone hole, first found above Antarctica.
Ultraviolet rays split chlorofluorocarbons to give chlorine free radicals. Each radical destroys ozone and is then regenerated, so one atom of chlorine destroys ozone molecule after molecule.
\[\mathrm{Cl} + \mathrm{O_3} \rightarrow \mathrm{ClO} + \mathrm{O_2} \qquad \mathrm{ClO} + \mathrm{O} \rightarrow \mathrm{Cl} + \mathrm{O_2}\]
Nitric oxide from aircraft fuel, and industrial chemicals such as methyl chloride, methyl bromide and carbon tetrachloride, also deplete ozone. The locking up of chlorine monoxide and chlorine radicals by nitrogen dioxide and methane is called scavenging.
Ozone depletion increases skin cancers and cataracts in human beings, weakens and stunts plants, and reduces the phytoplankton on which marine food chains depend.
Water pollution is the contamination of water bodies by harmful substances that make the water toxic. Its chief sources are industrial wastes, sewage, agricultural chemicals and wastes, thermal pollution, oil spills and plastic pollution.
Mercury discharged by thermometer and dry cell industries accumulates in fish and shellfish. Eating them caused Minamata disease in Japan, with numbness of the hands and feet, muscle weakness, impaired hearing and difficulty in speaking.
Hot water discharged from cooling systems causes thermal pollution, which lowers the dissolved oxygen of the water and kills cold-water species.
Natural water supplies two things life depends on: dissolved oxygen, which aquatic animals breathe, and essential minerals needed by plants and by our bones and teeth.
Aerobic bacteria decompose organic waste in water, consuming oxygen as they do so. The oxygen required is measured as the Biochemical Oxygen Demand (BOD), and a high BOD indicates heavy pollution.
Excess nutrients cause eutrophication, in which algae grow rapidly, then respire and decay and strip the water of oxygen. Chemicals from effluents undergo biomagnification, building up to higher concentrations at each step of the food chain.
Polluted water spreads typhoid, cholera and dysentery, carries toxic metals and PCBs into drinking supplies, damages crops and soil, and harms aquatic life through oil slicks and plastic debris.
Control of water pollution requires strict industrial and municipal wastewater treatment, proper agricultural management, responsible disposal of waste, reduced plastic consumption and the enforcement of environmental laws.
8.8 Key Terms
Acid rain — Any precipitation, whether rain, snow, fog or dew, that is more acidic than normal rain water.
Air pollution — The degradation of air quality caused by harmful contaminants that affect human, plant and animal life.
Biochemical Oxygen Demand (BOD) — The amount of dissolved oxygen used up by bacteria in decomposing the organic matter present in a water sample.
Biomagnification — The build-up of a pollutant to higher and higher concentrations at each successive level of a food chain.
Carboxyhaemoglobin — The compound formed when haemoglobin binds carbon monoxide instead of oxygen; it cannot carry oxygen to the tissues.
Catalytic converter — A device fitted in a vehicle’s exhaust system in which finely divided platinum or palladium converts oxides of nitrogen and carbon monoxide into harmless gases.
Dissolved oxygen — Oxygen gas dissolved in water, which aquatic animals take in through their gills.
Environment — The physical and biological world in which we live, that is, the air, water and land and the living things they support.
Environmental pollution — The effect of undesirable changes in our surroundings that harm plants, animals and human beings.
Eutrophication — The excessive growth of algae in a water body caused by an excess of nutrients, which eventually strips the water of oxygen.
Global warming — The long-term rise in the average surface temperature of the earth, caused by human activities that release greenhouse gases.
Greenhouse effect — The heating of the earth and its environment by solar radiation trapped by greenhouse gases in the atmosphere.
Greenhouse gases — The gases that trap the earth’s outgoing heat, namely carbon dioxide, water vapour, oxides of nitrogen, methane, ozone and the chlorofluorocarbons.
Groundwater — Water present beneath the earth’s surface, which feeds springs and wells.
Minamata disease — A disorder caused by mercury poisoning from eating fish and shellfish taken from mercury-contaminated water.
Ozone depletion — The gradual thinning of the ozone layer caused by chemicals such as chlorine and the chlorofluorocarbons.
Ozone hole — A region of extensive ozone depletion through which more ultraviolet radiation reaches the earth.
Ozone layer — The band of the stratosphere, about 16 km above the earth’s surface, in which ozone is concentrated and absorbs ultraviolet rays.
Particulates — Small solid particles and liquid droplets fine enough to remain suspended in air.
Pesticides — Chemicals applied to crops to kill pests, which reach water bodies in runoff from farmland.
Photochemical smog — Smog formed when nitrogen dioxide oxidises hydrocarbons in sunlight, causing eye irritation, asthma attacks and infections of the nose and throat.
Pollutant — A toxic or otherwise harmful substance that has an undesirable impact on the environment and on life forms.
