🧪 Chemistry · Topic 9 of 10
Start with the study notes on Air, Water & Pollution (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
Air is a mixture of gases, water vapour and dust. Its gases keep their own properties.
Remember: The amount of oxygen in air stays steady because plants release oxygen in photosynthesis, while breathing and burning use it up.
Air pollution is the contamination of air by harmful substances, called pollutants. The main sources are burning fuels in vehicles, factories and power plants, and burning waste.
Example: Asthma, bronchitis, allergies and, with long exposure, lung cancer.
Acid rain forms when sulphur dioxide and nitrogen oxides dissolve in rainwater, making acids. It harms crops, soil, lakes and buildings.
Example: Acid rain corrodes the marble of the Taj Mahal, turning it yellow (“marble cancer”). Industries nearby were told to switch to cleaner fuels such as CNG and LPG.
The greenhouse effect is the trapping of the Earth's heat by gases such as carbon dioxide and methane. Too much of these gases causes global warming: a rise in the Earth's average temperature.
Example: Glaciers and polar ice melt, sea levels rise, and weather becomes more extreme.
Example: The Bhopal gas tragedy of 1984, a leak of poisonous methyl isocyanate gas from a factory, killed thousands of people.
Water pollution is the addition of harmful substances that make water unsafe for drinking, bathing or aquatic life.
Fertilisers in a lake make algae grow too much. When the algae die and rot, bacteria use up the dissolved oxygen, and fish die.
Examples:
Water that is safe to drink is called potable water.
Example: At a city water works: alum and sedimentation, then filtration through sand beds, then chlorination.
| Misconception | Correct understanding |
|---|---|
| Air is mostly oxygen. | Air is about 78% nitrogen and only about 21% oxygen. |
| Air is a compound. | Air is a mixture: its gases can be separated, and their amounts vary. |
| Clear water is always safe to drink. | It can still contain germs or dissolved poisons; boil or purify it. |
| Global warming and the ozone hole are the same problem. | Global warming comes from greenhouse gases; ozone damage comes mainly from CFCs. |
| Fertilisers in lakes help fish. | They cause algae to grow, which uses up oxygen when it rots and kills fish. |
Which gas makes up most of the air?
Nitrogen, about 78%.
Why is carbon monoxide dangerous?
It stops the blood from carrying oxygen.
Which gases cause acid rain?
Sulphur dioxide and nitrogen oxides.
Why is the Taj Mahal turning yellow?
Acid rain is corroding its marble.
What does the ozone layer protect us from?
The Sun's harmful ultraviolet rays.
Name two diseases spread by polluted water.
Cholera and typhoid (also jaundice and dysentery).
What is the simplest way to make water safe at home?
Boiling it for several minutes.
Give two ways to save water.
Fix leaking taps, and harvest rainwater (or use drip irrigation).
Key takeaway: Most air and water pollution comes from human activity, so cleaner fuels, treating waste and saving water can fix much of it.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
Air is:
Answer: A — A mixture of gases, water vapour and dust particles
Air is a mixture, not a compound. Its components keep their own properties and can be separated by physical means such as fractional distillation of liquid air.
Q2. easy
Which gas is present in the LARGEST amount in air?
Answer: A — Nitrogen (about 78%)
Nitrogen makes up about 78% of air by volume, oxygen about 21%, and the remaining 1% is argon, carbon dioxide, water vapour and other gases.
Q3. medium
What percentage of air is oxygen?
Answer: A — About 21%
Oxygen makes up roughly 21% of air. It supports respiration and combustion, and its level is maintained by photosynthesis in plants.
Q4. medium
Carbon dioxide makes up about what fraction of air?
Answer: A — About 0.03-0.04%
Carbon dioxide is only a tiny fraction of air, but it is essential for photosynthesis and, in excess, is the main driver of global warming.
Q6. medium
Which of these is a MAJOR source of air pollution?
Answer: A — Burning of fossil fuels in vehicles and factories
Vehicles, thermal power plants, factories and the burning of fuels and waste release most of the carbon monoxide, sulphur dioxide, nitrogen oxides and soot in the air.
Q8. medium
Sulphur dioxide in the air comes mainly from:
Answer: A — Burning of coal and fuels containing sulphur, in power plants and refineries
Coal and crude oil contain sulphur compounds. Burning them releases sulphur dioxide, which irritates the lungs and causes acid rain.
Q10. medium
Acid rain harms the Taj Mahal because it:
Answer: A — Corrodes the marble (calcium carbonate), making it yellow and pitted
Acidic pollutants from the nearby Mathura refinery and industries react with the marble, corroding and discolouring it - a phenomenon called 'marble cancer'.
