Atmospheric Pollution Class 9 ICSE — Chemistry Chapter 8

Atmospheric pollution class 9 ICSE is the focus of Chapter 8 of the Chemistry textbook. This chapter explains what pollutants are, how gases and particulates degrade the air, and the chemistry behind acid rain, global warming, and ozone depletion. It covers the reactions that form acids in rain and the ways to control pollution. Each section below follows the book’s own order.

Download Atmospheric Pollution Class 9 ICSE Chemistry Chapter 8 PDF

Download the Atmospheric Pollution Class 9 ICSE Chemistry chapter PDF to revise offline or print for practice. It is the full chapter as printed in the book — 11 pages, 14 sections, 25 questions.

What is inside Chapter 8

11 pages · 14 sections · 25 exercise questions

• EXERCISE 8: 25 questions

8.1 INTRODUCTION

Environmental pollution is the effect of undesirable changes in our surroundings that harm plants, animals, and human beings. The word ‘pollution’ comes from the Latin ‘pollutes’, meaning “made dirty”. Substances that contaminate and degrade the environment are called pollutants.

Pollutants are classified by origin. Natural sources include volcanoes, decaying vegetation, forest fires, and wind or dust storms. Man-made sources include automobiles, factories, industrial processes, and the decay of crop residue in rural areas. This chapter sits inside the broader Class 9 Chemistry book.

8.2 AIR POLLUTION

Air pollution means the degradation of air quality by harmful contaminants. Clean dry air has a fixed composition. Nitrogen makes up 78.08%, oxygen 20.94%, argon 0.93%, and carbon dioxide 0.03%. Inert gases and natural pollutants appear in tiny amounts measured in parts per million (ppm).

One ppm is one part of a substance in one million parts of air. Pollutants fall into two classes. Gaseous pollutants include oxides of sulphur, nitrogen, and carbon, plus hydrocarbons. Particulate pollutants include dust, smoke, mist, spray, and fume.

8.2.1 Gaseous pollutants and their effects

Sulphur dioxide damages crops and lungs. Hydrogen sulphide slows crop growth and irritates eyes. Fluorides destroy vegetation and affect teeth and bones. Nitrogen oxides kill plants and may cause cancer. Carbon monoxide stops haemoglobin from carrying oxygen. Tobacco smoke causes lung cancer. Lead impairs metabolism. Cotton dust causes lung fibrosis, and smoke particles trigger asthma.

Smog is a dark, soot-laden fog formed when nitrogen and sulphur oxides mix with partially oxidised hydrocarbons from industry and automobiles. It reduces visibility, irritates the respiratory tract, and can suffocate.

Oxides of nitrogen form when air burns hot in furnaces, internal combustion engines, or lightning. Nitric oxide then reacts with oxygen and ozone to give nitrogen dioxide.

2\text{NO(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{NO}_2\text{(g)}

\text{NO(g)} + \text{O}_3\text{(g)} \rightarrow \text{NO}_2\text{(g)} + \text{O}_2\text{(g)}

Sulphur dioxide oxidises in air to sulphur trioxide, which reacts with water to form sulphuric acid. This acid is a main cause of acid rain.

2\text{SO}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{SO}_3\text{(g)}

\text{SO}_3\text{(g)} + \text{H}_2\text{O(l)} \rightarrow \text{H}_2\text{SO}_4\text{(aq)}

Carbon monoxide forms by incomplete combustion in homes, factories, and automobiles. It binds to haemoglobin more than 200 times more strongly than oxygen, forming carboxy haemoglobin. This starves the heart and brain of oxygen, and high levels can kill.

Control methods include electric cars, complete combustion, CNG and LNG fuel, and catalytic converters. Complete combustion of octane gives only carbon dioxide and water.

2\text{C}_8\text{H}_{18} + 25\text{O}_2 \rightarrow 16\text{CO}_2 + 18\text{H}_2\text{O}

Worked pattern: one mole of octane (114 g) produces eight moles of carbon dioxide. That is 8 \times 44 = 352 g of \text{CO}_2 from one mole of fuel.

In a catalytic converter, finely divided platinum reduces nitrogen oxides to nitrogen and oxidises carbon monoxide to carbon dioxide.

2\text{NO} \xrightarrow{\text{Pt}} \text{N}_2 + \text{O}_2

\text{CO} \xrightarrow[\text{[O]}]{\text{Pt}} \text{CO}_2

8.3 ACID RAIN

Acid rain describes all precipitation — rain, snow, fog, dew — that is more acidic than normal water. Normal rain has a pH near 5.6 because carbon dioxide reacts with water to form weak carbonic acid.

\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3

Acid rain usually has a pH between 5.6 and 3.5, and can fall as low as 2. A common mistake is to think normal rain is neutral. It is already slightly acidic; acid rain is far more acidic.