Primary pollutant — A pollutant released directly into the atmosphere from its source, which remains in the same form.
Scrubber — A device that absorbs gaseous pollutants, such as sulphur dioxide, from the gases rising up a chimney.
Secondary pollutant — A pollutant formed in the atmosphere itself by chemical reaction between primary pollutants and other substances present there.
Smog — A dark, thick fog laden with dust and soot, formed from oxides of nitrogen and sulphur together with partially oxidised hydrocarbons.
Thermal pollution — The harm done to aquatic life when hot water used for cooling machinery is discharged into a river or lake.
Water pollution — The contamination of water bodies by harmful substances that make the water toxic to human beings and to the environment.
8.9 Points To Ponder
Oxides of nitrogen have always been made by lightning, and the traces listed in ordinary dry air prove it. At what point does a substance that nature itself puts into the air become a pollutant — and who decides where that point lies?
A catalytic converter cleans the exhaust of one car, while a scrubber cleans the chimney of one factory. Both remove pollutants after they have been made. What would it take to design a fuel or a process that never made them in the first place, and why is that so much harder?
Sulphur dioxide released from a tall chimney may fall as acid rain hundreds of kilometres away, in a place that burned none of the coal. What does this tell you about treating air as something that belongs to a particular town or country?
Ozone protects life when it is 16 km up and harms it at ground level, and carbon dioxide keeps the earth habitable in one quantity and dangerous in another. Is ‘pollutant’ a property of a substance at all, or only of a substance in a place and an amount?
A single chlorine atom destroys ozone molecule after molecule and emerges unchanged each time — behaving exactly as a catalyst does. Why does this one property make a small leak of refrigerant so much more serious than a large release of an ordinary gas?
Warm water and clean fertiliser both kill fish, and neither is poisonous. What does that suggest about judging the safety of a discharge by asking only whether it is toxic?
8.10 Objective Questions
Multiple Choice
- The gas responsible for making normal rain slightly acidic is: [1]
- sulphur dioxide
- carbon dioxide
- nitric oxide
- hydrogen sulphide
- Peroxyacyl nitrate is classified as a secondary pollutant because it is: [1]
- released directly from vehicle exhaust
- present in ordinary dry air
- formed in the atmosphere by reaction between primary pollutants
- less harmful than a primary pollutant
- Carbon monoxide is dangerous at very low concentrations because it: [1]
- dissolves readily in the water of the blood
- binds haemoglobin far more strongly than oxygen does
- reacts with the oxygen present in the lungs
- has neither colour nor smell
- Which of the following is not a greenhouse gas? [1]
- Methane
- Nitrous oxide
- Argon
- Water vapour
- The reaction \(\mathrm{O} + \mathrm{O_2} \rightarrow \mathrm{O_3}\) takes place in the: [1]
- troposphere
- stratosphere
- exhaust of an internal combustion engine
- water of a polluted lake
- A single chlorine free radical can destroy a very large number of ozone molecules because it is: [1]
- heavier than an ozone molecule
- regenerated at the end of each cycle
- produced in very large quantity
- scavenged by methane
- A high value of Biochemical Oxygen Demand in a river sample indicates: [1]
- a large amount of organic waste in the water
- a high concentration of dissolved oxygen
- the presence of heavy metals only
- water fit for drinking
- Marble monuments are damaged by acid rain because marble contains: [1]
- calcium sulphate
- calcium carbonate
- calcium nitrate
- calcium oxide
Assertion–Reason
- Choose from the following codes:
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Assertion (A): Thermal pollution can kill fish although the discharged water contains no chemical waste. Reason (R): Warm water holds less dissolved oxygen than cold water. [1]
Assertion (A): Ozone present near the ground is treated as a pollutant. Reason (R): Ozone in the stratosphere absorbs the sun’s ultraviolet rays. [1]
Assertion (A): The greenhouse effect is harmful and should be removed altogether. Reason (R): Without greenhouse gases the earth would be as cold and barren as the moon. [1]
Assertion (A): Leaded petrol cannot be used in a vehicle fitted with a catalytic converter. Reason (R): Lead settles on the surface of the catalyst and puts it out of action. [1]
Match The Following
- Match each substance in column A with the effect it produces in column B. [5]
| a. Fluorides |
i. Lung fibrosis |
| b. Cotton dust |
ii. Numbness of hands and feet |
| c. Mercury in fish |
iii. Destruction of vegetation and damage to teeth and bones |
| d. Excess nitrates in a lake |
iv. Carboxyhaemoglobin in the blood |
| e. Carbon monoxide |
v. Excessive growth of algae |
Fill In The Blanks
The oxides of nitrogen and sulphur dissolved in rain water are the cause of ____. [1]
Ozone is concentrated in a layer about ____ km above the earth’s surface. [1]
The locking up of chlorine monoxide and chlorine free radicals in the stratosphere is called ____. [1]
