Chemical Composition of Air

 

Composition of Air – A Clear and Conceptual Understanding

Matter is defined as anything that occupies space and has mass. By this definition, air is matter, even though we cannot see it. Air surrounds us constantly and is so essential that life cannot exist even for a moment without it. The Earth is wrapped in a protective blanket of air known as the atmosphere, which supports life, regulates climate, and enables weather phenomena.

Every living organism—plants, animals, and humans—depends on air for survival. Beyond respiration, air plays a vital role in abiotic processes such as wind movement, rainfall, temperature regulation, and climate control. In this lesson, we will develop a clear understanding of the composition of air, its components, and its fundamental properties.

Chemical Composition of Air

Air is not a single substance; it is a homogeneous mixture of gases that collectively form Earth’s atmosphere. These gases are mostly colorless and odorless, which is why air is invisible, though we can easily feel its effects.

Think of the atmosphere as a vast ocean of gases surrounding our planet. The composition of air remains nearly constant up to about 10,000 meters above sea level. What changes with altitude is not the proportion of gases, but the number of air molecules, which decreases as elevation increases. This is why air becomes thinner at higher altitudes.

Major Components of Air (By Volume)

• Nitrogen (78%) – The most abundant gas; essential for plant growth and the nitrogen cycle.

• Oxygen (21%) – Vital for respiration in most living organisms.

• Other gases (1%) – Includes argon, carbon dioxide, and trace gases.

The amount of water vapor in air varies significantly depending on location and temperature. Humid regions contain more moisture than arid or desert regions. Warm air can hold more water vapor than cold air, which explains why humidity increases with temperature.

Important Atmospheric Facts

• The composition of air remains uniform up to ~10,000 meters.

• Air temperature decreases at an average rate of 0.6°C for every 100 meters of altitude.

• One Standard Atmosphere (1 atm) is the pressure exerted by a 760 mm column of mercury at sea level at 0°C.

Composition of Air (Detailed Table)

Gas	Volume (%)	Weight (%)	PPM (by Volume)	Symbol	Molecular Weight
Nitrogen	78.08	75.47	780,790	N₂	28.01
Oxygen	20.95	23.20	209,445	O₂	32.00
Argon	0.93	1.28	9,339	Ar	39.95
Carbon Dioxide	0.040	0.062	404	CO₂	44.01
Neon	0.0018	0.0012	18.21	Ne	20.18
Helium	0.0005	0.00007	5.24	He	4.00
Krypton	0.0001	0.0003	1.14	Kr	83.80
Hydrogen	0.00005	Negligible	0.50	H₂	2.02
Xenon	8.7 × 10⁻⁶	0.00004	0.087	Xe	131.30

Other Components of Air (Trace Gases)

In addition to the major gases, air contains several trace gases, present in very small quantities but often with significant environmental importance:

• Sulfur dioxide (SO₂): ~1.0 ppm

• Methane (CH₄): ~2.0 ppm

• Nitrous oxide (N₂O): ~0.5 ppm

• Ozone (O₃): 0–0.07 ppm

• Nitrogen dioxide (NO₂): ~0.02 ppm

• Iodine (I₂): ~0.01 ppm

• Carbon monoxide (CO): trace amounts

• Ammonia (NH₃): trace amounts

These gases play key roles in atmospheric chemistry, air pollution, and climate change.

Properties of Air

Since air is a form of matter, it exhibits properties common to all gases.

1. Colourless and Odourless

Air has no natural color or smell, making it invisible. However, we can feel moving air, which we call wind. Odors we sense are actually due to substances mixed with air.

2. Air Occupies Space

Air takes up space just like solids and liquids. When you blow air into a balloon, it expands because air fills the empty space inside.

3. Air Exerts Pressure

Air has weight, and the force exerted by this weight is known as air pressure. Air near Earth’s surface is denser due to gravity, which is why air pressure is higher at sea level and lower in mountainous regions.

4. Expansion on Heating

When air is heated, it expands and becomes less dense. Warm air rises, while cooler air sinks. This principle explains wind circulation and many weather patterns.

Frequently Asked Questions (FAQs)

Q1. Does air occupy space?

Yes. Air has mass and occupies space, which makes it a form of matter.

Q2. What are the main properties of air?

• Colourless and odourless

• Occupies space

• Exerts pressure

• Expands when heated

Q3. What are the major components of air?

• Nitrogen

• Oxygen

• Argon

• Carbon dioxide

• Neon and other trace gases

Q4. What is meant by the atmosphere?

The atmosphere is a protective layer of gases surrounding Earth. It regulates temperature, supports life, and shields the planet from harmful solar radiation.

Q5. What are the layers of Earth’s atmosphere?

• Troposphere

• Stratosphere

• Mesosphere

• Thermosphere

• Exosphere

Visit HN Series