Q12. medium
The greenhouse effect is:
Answer: A — The trapping of the Earth's heat by gases like carbon dioxide in the atmosphere
Greenhouse gases let sunlight in but absorb the heat radiated back by the Earth. A natural greenhouse effect keeps the planet habitable, but excess gases cause harmful warming.
Q14. medium
The rise in the average temperature of the Earth's atmosphere is called:
Answer: A — Global warming
Global warming causes melting of glaciers and polar ice, rising sea levels, changed rainfall patterns and more extreme weather.
Q16. medium
A mixture of smoke and fog that reduces visibility is called:
Answer: A — Smog
Smog forms in cold, humid weather when smoke particles mix with fog. It causes breathing difficulties and reduces visibility, disrupting traffic and flights.
Q18. medium
CFCs used in refrigerators and air conditioners damage:
Answer: A — The ozone layer
Chlorofluorocarbons break down ozone molecules in the stratosphere, thinning the ozone layer that shields us from harmful ultraviolet radiation.
Q20. medium
Which of these helps REDUCE air pollution?
Answer: A — Using public transport, planting trees and switching to CNG
Fewer vehicles on the road, more trees to absorb carbon dioxide, and cleaner fuels all reduce pollutants entering the air.
Q22. medium
Water that is unfit for use because harmful substances have been added is said to be:
Answer: A — Polluted
Water pollution occurs when sewage, industrial waste, chemicals or pesticides enter water bodies, making the water unsafe for drinking, bathing or aquatic life.
Q24. medium
Which is a MAJOR source of water pollution in India?
Answer: A — Untreated sewage and industrial effluents discharged into rivers
Cities discharge sewage and factories release chemical effluents directly into rivers. Agricultural runoff of fertilisers and pesticides adds to the problem.
Q26. medium
Which harmful chemicals from farms reach rivers with rainwater?
Answer: A — Fertilisers and pesticides
Rain washes excess fertilisers and pesticides from fields into rivers and lakes. They poison aquatic life and can enter our bodies through the food chain.
Q28. medium
Hot water released from factories and power plants into rivers causes:
Answer: A — Thermal pollution, which harms fish and aquatic organisms
Warm water holds less dissolved oxygen and disturbs the breeding and survival of aquatic species. This is called thermal pollution.
Q30. medium
The simplest household method to make water safe for drinking is:
Answer: A — Boiling it for several minutes to kill germs
Boiling kills bacteria, viruses and parasites. It is the cheapest and most reliable way to make water microbiologically safe at home.
Q32. medium
Which method of water purification uses ultraviolet radiation?
Answer: A — UV filters, which kill germs without adding any chemicals
Ultraviolet light damages the DNA of micro-organisms so they cannot multiply. UV purifiers work quickly and leave no taste or chemical residue.
Q34. medium
Which of these is a way to CONSERVE water at home?
Answer: A — Repairing leaking taps and turning off the tap while brushing
A dripping tap can waste thousands of litres a year. Fixing leaks, using buckets instead of hoses and reusing water are simple, effective conservation steps.
Q36. medium
Drip irrigation conserves water because it:
Answer: A — Delivers water drop by drop directly to the roots, reducing wastage
Narrow pipes with tiny holes drip water at the base of each plant. Very little is lost to evaporation or runoff, so crops need far less water.
Q38. medium
The oxygen dissolved in water that fish need is called:
Answer: A — Dissolved oxygen
Aquatic animals breathe oxygen dissolved in the water through their gills. Pollution and warm water reduce dissolved oxygen and can suffocate fish.
Q40. medium
Which of these is the BEST way to deal with domestic sewage?
Answer: A — Treat it at a sewage treatment plant before releasing it
Treatment removes solids, breaks down organic matter with bacteria and disinfects the water, so what is released does not pollute rivers or spread disease.
Q42. medium
Which of these vehicles causes the LEAST air pollution?
Answer: A — A bicycle
A bicycle needs no fuel and produces no emissions at all. Cycling for short distances is one of the simplest ways to reduce urban air pollution.
Q44. medium
Unleaded petrol is used because lead in petrol:
Answer: A — Is a poisonous pollutant that damages the nervous system
Lead compounds released in exhaust are toxic, especially to children's brain development. India phased out leaded petrol, which also allows catalytic converters to work.
Q46. medium
Which fuel is being encouraged in cities to reduce vehicular pollution?
Answer: A — CNG
CNG burns cleanly, producing very little carbon monoxide, sulphur dioxide or soot. Delhi's public transport switched to CNG for exactly this reason.
Q48. medium
Air pollution can cause which health problems?
Answer: A — Asthma, bronchitis, allergies and lung cancer
Fine particles and toxic gases irritate and damage the respiratory system, causing coughing, asthma attacks, chronic bronchitis and, with long exposure, cancer.
Q50. medium
Which of these is NOT a water pollutant?