The build-up of a pollutant to higher concentrations at each level of a food chain is called ____. [1]
True Or False
Ordinary dry air contains no oxides of nitrogen at all. [1]
Sulphur enters the atmosphere because it is present in coal and fuel oil. [1]
Water that looks perfectly clear is always safe to drink. [1]
Name The Following
The gas produced by microbial action on organic matter in soil. [1]
The metal, once added to petrol, that impairs the body’s metabolic activities. [1]
The compound formed when chlorine monoxide is scavenged by nitrogen dioxide. [1]
The process by which aerobic bacteria break organic waste in water into simpler substances. [1]
Give One Word Or Phrase For
A pollutant that is formed in the air itself rather than released from a source. [1]
A device that removes sulphur dioxide from the gases rising up a chimney. [1]
Precipitation with a pH lower than 5.6. [1]
Odd One Out, With A Reason
Volcanoes, forest fires, dust storms, fertiliser factories. [1]
Carbon dioxide, methane, nitrous oxide, hydrogen sulphide. [1]
Mercury, cadmium, lead, chlorofluorocarbon. [1]
8.11 Check Your Concepts
Define a pollutant, and state how it differs from pollution. [2]
State two natural sources of air pollutants, naming one pollutant released by each. [2]
Why is ppm, rather than percentage by volume, used for the trace gases listed in Table 8.2? [2]
Give a reason why nitrogen, though it makes up more than three-quarters of the air, forms oxides only in furnaces, engines and lightning. [2]
State the function of ozone in the atmosphere, and name two harmful effects that follow if it is depleted. [3]
Differentiate between primary and secondary pollutants, stating the basis of difference and giving one example of each. [3]
Explain, with an example, why photochemical smog builds up over a city on a bright afternoon rather than on a dark night. [3]
Give a reason for each of the following. [3]
- Rain water is slightly acidic even in unpolluted air.
- Lime is spread on soil affected by acid rain.
- A small leak of a chlorofluorocarbon does damage out of all proportion to its quantity.
What would happen to the fish in a lake if a large quantity of fertiliser were washed into it? Trace the sequence of changes that leads to their death. [4]
Explain the mechanism of the greenhouse effect, making clear why the atmosphere lets the sun’s radiation in but holds the earth’s radiation back. [4]
Differentiate between the greenhouse effect and global warming, stating the basis of difference, and explain why greenhouse gases are described as both essential and dangerous. [4]
Describe how mercury discharged by an industry reaches human beings, and state the disease that resulted from this in Japan, together with two of its symptoms. [4]
Acid rain falls on a region that burns no coal of its own. Explain how this is possible, and describe its impact on the soil, on water bodies and on stone buildings of that region. [5]
State five ways in which global warming can be reduced, and explain, for any two of them, exactly which greenhouse gas is being kept out of the atmosphere. [5]
8.12 Equations, Conversions & Identification
Write balanced equations for the following. [3]
- The combination of atmospheric nitrogen and oxygen in a lightning stroke.
- The oxidation of sulphur dioxide by atmospheric oxygen.
- The action of sulphuric acid in rain on the calcium carbonate of a marble slab.
Complete and balance the following equations. [3]
- \(\mathrm{SO_3} + \mathrm{H_2O} \rightarrow\)
- \(\mathrm{NO_2} + \mathrm{H_2O} \rightarrow\)
- \(\mathrm{Cl} + \mathrm{O_3} \rightarrow\)
Write the word equation and the balanced chemical equation for the complete combustion of octane, and state why a pollution control device is designed to make this reaction go to completion. [3]
How would you obtain the following? Give the equations involved. [4]
- Nitric acid in rain water, starting from the nitrogen of the air.
- Sulphuric acid in rain water, starting from the sulphur present in coal.
Write the three equations by which a chlorofluorocarbon molecule leads to the destruction of ozone, and state what happens to the chlorine atom at the end of the sequence. [3]
A colourless, odourless gas is released from a stove burning with a yellow, smoky flame in a closed kitchen. A person in the room becomes dizzy. Identify the gas, name the compound it forms in the blood, and state the change in the fuel supply that would prevent it. [3]
A water sample from below a factory outfall is warm, has a low dissolved oxygen content and contains no toxic chemical. Identify the type of pollution, and state the effect on cold-water fish. [2]
Study Fig. 8.3-2. Name the labelled parts through which the gas enters and the water enters, and state the purpose of the falling spray. [3]
Study Fig. 8.2-3. Name the metals coated on the honeycomb block, state the purpose of the honeycomb shape, and name the gases that leave the outlet. [3]
State what you would observe in each case. [3]
- A marble statue is left standing in a city where the rain has a pH of 3.5 for many years.
- Iron gates are exposed to acid rain over several seasons.
- A lake receives a heavy run-off of fertiliser through one summer.