Why the Oxygen in Air Never Runs Out
Learn how the oxygen cycle keeps air constant, follow nitrogen fixation and denitrification, name the major air pollutants and their sources, and explain acid rain and global warming.
Why has the oxygen in air not run out after millions of breaths?
Because it is put back as fast as it is taken out. Every animal that breathes and every fire that burns removes oxygen, and green plants return it through photosynthesis.
Air is not a stock being used up; it is a balance being maintained. This page covers everything in the ICSE Class 7 Chemistry chapter's third part: the oxygen cycle, the nitrogen cycle, air pollutants and their sources, and acid rain, the greenhouse effect and global warming.
Air is not a stock being used up; it is a balance being maintained. This page covers everything in the ICSE Class 7 Chemistry chapter's third part: the oxygen cycle, the nitrogen cycle, air pollutants and their sources, and acid rain, the greenhouse effect and global warming.
How does the oxygen cycle keep the air constant?
Four processes push oxygen and carbon dioxide in opposite directions, and together they balance out.
Oxygen is removed and carbon dioxide added by:
- Respiration — animals and plants take in oxygen and release carbon dioxide.
- Combustion — burning wood, coal, petrol and cooking gas uses oxygen and releases carbon dioxide.
- Decay — dead plants and animals are broken down by bacteria, using oxygen.
Oxygen is returned and carbon dioxide removed by:
- Photosynthesis — green plants take in carbon dioxide and water in sunlight, make food, and release oxygen.
So photosynthesis is the single process that reverses the other three, which is why the percentages stay near 21 percent oxygen and 0.03 percent carbon dioxide.
A garden shows both halves running at once: the plants photosynthesise all day while the people, insects and soil bacteria around them respire.
That is also why deforestation matters chemically and not only scenically. Cutting forests removes the return arm of the cycle while respiration and combustion continue, so carbon dioxide accumulates — the direct link to the warming discussed later on this page.
Oxygen is removed and carbon dioxide added by:
- Respiration — animals and plants take in oxygen and release carbon dioxide.
- Combustion — burning wood, coal, petrol and cooking gas uses oxygen and releases carbon dioxide.
- Decay — dead plants and animals are broken down by bacteria, using oxygen.
Oxygen is returned and carbon dioxide removed by:
- Photosynthesis — green plants take in carbon dioxide and water in sunlight, make food, and release oxygen.
So photosynthesis is the single process that reverses the other three, which is why the percentages stay near 21 percent oxygen and 0.03 percent carbon dioxide.
A garden shows both halves running at once: the plants photosynthesise all day while the people, insects and soil bacteria around them respire.
That is also why deforestation matters chemically and not only scenically. Cutting forests removes the return arm of the cycle while respiration and combustion continue, so carbon dioxide accumulates — the direct link to the warming discussed later on this page.
How does nitrogen get from the air into a plant?
Plants cannot use nitrogen gas directly, so it must first be fixed — converted into compounds they can absorb. That conversion and its reverse make up the nitrogen cycle.
Nitrogen fixation happens in two natural ways:
- By lightning — the enormous energy of a lightning flash makes nitrogen and oxygen in the air combine into oxides of nitrogen, which dissolve in rain and reach the soil as nitrates.
- By symbiotic bacteria — bacteria living in the root nodules of leguminous plants such as gram, peas, beans and groundnut convert atmospheric nitrogen into compounds the plant can use.
Nitrification follows: soil bacteria convert ammonia and ammonium compounds into nitrites and then into nitrates, the form plant roots absorb. Plants use nitrates to build proteins, and animals get their nitrogen by eating plants.
Denitrification completes the circle: denitrifying bacteria break nitrates in the soil back down into free nitrogen gas, returning it to the air.
This is why farmers grow a legume such as gram between two cereal crops — the root nodules leave the field richer in nitrogen than they found it, with no fertiliser added.
The misconception to correct is that plants breathe in nitrogen from the air. They do not. Nitrogen is taken up through the roots as nitrates, which is precisely why fixation is necessary at all.
Nitrogen fixation happens in two natural ways:
- By lightning — the enormous energy of a lightning flash makes nitrogen and oxygen in the air combine into oxides of nitrogen, which dissolve in rain and reach the soil as nitrates.
- By symbiotic bacteria — bacteria living in the root nodules of leguminous plants such as gram, peas, beans and groundnut convert atmospheric nitrogen into compounds the plant can use.
Nitrification follows: soil bacteria convert ammonia and ammonium compounds into nitrites and then into nitrates, the form plant roots absorb. Plants use nitrates to build proteins, and animals get their nitrogen by eating plants.
