Four-Fifths of Every Breath You Take Is Useless to You
Learn the composition of clean dry air, four proofs that air is a mixture and not a compound, why the atmosphere matters, and one important use of each gas in it.
What is air actually made of?
Mostly nitrogen — about 78 percent of every breath, which your body does not use at all. Only about 21 percent is the oxygen you actually need.
Air is far less uniform than it feels, and it is not even a single substance. This page covers everything in the ICSE Class 7 Chemistry chapter's first part: the composition of air, the proof that air is a mixture, the importance of the atmosphere, and the uses of its components.
Air is far less uniform than it feels, and it is not even a single substance. This page covers everything in the ICSE Class 7 Chemistry chapter's first part: the composition of air, the proof that air is a mixture, the importance of the atmosphere, and the uses of its components.
What is the composition of clean dry air by volume?
Clean dry air has a nearly fixed set of proportions by volume:
- Nitrogen — about 78%
- Oxygen — about 21%
- Inert (noble) gases — about 0.94%, mainly argon
- Carbon dioxide — about 0.03%
Those four make up the permanent components, present almost everywhere in roughly the same amounts.
Air also carries variable components, whose amounts change from place to place and hour to hour:
- Water vapour — much higher in coastal Mumbai than in dry Rajasthan, and higher during the monsoon than in summer.
- Dust — from soil, roads and construction.
- Smoke — from vehicles, factories and burning fuel.
A dusty ceiling fan blade and the haze over a busy road are both variable components made visible.
The figure students under-rate is carbon dioxide's. At only 0.03 percent it sounds negligible, yet it is the raw material of photosynthesis and therefore the starting point of all food. Small does not mean unimportant.
- Nitrogen — about 78%
- Oxygen — about 21%
- Inert (noble) gases — about 0.94%, mainly argon
- Carbon dioxide — about 0.03%
Those four make up the permanent components, present almost everywhere in roughly the same amounts.
Air also carries variable components, whose amounts change from place to place and hour to hour:
- Water vapour — much higher in coastal Mumbai than in dry Rajasthan, and higher during the monsoon than in summer.
- Dust — from soil, roads and construction.
- Smoke — from vehicles, factories and burning fuel.
A dusty ceiling fan blade and the haze over a busy road are both variable components made visible.
The figure students under-rate is carbon dioxide's. At only 0.03 percent it sounds negligible, yet it is the raw material of photosynthesis and therefore the starting point of all food. Small does not mean unimportant.
How can you prove air is a mixture and not a compound?
Four arguments prove it, and a complete answer gives all four.
Variable composition. A compound has a fixed proportion by mass. Air's proportions vary — its water vapour, dust and carbon dioxide all differ from place to place and season to season.
Components retain their properties. In air, oxygen still supports burning, nitrogen still does not, and carbon dioxide still turns limewater milky. In a compound the constituents lose their own properties entirely.
No energy change on mixing. Mixing nitrogen and oxygen in the laboratory gives off no heat or light. Forming a compound always involves an energy change.
Separation by physical means. Air is separated into nitrogen, oxygen and argon by fractional distillation of liquid air — a physical process. A compound's elements need chemical means.
The contrast that settles it is with water. Water is a compound of hydrogen and oxygen in a fixed 1 : 8 ratio by mass, it puts fires out rather than supporting burning, and only an electric current will split it.
So although air looks like one clear, uniform substance, every chemical test shows it is a homogeneous mixture.
Variable composition. A compound has a fixed proportion by mass. Air's proportions vary — its water vapour, dust and carbon dioxide all differ from place to place and season to season.
Components retain their properties. In air, oxygen still supports burning, nitrogen still does not, and carbon dioxide still turns limewater milky. In a compound the constituents lose their own properties entirely.
No energy change on mixing. Mixing nitrogen and oxygen in the laboratory gives off no heat or light. Forming a compound always involves an energy change.
Separation by physical means. Air is separated into nitrogen, oxygen and argon by fractional distillation of liquid air — a physical process. A compound's elements need chemical means.