Answer: A — Dissolved oxygen
Dissolved oxygen is essential for aquatic life - its presence is a sign of healthy water. Effluents, sewage and pesticides are all pollutants.
Q52. medium
Which of these best describes 'reduce, reuse, recycle' in the context of pollution?
Answer: A — A three-step approach that cuts waste at source, extends product life and recovers materials
Reducing consumption prevents waste, reusing extends the life of items, and recycling turns waste into new products - together they cut both pollution and resource use.
Q54. medium
Which of these is a good practice to keep water bodies clean?
Answer: A — Not throwing garbage, plastic or religious offerings into rivers and lakes
Solid waste dumped into water bodies chokes them, harms aquatic life and spreads disease. Keeping rivers and lakes free of litter is a basic civic duty.
Q56. medium
The percentage of oxygen in air stays roughly constant because of:
Answer: A — The balance between respiration/combustion and photosynthesis by plants
Animals and burning consume oxygen and release carbon dioxide; plants do the reverse during photosynthesis. This oxygen-carbon dioxide cycle keeps the levels stable - which is why deforestation is so damaging.
Q58. medium
Bursting a large number of firecrackers during festivals causes:
Answer: A — A sharp rise in smoke, harmful gases and particulate matter in the air
Crackers release sulphur dioxide, nitrogen oxides and fine particles, along with heavy metal salts used for colour. Air quality in many Indian cities drops severely for days after Diwali.
Q60. medium
Why should used cooking oil and paint NOT be poured down the drain?
Answer: A — They pollute water bodies and are hard to remove at treatment plants
Oils form a film on water that blocks oxygen, and paint contains heavy metals and solvents that poison aquatic life. Both should be collected separately for proper disposal.
Q5. hard
The contamination of air by unwanted harmful substances is called:
Answer: A — Air pollution
Air pollution occurs when gases, smoke and particles harmful to living beings are added to air. The substances that cause it are called air pollutants.
Q7. hard
Which air pollutant is a colourless, odourless gas that reduces the oxygen-carrying capacity of blood?
Answer: A — Carbon monoxide
Carbon monoxide, produced by incomplete combustion in vehicles and stoves, binds strongly to haemoglobin, preventing it from carrying oxygen. It can be fatal.
Q9. hard
Acid rain is caused when which gases dissolve in rainwater?
Answer: A — Sulphur dioxide and nitrogen oxides
SO2 and NOx dissolve in atmospheric moisture to form sulphuric and nitric acids, which fall as acid rain and damage crops, soil, water bodies and monuments.
Q11. hard
Which measures were taken to protect the Taj Mahal from air pollution?
Answer: A — Industries around it were asked to switch to cleaner fuels like CNG and LPG, and vehicles were restricted
The Supreme Court directed industries in the Taj Trapezium Zone to use cleaner fuels, and petrol and diesel vehicles are kept away from the monument.
Q13. hard
Which of these is a greenhouse gas?
Answer: A — Methane
Carbon dioxide, methane, nitrous oxide, water vapour and CFCs are greenhouse gases. Methane comes from paddy fields, cattle and decaying waste.
Q15. hard
Which of these is a CONSEQUENCE of global warming?
Answer: A — Melting of polar ice and a rise in sea level, threatening coastal areas
Warmer temperatures melt glaciers and ice caps and cause sea water to expand. Low-lying cities and islands risk flooding, and weather patterns become erratic.
Q17. hard
The Bhopal gas tragedy of 1984 was caused by the leak of:
Answer: A — Methyl isocyanate (MIC) gas
MIC leaked from a pesticide plant in Bhopal on the night of 2-3 December 1984, killing thousands and leaving many more with lifelong illnesses - the worst industrial disaster in history.
Q19. hard
The ozone layer is important because it:
Answer: A — Absorbs harmful ultraviolet radiation from the Sun
Ozone in the stratosphere blocks most of the Sun's ultraviolet rays, which would otherwise cause skin cancer, cataracts and damage to crops and marine life.
Q21. hard
Planting trees helps control air pollution because trees:
Answer: A — Absorb carbon dioxide, release oxygen and trap dust particles
Through photosynthesis, trees remove carbon dioxide and add oxygen. Their leaves also trap dust and other particulates, cleaning the air near the ground.
Q23. hard
Water that is safe for drinking is called:
Answer: A — Potable water
Potable water is clean, free from harmful germs and chemicals, and safe to drink. Making water potable usually needs filtration and disinfection.
Q25. hard
The Ganga Action Plan was launched in 1985 to:
Answer: A — Reduce pollution of the river Ganga
The Ganga Action Plan aimed to clean the heavily polluted river by treating sewage and controlling industrial discharge along its course.