Denitrification completes the circle: denitrifying bacteria break nitrates in the soil back down into free nitrogen gas, returning it to the air.
This is why farmers grow a legume such as gram between two cereal crops — the root nodules leave the field richer in nitrogen than they found it, with no fertiliser added.
The misconception to correct is that plants breathe in nitrogen from the air. They do not. Nitrogen is taken up through the roots as nitrates, which is precisely why fixation is necessary at all.
What are the major air pollutants and where do they come from?
An air pollutant is any substance present in air in amounts harmful to living things. Six matter most, each with its chief source.
- Carbon monoxide — incomplete burning of fuel, mainly from vehicle exhaust. It is poisonous because it combines with the blood's haemoglobin and prevents it from carrying oxygen.
- Sulphur dioxide — burning of coal and other sulphur-containing fuels in power stations and factories. It irritates the lungs and causes acid rain.
- Oxides of nitrogen — high-temperature combustion in vehicle and factory engines. They also cause acid rain.
- Hydrogen sulphide — decaying organic matter, sewage and some industries. It smells of rotten eggs.
- Smoke — burning of fuels, crop residue and rubbish; it carries fine soot particles into the lungs.
- Dust — construction, mining, road traffic and dry soil.
Anyone standing at a busy Indian crossroads meets several at once: exhaust fumes, road dust and the smoke of nearby burning.
Carbon monoxide is the one most often described loosely. It is dangerous not because it is corrosive but because it blocks oxygen transport in the blood — which is why a coal fire or generator running in a closed room is so hazardous.
- Carbon monoxide — incomplete burning of fuel, mainly from vehicle exhaust. It is poisonous because it combines with the blood's haemoglobin and prevents it from carrying oxygen.
- Sulphur dioxide — burning of coal and other sulphur-containing fuels in power stations and factories. It irritates the lungs and causes acid rain.
- Oxides of nitrogen — high-temperature combustion in vehicle and factory engines. They also cause acid rain.
- Hydrogen sulphide — decaying organic matter, sewage and some industries. It smells of rotten eggs.
- Smoke — burning of fuels, crop residue and rubbish; it carries fine soot particles into the lungs.
- Dust — construction, mining, road traffic and dry soil.
Anyone standing at a busy Indian crossroads meets several at once: exhaust fumes, road dust and the smoke of nearby burning.
Carbon monoxide is the one most often described loosely. It is dangerous not because it is corrosive but because it blocks oxygen transport in the blood — which is why a coal fire or generator running in a closed room is so hazardous.
What causes acid rain and global warming?
Both are consequences of pollutants, but by two quite different mechanisms.
Acid rain. Sulphur dioxide and oxides of nitrogen released from burning fuels dissolve in the water vapour of clouds, forming sulphuric and nitric acids. The rain that falls is acidic. It corrodes metal structures, damages the stone of buildings and monuments, makes soil acidic so crops fail, and kills fish by acidifying lakes and rivers.
The greenhouse effect. Carbon dioxide, methane and water vapour let the Sun's rays reach the Earth but trap part of the heat radiated back from the surface. This natural effect keeps the planet warm enough for life.
Global warming is what happens when that effect is strengthened. Burning fossil fuels and cutting forests raise the carbon dioxide level, so more heat is trapped and average temperatures rise — melting glaciers and polar ice, raising sea levels, and disturbing rainfall and the monsoon.
At least three control measures should be named:
- Use unleaded petrol, CNG and public transport instead of private vehicles.
- Plant trees and protect existing forests, restoring the oxygen cycle's return arm.
- Fit filters, scrubbers and electrostatic precipitators to factory chimneys.
- Use cleaner energy — solar, wind and hydroelectric — instead of coal.
- Stop the burning of rubbish and crop residue.
The distinction examiners test is that the greenhouse effect is natural and necessary, while global warming is its harmful intensification. Calling the greenhouse effect itself a pollution problem loses the mark.
Acid rain. Sulphur dioxide and oxides of nitrogen released from burning fuels dissolve in the water vapour of clouds, forming sulphuric and nitric acids. The rain that falls is acidic. It corrodes metal structures, damages the stone of buildings and monuments, makes soil acidic so crops fail, and kills fish by acidifying lakes and rivers.
The greenhouse effect. Carbon dioxide, methane and water vapour let the Sun's rays reach the Earth but trap part of the heat radiated back from the surface. This natural effect keeps the planet warm enough for life.
Global warming is what happens when that effect is strengthened. Burning fossil fuels and cutting forests raise the carbon dioxide level, so more heat is trapped and average temperatures rise — melting glaciers and polar ice, raising sea levels, and disturbing rainfall and the monsoon.