8.3.1 Composition of Acid Rain

Acid rain is mainly a mixture of nitric acid and sulphuric acid in polluted air. These acids form when oxides of nitrogen and sulphur come into contact with rain water.

8.3.2 Causes of Acid Rain

Fossil fuels contain nitrogen and sulphur compounds. Burning them releases oxides into the air. These oxides dissolve in rain water and form mineral acids.

Nitric acid forms in three steps. Nitrogen and oxygen combine at high temperature during thunder and lightning.

\text{N}_2 + \text{O}_2 \xrightarrow{\text{high temperature}} 2\text{NO}

Nitric oxide oxidises to nitrogen dioxide, which reacts with water to give nitrous and nitric acids.

2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2

2\text{NO}_2 + \text{H}_2\text{O} \rightarrow \text{HNO}_2 + \text{HNO}_3

Sulphuric acid forms when sulphur in coal burns. Coal used in power plants can hold up to 4% sulphur.

\text{S} + \text{O}_2 \rightarrow \text{SO}_2

2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3

\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4

Worked pattern: trace sulphur through the route with new numbers. A power plant burns coal that is 2% sulphur. In 100 g of coal there are 2 g of sulphur, which is \frac{2}{32} = 0.0625 mol. Each mole of sulphur gives one mole of sulphuric acid. The acid mass is 0.0625 \times 98 = 6.125 g. That mass falls as acid rain.

8.3.3 Impact of Acid Rain

Acid rain leaches calcium and potassium from soil, so the soil loses fertility. It acidifies lakes and rivers, harming aquatic life. It damages marble, limestone, slate, and mortar, weakening them.

Calcium carbonate reacts with both sulphuric and nitric acids.

\text{CaCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{CO}_2 + \text{H}_2\text{O}

\text{CaCO}_3 + 2\text{HNO}_3 \rightarrow \text{Ca(NO}_3)_2 + \text{CO}_2 + \text{H}_2\text{O}

The Taj Mahal faces this problem. Acid rain also corrodes metals, such as iron gates and frames, and irritates human breathing.

Worked pattern: a marble surface of 100 g is calcium carbonate. One mole of \text{CaCO}_3 (molar mass 100 g/mol) reacts with sulphuric acid to give one mole of calcium sulphate. The product mass is 1 \times 136 = 136 g.

Reduction attacks the root cause by cutting sulphur and nitrogen oxide emissions. Use coal or oil with low sulphur content. A scrubber sprays fine water through rising stack gas, and the water absorbs sulphur dioxide.

8.4 GLOBAL WARMING

About three-fourths of the sun’s energy reaching the earth is absorbed by the surface, which raises its temperature. Greenhouse gases — carbon dioxide, methane, nitrous oxide, ozone, CFCs, and water vapour — trap some of that heat. The trapped heat causes global warming.

8.4.1 Green house effect

The greenhouse effect is the heating of the earth by solar radiation trapped by carbon dioxide and water vapour. The name comes from glass greenhouses used to grow plants in cold regions. Glass lets sunlight in but stops radiated heat from escaping, so the inside warms.

8.4.2 Greenhouse gases

Greenhouse gases contribute in set proportions: \text{CO}_2 50%, \text{CH}_4 19%, CFC 17%, \text{O}_3 8%, \text{N}_2\text{O} 4%, \text{H}_2\text{O} 2%. The mnemonic “Clean Cars Cut Ozone, No Harm” keeps the order. The first letters C-C-C-O-N-H match the gases from largest to smallest contribution.

Carbon dioxide comes from burning fossil fuels, industrial processes, plant decay, and respiration. Water vapour forms when hydrocarbons burn. Nitrogen oxides form in hot combustion. Methane comes from anaerobic decay of organic matter.

The greenhouse effect supports life. Without it, the earth would be as cold and barren as the moon. It drives evaporation and rain. But there is an optimum level. If greenhouse gases rise, the earth may warm by 1 to 3 degrees Celsius by 2050.

8.5 OZONE

Ozone is a light bluish gas in the stratosphere, formed by ultraviolet rays acting on oxygen.

3\text{O}_2\text{(g)} \rightarrow 2\text{O}_3\text{(g)}

8.5.1 Function of ozone in the atmosphere

The ozone layer acts as a blanket above 16 km. It absorbs harmful ultraviolet rays that can cause skin cancer and destroy organisms.

High-energy UV breaks oxygen molecules into atoms. Each atom reacts with an oxygen molecule to form ozone.

\text{O}_2 + \text{UV} \rightarrow \text{O} + \text{O}

\text{O} + \text{O}_2 \rightarrow \text{O}_3

Net reaction: 3\text{O}_2 + \text{UV} \rightarrow 2\text{O}_3. Ozone also absorbs longer UV, splitting back into oxygen and an oxygen atom.

