11.1 Composition of Atmosphere

This topic explains what the atmosphere is and describes its composition — the main gases and other substances that make up the air around us.

Definition

Atmosphere

The envelope of gases and water vapour surrounding the planet Earth is called the atmosphere.

Simple explanation: The atmosphere is the layer of air around the Earth.

Air

Air is a mixture of gases.

Simple explanation: Air is not a single substance — it is a mixture of many gases.

Main Idea

• Nitrogen
• Oxygen
• Hydrogen
• Carbon dioxide
• Noble gases
• Water vapour

• 78% nitrogen
• 21% oxygen
• The remainder is a mixture of noble gases and carbon dioxide.

Simple explanation: Air is mostly nitrogen and oxygen, with small amounts of other gases.

Core Concept

Composition of Dry Air (Figure 11.1)

The pie chart in the book shows the composition of dry air by volume:

Simple explanation: Dry air is mostly nitrogen (78%) and oxygen (21%). The remaining 1% contains argon, carbon dioxide, and other trace gases.

Key Components

Nitrogen (78%) — the most abundant gas in air. Oxygen (21%) — the second most abundant; essential for respiration and combustion. Argon (0.93%) — the most abundant noble gas in air. Carbon dioxide (0.038%) — important for photosynthesis and a greenhouse gas. Other gases — neon, helium, methane, krypton, hydrogen (in very small amounts).

Water Vapour in Air

Besides gases, there are varying amounts of water vapour in the air.

• Little water in the air over the desert.
• In the tropical rainforest, the air may contain up to 4% water vapour.

This means the amount of water vapour in the air varies from place to place and from time to time.

Simple explanation: The amount of water vapour in air is not fixed — it depends on location and weather.

Detailed Explanation

Why the Atmosphere Is Important

• Provides oxygen for respiration.
• Provides carbon dioxide for photosynthesis.
• Protects us from harmful solar radiation.
• Helps regulate Earth's temperature.
• Is the medium for weather and climate.

Simple explanation: The atmosphere makes life on Earth possible.

The Main Gases

• Most abundant gas in air.
• Chemically quite unreactive under normal conditions.
• Essential for proteins and DNA (through nitrogen fixation).

• Essential for respiration in most living things.
• Needed for combustion (burning).
• Chemically reactive.

• Most abundant noble gas.
• Very unreactive.
• Used in welding and light bulbs.

• Used by plants for photosynthesis.
• A greenhouse gas.
• Released by respiration and combustion.

5. Other trace gases — neon, helium, methane, krypton, hydrogen.

What the Book Asks

The book asks: "Can you name the main gases that make up the air?"

• Nitrogen (78%)
• Oxygen (21%)
• Argon (0.93%)
• Carbon dioxide (0.038%)
• Other trace gases

What the Book Asks (Concept Assessment Exercise 11.1)

"What two gases make up most of the air?"

Answer: Nitrogen (78%) and oxygen (21%) make up most of the air.

Intermediate Level

Why the Composition Is Important

The composition of the atmosphere is crucial for life:

Simple explanation: Each gas in the atmosphere plays a specific role.

Dry Air vs Moist Air

• Dry air — air without water vapour.
• Moist air — air containing water vapour.

The book gives the composition of dry air because water vapour content varies greatly.

• Desert: very little water vapour
• Tropical rainforest: up to 4% water vapour

Simple explanation: The composition of dry air is fixed, but the amount of water vapour varies.

Comparison of Gases in Dry Air

Why the Atmosphere Contains These Gases

• Volcanic activity (released early gases)
• Photosynthesis (produced oxygen)
• Respiration and combustion (add CO₂)
• Natural cycles (nitrogen cycle, carbon cycle, water cycle)

Simple explanation: The atmosphere has evolved over billions of years.

Important Points

• - Atmosphere: envelope of gases and water vapour surrounding the Earth.
• - Air: a mixture of gases.
• - Main gases in air: nitrogen (78%), oxygen (21%), argon (0.93%), carbon dioxide (0.038%).
• - Other trace gases: neon, helium, methane, krypton, hydrogen.
• - Water vapour: varies from place to place (up to 4% in tropical rainforests).
• - Composition of dry air: approximately 78% nitrogen, 21% oxygen, remainder as noble gases and carbon dioxide.
• - Nitrogen is the most abundant gas in air.
• - Oxygen is the second most abundant gas in air.

Remember

• - Atmosphere = envelope of gases and water vapour.
• - **Air** = mixture of gases.
• - Nitrogen = 78% (most abundant).
• - Oxygen = 21% (second most abundant).
• - **Argon** = 0.93%.
• - **Carbon dioxide** = 0.038%.
• - Water vapour varies (up to 4% in tropical rainforests).
• - Composition of dry air: 78% N₂, 21% O₂, 1% other gases.

Common Mistakes

• 1. Thinking air is a single substance. Air is a mixture of gases.
• 2. Thinking oxygen is the most abundant gas. Nitrogen is the most abundant (78%).
• 3. Forgetting water vapour. Water vapour is a variable component of air.
• 4. Thinking the composition of dry air is the same as moist air. Dry air excludes water vapour; moist air includes it.
• 5. Confusing the percentages. Nitrogen = 78%; oxygen = 21%.
• 6. Thinking carbon dioxide is a major component. CO₂ is only 0.038% — a trace gas.

Quick Revision

• Atmosphere: the envelope of gases and water vapour surrounding the planet Earth.
• Air: a mixture of gases including nitrogen, oxygen, hydrogen, carbon dioxide, noble gases, and water vapour.
• Composition of clean, dry air:
• Nitrogen: 78%
• Oxygen: 21%
• Argon: 0.93%
• Carbon dioxide: 0.038%
• Other trace gases: neon, helium, methane, krypton, hydrogen
• Water vapour in air varies from place to place and time to time (up to 4% in tropical rainforests).
• Two gases that make up most of the air: nitrogen and oxygen.

60-Second Revision

• Atmosphere = envelope of gases and water vapour.
• **Air** = mixture of gases.
• **Nitrogen** = 78%; **oxygen** = 21%.
• **Argon** = 0.93%; **CO₂** = 0.038%.
• Warning: nitrogen is the most abundant gas, not oxygen.