Hello, and welcome to today's chemistry lesson! Today, we are going to explore something that surrounds us every single moment of our lives — air and the atmosphere. By the end of this lesson, you will understand what air is made of, why it is essential for life, how living things use it, and why we must keep it clean.
Let us begin with a simple truth: air is a kind of matter. It is invisible, but it is very real. You cannot survive without it — not even for a few minutes. While you can live without food or water for some time, air is absolutely essential for life.
Air is present everywhere — over land, over oceans, and all around you. The Earth is wrapped in a thick blanket of air called the atmosphere. This blanket extends up to about three hundred twenty kilometres above the Earth's surface. However, most of the air is concentrated within the first fifteen kilometres. As you go higher, the air becomes thinner and less dense.
The atmosphere is our protector. It shields us from harmful rays of the sun. Without it, Earth would become unbearably hot during the day and freezing cold at night. Life as we know it would be impossible.
Air is colourless, odourless, tasteless, and transparent. You cannot see it, but you can feel it when it moves — what we call wind. Fast-moving air is wind. Air has mass and occupies space, just like any other form of matter.
Now, let us understand what air is actually made of. Long ago, people thought air was a single element. But in 1774, a French chemist named Lavoisier proved that air is a mixture of gases.
The main components of air are nitrogen and oxygen. By volume, nitrogen makes up seventy-eight percent of air, while oxygen makes up twenty-one percent. There are also small amounts of other substances: carbon dioxide at about zero point zero three to zero point zero four percent, inert gases at about zero point nine percent, and variable amounts of water vapour, dust particles, and other impurities.
Air is a mixture, not a compound. This is important to understand. The composition of air varies from place to place and from time to time. For example, during the rainy season, air contains more water vapour. The amount of dust and pollutants also changes depending on location. If air were a compound, its composition would be fixed. Additionally, no energy change occurs when gases mix to form air, and each gas keeps its own properties. These are clear signs that air is a mixture.
Let us now look at the two major components of air in detail.
First, oxygen — the active part of air. Oxygen supports burning and is essential for respiration. It makes up about one-fifth of air by volume. When substances burn in air, they are actually reacting with oxygen. This process is called combustion.
Second, nitrogen — the inactive part of air. Nitrogen makes up about four-fifths of air by volume. It does not support burning, but it plays a crucial role. Nitrogen controls burning by diluting oxygen. If nitrogen were not present, everything would catch fire because oxygen is highly reactive.
Nitrogen is vital for life in another way too. It is needed to make proteins, which are essential for growth. Plants cannot absorb nitrogen directly from air. It must first be converted into compounds like nitrites and nitrates in the soil — a process called nitrogen fixation. Some bacteria living in the root nodules of leguminous plants, like peas and beans, help fix nitrogen naturally.
Carbon dioxide, though present in very small amounts, is extremely important. Green plants use carbon dioxide during photosynthesis to make their food. In this process, plants release oxygen, which replenishes the air. Carbon dioxide also helps maintain Earth's temperature by trapping heat — keeping the planet warm at night. However, excess carbon dioxide contributes to global warming.
Water vapour in air provides moisture for living things and helps predict weather conditions. Dust particles, though they seem like impurities, actually help form rain by providing surfaces for water vapour to condense on.
Inert gases like helium, neon, and argon have special uses. Helium fills weather balloons. Neon lights up fluorescent bulbs and traffic signals. Argon is filled in electric bulbs to prevent the filament from burning out.
Now, let us understand how living things use air.
Respiration is the process by which living organisms obtain energy from food. Here is the precise definition: respiration is a chemical process that takes place in all living beings, in which oxygen reacts with digested food in the body, releasing energy, carbon dioxide, and water.
Without oxygen, your body cannot use the food you eat to get energy. This is why breathing is so important. Even plants respire — they take in oxygen and release carbon dioxide, just like animals. During photosynthesis, plants do absorb carbon dioxide and release oxygen, but respiration happens continuously in plants, day and night.
Air is also dissolved in water. Aquatic animals and plants use this dissolved air for respiration. That is why fish can live underwater — they extract oxygen from the water around them.
Soil also contains trapped air in spaces between particles. Earthworms and plant roots use this air for respiration. During heavy rains, when water fills these spaces, earthworms come to the surface to breathe.