\text{O}_3 \rightarrow \text{O}_2 + \text{O}

8.5.2 Destruction of ozone layer

Ozone depletion is a decrease in ozone in the upper atmosphere, which leads to an ozone hole. Through the hole, ultraviolet rays reach the earth and cause skin cancer. In 1980, scientists detected an ozone hole above Antarctica.

Normally, ozone generation and destruction balance, so the concentration stays constant. Pollutants like nitrogen oxides and free chlorine radicals react with ozone and break that balance. This causes depletion.

8.5.3 Chemicals responsible for destruction of ozone layer

Chlorofluorocarbons (CFCs) enter the air from solvents, aerosol sprays, propellants, refrigerants, and plastic foam blowing agents. Ultraviolet rays decompose them, releasing atomic chlorine.

\text{CF}_2\text{Cl}_2\text{(g)} \xrightarrow{\text{UV rays}} \text{CF}_2\text{Cl(g)} + \text{Cl(g)}

The chlorine radical attacks ozone, forming chlorine monoxide. This reacts with atomic oxygen, regenerating chlorine.

\text{Cl(g)} + \text{O}_3\text{(g)} \rightarrow \text{ClO(g)} + \text{O}_2\text{(g)}

\text{ClO(g)} + \text{O(g)} \rightarrow \text{Cl(g)} + \text{O}_2\text{(g)}

Worked pattern: because chlorine is regenerated, one atom can destroy many ozone molecules. If a single chlorine radical cycles ten times, it removes ten ozone molecules before it is stopped.

Aircraft fuel is a second cause. Burning it releases nitric oxide, which reacts with ozone.

\text{NO(g)} + \text{O}_3\text{(g)} \rightarrow \text{NO}_2\text{(g)} + \text{O}_2\text{(g)}

\text{NO}_2\text{(g)} + \text{O}_3\text{(g)} \rightarrow \text{NO}_3\text{(g)} + \text{O}_2\text{(g)}

Scavenging locks the reactive species. Nitrogen dioxide captures chlorine monoxide as \text{ClONO}_2, and methane captures chlorine atoms.

\text{ClO(g)} + \text{NO}_2\text{(g)} \rightarrow \text{ClONO}_2\text{(g)}

Ground-level ozone is harmful, unlike the protective stratospheric layer. It causes respiratory problems, damages plants and trees, and harms tyres and asphalt. This is the second common mistake: ozone protects high up but pollutes at ground level.

Table 8.2 lists major air pollutants with their origin and health impact. Exam questions often ask you to match a pollutant to its source and effect. Carbon monoxide, CFCs, lead, and sulphur dioxide all appear in this table.

What to practise from Atmospheric Pollution Class 9 ICSE

This chapter lists one exercise block. It covers definitions, sources and effects of pollutants, reaction writing, fill-in-the-blanks, and multiple choice.

• EXERCISE 8 — 25 questions

Attempt the pollutant-effect questions and the reaction-writing questions first. The table of major air pollutants is a common source for these. Check the syllabus on the official CISCE website. Other Class 9 chapters appear in the Class 9 books collection.

Atmospheric Pollution Class 9 ICSE frequently asked questions

What is the difference between normal rain and acid rain in Class 9 ICSE Chemistry?

Normal rain has a pH near 5.6 because carbon dioxide forms weak carbonic acid. Acid rain has a pH between 5.6 and 3.5, and can fall as low as 2. The extra acidity comes from nitric and sulphuric acids.

Why is carbon monoxide so dangerous in atmospheric pollution class 9 ICSE?

Carbon monoxide binds to haemoglobin more than 200 times more strongly than oxygen. It forms carboxy haemoglobin, which stops blood from carrying oxygen. The heart and brain suffer first, and high levels can kill.

How do CFCs destroy the ozone layer step by step in this ICSE chapter?

Ultraviolet rays split CFCs, releasing chlorine atoms. Each chlorine atom reacts with ozone to form chlorine monoxide. Chlorine monoxide then reacts with atomic oxygen to regenerate chlorine, so one atom can destroy many ozone molecules.

Which gases cause the greenhouse effect in the Class 9 ICSE syllabus?

Carbon dioxide contributes 50%, methane 19%, CFCs 17%, ozone 8%, nitrous oxide 4%, and water vapour 2%. Carbon dioxide is the largest single contributor.

What are the main ways to reduce acid rain and global warming in this ICSE chapter?

For acid rain, use low-sulphur fuel and scrubbers to cut emissions. For global warming, use public transport, plant trees, avoid burning leaves, and spread awareness.

Reference: ICSE Class 9 Chemistry textbook material, chapter 8: Atmospheric Pollution — CISCE-aligned.

More from this book

• Chapter 1: The Language of Chemistry
• Chapter 2: Chemical Changes and Reactions
• Chapter 3: Water
• Chapter 4: Atomic Structure and Chemical Bonding
• Chapter 5: The Periodic Table
• Chapter 6: Study of the First Element Hydrogen
• Chapter 7: Study of Gas Laws