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Chemistry · Class 10 · Chapter 13: Environmental Chemistry I: The Atmosphere

8 min

Key points

The atmosphere is a layer of gases surrounding the Earth, held by gravitational force
Troposphere (0-12 km) contains 75% of atmospheric mass and all weather phenomena occur here
Stratosphere (12-50 km) contains the ozone layer which protects Earth from harmful UV radiations
Greenhouse effect is the warming of Earth's surface due to absorption of infrared radiation by greenhouse gases
Major greenhouse gases are CO₂, CH₄, N₂O, CFCs and water vapors
Ozone depletion is caused by CFCs (Chlorofluorocarbons) which release chlorine atoms in stratosphere
Air pollution in cities like Karachi is mainly due to vehicle emissions, industrial waste and burning of fossil fuels
Acid rain is formed when SO₂ and NO₂ react with water vapor to form H₂SO₄ and HNO₃

Definitions

Atmosphere: The atmosphere is a layer of gases that surrounds the Earth and is held in place by Earth's gravitational force
Greenhouse Effect: The greenhouse effect is the process by which certain gases in the atmosphere absorb infrared radiation from Earth's surface and re-radiate it back, causing warming of the planet
Ozone Layer: The ozone layer is a region in the stratosphere containing high concentration of ozone (O₃) molecules that absorbs most of the Sun's harmful ultraviolet radiation
Air Pollution: Air pollution is the contamination of air by harmful substances that are not normally present or are present in abnormal quantities

Worked example

Define greenhouse effect. Name any two greenhouse gases and explain how CFCs cause ozone depletion. (7 marks)

Greenhouse effect is the process by which certain gases in the atmosphere trap heat from the sun, causing the Earth's temperature to rise. Two greenhouse gases are: (1) Carbon dioxide (CO₂) (2) Methane (CH₄). CFCs cause ozone depletion because when CFCs reach the stratosphere, UV radiation breaks them down releasing chlorine atoms. These chlorine atoms react with ozone molecules: Cl + O₃ → ClO + O₂. This destroys the protective ozone layer.

Common mistakes

Students confuse troposphere with stratosphere - remember troposphere is lowest layer where we live
Writing incomplete chemical reactions for acid rain formation without balancing equations
Mixing up greenhouse effect (good) with global warming (excessive greenhouse effect)
Not drawing proper labeled diagram of atmospheric layers with correct heights
Forgetting to mention Pakistani context like Karachi's air pollution in answers

Quick recap

Atmosphere has four layers: troposphere, stratosphere, mesosphere, thermosphere. Greenhouse gases (CO₂, CH₄, H₂O vapor) trap heat causing global warming. CFCs destroy ozone layer in stratosphere by releasing chlorine atoms. Clean air contains 78% nitrogen, 21% oxygen, and 1% other gases including noble gases and CO₂.

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