The contrast that settles it is with water. Water is a compound of hydrogen and oxygen in a fixed 1 : 8 ratio by mass, it puts fires out rather than supporting burning, and only an electric current will split it.
So although air looks like one clear, uniform substance, every chemical test shows it is a homogeneous mixture.
Why does the atmosphere matter so much?
The atmosphere is the envelope of air surrounding the Earth, held in place by gravity and extending for hundreds of kilometres upward, growing thinner with height.
It does four jobs the syllabus asks you to name.
Supports respiration and combustion. Its oxygen keeps every animal and plant alive and lets fuels burn.
Shields the Earth. The ozone in the upper atmosphere absorbs most of the harmful ultraviolet radiation from the Sun, and the air burns up most meteorites before they reach the ground.
Regulates temperature. It keeps the Earth warm at night by trapping some heat, and moderates the day's heat by reflecting part of it away — so temperatures stay within the range life can survive.
Carries water. Water vapour in the air forms clouds and brings rainfall, which is why the monsoon exists at all.
Mountaineers climbing high peaks carry oxygen cylinders precisely because the atmosphere thins with height, and the same thinning is why aircraft cabins must be pressurised.
The temperature-regulating role is the one to state carefully. Without an atmosphere the Earth would be blisteringly hot by day and freezing by night, as airless surfaces are — so the atmosphere does not merely warm the planet, it evens out its temperature.
It does four jobs the syllabus asks you to name.
Supports respiration and combustion. Its oxygen keeps every animal and plant alive and lets fuels burn.
Shields the Earth. The ozone in the upper atmosphere absorbs most of the harmful ultraviolet radiation from the Sun, and the air burns up most meteorites before they reach the ground.
Regulates temperature. It keeps the Earth warm at night by trapping some heat, and moderates the day's heat by reflecting part of it away — so temperatures stay within the range life can survive.
Carries water. Water vapour in the air forms clouds and brings rainfall, which is why the monsoon exists at all.
Mountaineers climbing high peaks carry oxygen cylinders precisely because the atmosphere thins with height, and the same thinning is why aircraft cabins must be pressurised.
The temperature-regulating role is the one to state carefully. Without an atmosphere the Earth would be blisteringly hot by day and freezing by night, as airless surfaces are — so the atmosphere does not merely warm the planet, it evens out its temperature.
What is each gas in air actually used for?
Every component has at least one important use.
Nitrogen — used to manufacture ammonia and nitrogenous fertilisers, and as an unreactive gas to pack food so it does not spoil. Liquid nitrogen is used for freezing and for cold storage.
Oxygen — needed for respiration, supplied in cylinders to patients and mountaineers, used in the oxy-acetylene flame for welding and cutting metals, and required for the burning of all fuels.
Carbon dioxide — used by plants in photosynthesis, in fire extinguishers because it does not support burning, in aerated soft drinks, and as dry ice for refrigeration.
Water vapour — brings rainfall, keeps the air humid enough for plants and skin, and is part of the water cycle.
Inert gases:
- Helium — fills weather balloons and airships, since it is light and does not catch fire.
- Neon — glows red-orange in glass display tubes and advertising signs.
- Argon — fills electric bulbs, where its unreactive nature stops the filament from burning away.
The reason the inert gases are useful is exactly the reason they seem useless: they do not react. Argon protects a bulb filament, and helium lifts a balloon without the fire risk that hydrogen would bring, precisely because neither takes part in any chemistry.
Nitrogen — used to manufacture ammonia and nitrogenous fertilisers, and as an unreactive gas to pack food so it does not spoil. Liquid nitrogen is used for freezing and for cold storage.
Oxygen — needed for respiration, supplied in cylinders to patients and mountaineers, used in the oxy-acetylene flame for welding and cutting metals, and required for the burning of all fuels.
Carbon dioxide — used by plants in photosynthesis, in fire extinguishers because it does not support burning, in aerated soft drinks, and as dry ice for refrigeration.
Water vapour — brings rainfall, keeps the air humid enough for plants and skin, and is part of the water cycle.