Burning, or combustion, is another process that uses oxygen from air. Burning is defined as a process in which a substance reacts chemically with oxygen and gets oxidised, with the release of energy in the form of heat and light.
Substances that burn to produce heat and light are called fuels. Common fuels include wood, coal, petrol, kerosene, diesel, compressed natural gas or CNG, and liquefied petroleum gas or LPG. LPG is what you commonly know as cooking gas.
Both respiration and burning require oxygen and produce carbon dioxide and water vapour. However, they differ in important ways. Respiration releases a small amount of energy slowly, at body temperature, and happens continuously as a natural process. Burning releases a large amount of energy quickly as heat and light, occurs at high temperatures, and is an artificial, discontinuous process.
Here is a fascinating question: if respiration and burning constantly use up oxygen and produce carbon dioxide, why does the composition of air remain stable?
The answer lies in photosynthesis. Photosynthesis is defined as the process through which green plants prepare their food in presence of sunlight with the help of carbon dioxide and water. During this process, plants use carbon dioxide from the air and release oxygen.
This creates a beautiful balance in nature. Animals and burning processes consume oxygen and release carbon dioxide. Plants consume carbon dioxide and release oxygen. This oxygen-carbon dioxide balance is essential for life on Earth.
Let us now discuss rusting — a slow chemical process that also involves air.
Rusting is defined as the slow conversion of iron into its hydrated oxide in the presence of moisture and air. The chemical name of rust is hydrated iron oxide, with the formula Fe₂O₃·xH₂O.
Two conditions are necessary for rusting: oxygen from air, and moisture or water. Rusting is a wasteful process that damages iron objects. It can be prevented by painting, greasing, coating with other materials, or alloying iron with non-corrosive metals.
Finally, we must talk about air pollution — a serious problem facing our world today.
Air pollution occurs when harmful and undesirable substances mix with air. These pollutants include smoke, dust, soot, sulphur dioxide, carbon monoxide, excess carbon dioxide, ammonia, and methane.
The effects of air pollution are devastating. It causes respiratory diseases, skin problems, and eye irritation. It can even lead to cancer. Plants and crops are damaged, causing economic losses. Pollutants like sulphur dioxide and nitrogen dioxide dissolve in rainwater to form acid rain, which damages buildings, crops, and aquatic life.
Global warming is another major concern. Gases like carbon dioxide, methane, and nitrous oxide trap heat from sunlight, causing Earth's temperature to rise. These are called greenhouse gases.
The ozone layer, which protects Earth from harmful ultraviolet rays, is also under threat. Chemicals called chlorofluorocarbons, or CFCs, react with ozone and deplete this protective layer. This allows dangerous ultraviolet radiation to reach Earth's surface, causing skin diseases and cancer.
Air pollution has many causes: growing population, deforestation, burning of fossil fuels, vehicle emissions, industrial activities, agricultural chemicals, and even natural events like volcanic eruptions.
What can we do to control air pollution? We can plant more trees, use tall chimneys in factories, switch to cleaner fuels like CNG, use unleaded petrol, adopt solar and other renewable energies, and locate industries away from residential areas. Every small step helps keep our air clean and breathable.
Let us quickly recap the key points from today's lesson.
First, air is a mixture of gases, mainly nitrogen and oxygen, with small amounts of carbon dioxide, water vapour, inert gases, and dust particles.
Second, oxygen supports burning and respiration, while nitrogen controls burning and is essential for making proteins.
Third, photosynthesis by green plants maintains the balance of oxygen and carbon dioxide in the atmosphere.
Fourth, rusting of iron occurs in the presence of both air and moisture, and it can be prevented by protective coatings.
Fifth, air pollution harms health, damages the environment, and disrupts the natural balance — but it can be controlled through conscious efforts and cleaner technologies.
And finally, air is essential for life, and it is our responsibility to keep it clean for ourselves and for future generations.
That brings us to the end of today's lesson on air and atmosphere. I hope you now appreciate this invisible yet precious resource that sustains all life on Earth. Remember, every breath you take connects you to the natural world around you. Stay curious, keep learning, and take care of the air we all share. Until next time, goodbye!