At least three control measures should be named:
- Use unleaded petrol, CNG and public transport instead of private vehicles.
- Plant trees and protect existing forests, restoring the oxygen cycle's return arm.
- Fit filters, scrubbers and electrostatic precipitators to factory chimneys.
- Use cleaner energy — solar, wind and hydroelectric — instead of coal.
- Stop the burning of rubbish and crop residue.
The distinction examiners test is that the greenhouse effect is natural and necessary, while global warming is its harmful intensification. Calling the greenhouse effect itself a pollution problem loses the mark.
Exam tip
Exam tip: matching each pollutant to its own source
Pollution questions are marked as pairs, so always give the pollutant and its source together.
Sulphur dioxide, from the burning of coal. Carbon monoxide, from vehicle exhaust. A list of pollutant names with no sources scores half.
For acid rain, name both gases — sulphur dioxide and oxides of nitrogen — and then the acids they form. Naming only carbon dioxide is wrong: carbon dioxide causes warming, not acid rain.
For the nitrogen cycle, use the three technical terms in order: fixation, nitrification, denitrification, and mention root nodules of leguminous plants by name.
For control measures, give the number the question asks for and make each one practical — planting trees, using CNG, fitting chimney filters.
And keep the greenhouse effect and global warming distinct: the first is natural, the second is its harmful increase.
Sulphur dioxide, from the burning of coal. Carbon monoxide, from vehicle exhaust. A list of pollutant names with no sources scores half.
For acid rain, name both gases — sulphur dioxide and oxides of nitrogen — and then the acids they form. Naming only carbon dioxide is wrong: carbon dioxide causes warming, not acid rain.
For the nitrogen cycle, use the three technical terms in order: fixation, nitrification, denitrification, and mention root nodules of leguminous plants by name.
For control measures, give the number the question asks for and make each one practical — planting trees, using CNG, fitting chimney filters.
And keep the greenhouse effect and global warming distinct: the first is natural, the second is its harmful increase.
Did you know
Why does lightning act like a natural fertiliser factory?
Because it supplies enough energy to force two gases together that normally ignore each other.
Nitrogen and oxygen sit side by side in air all day without reacting — nitrogen molecules are held too tightly to combine easily. A lightning flash delivers a burst of energy fierce enough to break them apart, and the two gases form oxides of nitrogen.
Those oxides dissolve in the rain that follows and soak into the soil as nitrates, exactly the form plant roots can absorb. So a thunderstorm genuinely leaves fields better fed than it found them.
Nitrogen and oxygen sit side by side in air all day without reacting — nitrogen molecules are held too tightly to combine easily. A lightning flash delivers a burst of energy fierce enough to break them apart, and the two gases form oxides of nitrogen.
Those oxides dissolve in the rain that follows and soak into the soil as nitrates, exactly the form plant roots can absorb. So a thunderstorm genuinely leaves fields better fed than it found them.
Key takeaways
Air cycles and pollution: quick revision
- Respiration, combustion and decay remove oxygen and add carbon dioxide; photosynthesis reverses all three, keeping the percentages nearly constant.
- Plants cannot use nitrogen gas — it is fixed by lightning and by symbiotic bacteria in root nodules of leguminous plants such as gram and peas.
- Nitrification turns ammonia into nitrates that roots absorb; denitrification returns free nitrogen to the air.
- Major pollutants and sources: carbon monoxide from vehicle exhaust, sulphur dioxide from burning coal, oxides of nitrogen from high-temperature combustion, hydrogen sulphide from decay, plus smoke and dust.
- Acid rain comes from sulphur dioxide and oxides of nitrogen dissolving in rain, corroding buildings and acidifying soil and lakes.
- The greenhouse effect is natural and necessary; global warming is its harmful intensification by extra carbon dioxide from fossil fuels and deforestation.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Plants cannot use nitrogen gas — it is fixed by lightning and by symbiotic bacteria in root nodules of leguminous plants such as gram and peas.
- Nitrification turns ammonia into nitrates that roots absorb; denitrification returns free nitrogen to the air.
- Major pollutants and sources: carbon monoxide from vehicle exhaust, sulphur dioxide from burning coal, oxides of nitrogen from high-temperature combustion, hydrogen sulphide from decay, plus smoke and dust.
- Acid rain comes from sulphur dioxide and oxides of nitrogen dissolving in rain, corroding buildings and acidifying soil and lakes.
- The greenhouse effect is natural and necessary; global warming is its harmful intensification by extra carbon dioxide from fossil fuels and deforestation.
You will remember all of this far better after answering five questions on it than after reading it twice.