Inert gases:
- Helium — fills weather balloons and airships, since it is light and does not catch fire.
- Neon — glows red-orange in glass display tubes and advertising signs.
- Argon — fills electric bulbs, where its unreactive nature stops the filament from burning away.
The reason the inert gases are useful is exactly the reason they seem useless: they do not react. Argon protects a bulb filament, and helium lifts a balloon without the fire risk that hydrogen would bring, precisely because neither takes part in any chemistry.
Exam tip
Exam tip: giving all four proofs that air is a mixture
The "prove air is a mixture" question is worth several marks, awarded one per argument.
Write them as four separate points — variable composition, components retain properties, no energy change on mixing, and separable by fractional distillation. Three points earn three marks; a single paragraph saying "because it has many gases" earns one.
Quote the percentages when asked for composition, and label them by volume. Nitrogen 78, oxygen 21, inert gases 0.94, carbon dioxide 0.03.
Keep the permanent and variable components separate. Water vapour, dust and smoke belong in the variable list, and putting them in the main percentages is a common error.
And when naming a use, tie it to the property: argon fills bulbs because it is unreactive. The reason is usually the mark.
Write them as four separate points — variable composition, components retain properties, no energy change on mixing, and separable by fractional distillation. Three points earn three marks; a single paragraph saying "because it has many gases" earns one.
Quote the percentages when asked for composition, and label them by volume. Nitrogen 78, oxygen 21, inert gases 0.94, carbon dioxide 0.03.
Keep the permanent and variable components separate. Water vapour, dust and smoke belong in the variable list, and putting them in the main percentages is a common error.
And when naming a use, tie it to the property: argon fills bulbs because it is unreactive. The reason is usually the mark.
Did you know
Why is a tiny 0.03 percent of carbon dioxide enough to feed the whole planet?
Because it is constantly replaced as fast as it is used.
Plants take carbon dioxide from the air to make food, and respiration, burning and decay put it straight back. The small amount present at any moment is not a stock being used up — it is a flow passing through.
So the figure stays near 0.03 percent even though enormous quantities move through it every day. That balance is what the oxygen and carbon cycles maintain, and it is why a change of even a fraction of a percent in that figure matters far more than its size suggests.
Plants take carbon dioxide from the air to make food, and respiration, burning and decay put it straight back. The small amount present at any moment is not a stock being used up — it is a flow passing through.
So the figure stays near 0.03 percent even though enormous quantities move through it every day. That balance is what the oxygen and carbon cycles maintain, and it is why a change of even a fraction of a percent in that figure matters far more than its size suggests.
Key takeaways
Air and the atmosphere: quick revision
- Clean dry air by volume: nitrogen about 78%, oxygen about 21%, inert gases about 0.94% and carbon dioxide about 0.03%.
- Water vapour, dust and smoke are the variable components, differing with place and season.
- Air is a mixture because its composition varies, its components keep their properties, no energy change occurs on mixing, and it separates by fractional distillation of liquid air.
- The atmosphere supports respiration and combustion, shields the Earth from ultraviolet radiation and meteorites, regulates temperature and carries water for rainfall.
- Nitrogen makes fertilisers, oxygen supports respiration and welding, carbon dioxide is used in photosynthesis and fire extinguishers.
- Helium fills balloons, neon glows in display tubes and argon fills bulbs — all because they are unreactive.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Water vapour, dust and smoke are the variable components, differing with place and season.
- Air is a mixture because its composition varies, its components keep their properties, no energy change occurs on mixing, and it separates by fractional distillation of liquid air.
- The atmosphere supports respiration and combustion, shields the Earth from ultraviolet radiation and meteorites, regulates temperature and carries water for rainfall.
- Nitrogen makes fertilisers, oxygen supports respiration and welding, carbon dioxide is used in photosynthesis and fire extinguishers.
- Helium fills balloons, neon glows in display tubes and argon fills bulbs — all because they are unreactive.
You will remember all of this far better after answering five questions on it than after reading it twice.