Q27. hard
Excess fertilisers in a lake cause excessive growth of algae. This is called:
Answer: A — Eutrophication - it uses up dissolved oxygen and kills fish
Nutrients from fertilisers make algae grow rapidly. When the algae die and decompose, bacteria consume the dissolved oxygen, suffocating fish and other aquatic life.
Q29. hard
Which disease is spread by drinking water contaminated with germs?
Answer: A — Cholera and typhoid
Cholera, typhoid, dysentery, jaundice and diarrhoea are water-borne diseases caused by bacteria and viruses in contaminated water.
Q31. hard
Chlorination of water means:
Answer: A — Adding chlorine tablets or bleaching powder to kill germs
Chlorine is a powerful disinfectant. Adding the correct amount kills disease-causing micro-organisms without harming humans, so it is used in municipal supplies.
Q33. hard
At a municipal water works, the correct order of treatment is:
Answer: A — Sedimentation with alum, filtration through sand beds, then chlorination
Alum makes suspended particles settle, sand and gravel beds filter out remaining solids, and chlorine finally kills the germs before the water reaches homes.
Q35. hard
Rainwater harvesting means:
Answer: A — Collecting and storing rainwater or letting it recharge groundwater
Rooftop rainwater is collected in tanks or directed into recharge pits so it seeps into the ground, raising the water table and providing water in dry months.
Q37. hard
Which of these processes purifies water in nature?
Answer: A — The water cycle - evaporation followed by condensation as rain
When water evaporates, dissolved salts and impurities are left behind. The vapour condenses into clean rainwater - nature's own distillation.
Q39. hard
Oil spills in the sea are harmful because the oil layer:
Answer: A — Blocks sunlight and oxygen from entering the water, killing marine life
A floating oil film cuts off gas exchange with the air and blocks light needed by marine plants. It also coats birds' feathers, destroying their insulation.
Q41. hard
Which gas released from decaying garbage and paddy fields contributes to global warming?
Answer: A — Methane
Methane is produced when organic matter decays without oxygen, as in landfills, marshes, paddy fields and the stomachs of cattle. It traps far more heat than carbon dioxide.
Q43. hard
Catalytic converters fitted in vehicles help by:
Answer: A — Converting harmful exhaust gases into less harmful ones
A catalytic converter changes carbon monoxide into carbon dioxide and reduces nitrogen oxides, cutting the harmful emissions leaving the exhaust pipe.
Q45. hard
The 'Van Mahotsav' festival encourages people to:
Answer: A — Plant trees
Van Mahotsav is a tree-planting festival held every year in India. Increasing green cover reduces carbon dioxide, dust and temperature.
Q47. hard
Which of these is an example of using an alternative, non-polluting energy source?
Answer: A — Solar cookers and solar water heaters
Solar devices use sunlight directly, producing no smoke or greenhouse gases. Wind, hydro and biogas are other clean alternatives.
Q49. hard
Which particulate pollutant is most dangerous because it penetrates deep into the lungs?
Answer: A — Fine suspended particulate matter (PM2.5)
Particles smaller than 2.5 micrometres bypass the nose and throat defences and lodge deep in the lungs, even entering the bloodstream. They come mainly from combustion.
Q51. hard
Heavy metals like mercury and lead in river water are dangerous because they:
Answer: A — Accumulate in fish and enter our bodies through the food chain
Heavy metals are not broken down. They build up in the tissues of fish, and their concentration increases up the food chain, poisoning people who eat the fish.
Q53. hard
Why is groundwater becoming polluted in many areas?
Answer: A — Chemicals, sewage and pesticides seep down through the soil into the water table
Untreated waste dumped on land, leaking septic tanks and agricultural chemicals gradually percolate downwards, contaminating aquifers that are very hard to clean.
Q55. hard
Nitrogen from the air is made available to plants mainly by:
Answer: A — Nitrogen-fixing bacteria in the soil and root nodules of legumes, and by lightning
Plants cannot use nitrogen gas directly. Rhizobium bacteria in legume root nodules and free-living soil bacteria convert it into nitrates, and lightning fixes a small amount too.
Q57. hard
Which overall statement about air and water pollution is CORRECT?
Answer: A — Both are caused mainly by human activities, harm health and ecosystems, and can be reduced by cleaner fuels, treatment of waste and conservation
This sums up the topic: burning fuels, industries, sewage and chemicals are the main culprits; the effects range from asthma and acid rain to global warming and dead rivers; and the remedies are cleaner technology, treatment, tree planting and careful use of resources.
Q59. hard
Burning crop stubble in fields after harvest is discouraged because it:
Answer: A — Releases huge amounts of smoke and destroys useful soil organisms and nutrients
Stubble burning fills the air with smoke and particulates - a major cause of winter smog in north India - and also kills the microbes and burns the organic matter that keep soil healthy.