Chapter 13 · Question 7

What is ozone? How is it formed, and what is its role in protecting life on Earth? Explain how CFCs cause ozone depletion and what steps have been taken to address this problem.

Answer Revealed

Direct Answer:

Ozone (

\text{O}_3

) is a molecule made of three oxygen atoms. At ground level it is a poisonous gas, but in the stratosphere it forms a protective layer that shields Earth from harmful ultraviolet (UV) radiation from the Sun. Ozone is formed when high-energy UV radiation splits molecular oxygen (

\text{O}_2

) into free oxygen atoms:

\text{O}_2 \xrightarrow{\text{UV}} \text{O} + \text{O}

. These free atoms then combine with molecular oxygen:

\text{O} + \text{O}_2 \rightarrow \text{O}_3

. Ozone depletion is caused primarily by chlorofluorocarbons (CFCs), synthetic chemicals used as refrigerants and in fire extinguishers. CFCs released into the atmosphere break down ozone molecules. In 1987, UNEP forged an agreement to freeze CFC production at 1986 levels. Today, it is mandatory for manufacturers to produce CFC-free refrigerators worldwide.

Simple Explanation

Ozone is a special form of oxygen with three atoms instead of two. High up in the atmosphere, it forms a natural sunscreen that blocks harmful UV rays from reaching us. Without it, skin cancer and other damage would be much more common. In the 1980s, scientists discovered that chemicals called CFCs — used in old refrigerators and spray cans — were destroying the ozone layer. The good news is that countries around the world signed an agreement to stop using CFCs, and now all new refrigerators are made without them.

Exam-Ready Structure

The ozone layer and its depletion represent one of the earliest and most successful examples of global environmental cooperation. NCERT Class 10 Chapter 13 covers this under Section 13.2.1. 1. What is ozone? Ozone (

\text{O}_3

) is a molecule consisting of three oxygen atoms. While ordinary diatomic oxygen (

\text{O}_2

) is essential for all aerobic forms of life, ozone is a deadly poison at ground level and is known to cause skin cancer in human beings. However, at higher levels of the atmosphere (the stratosphere), it performs a vital protective function. 2. Ozone formation: High-energy ultraviolet (UV) radiation from the Sun strikes oxygen molecules in the stratosphere: (a) Step 1:

\text{O}_2 \xrightarrow{\text{UV}} \text{O} + \text{O}

(UV splits molecular oxygen into free oxygen atoms). (b) Step 2:

\text{O} + \text{O}_2 \rightarrow \text{O}_3

(a free oxygen atom combines with an O₂ molecule to form ozone). This is a continuous, dynamic process — ozone is constantly created and destroyed in the stratosphere, maintaining a natural balance. 3. Protective role: The ozone layer absorbs the most harmful wavelengths of ultraviolet (UV) radiation from the Sun, particularly UV-B and UV-C. Without the ozone shield, these high-energy rays would reach the Earth's surface, causing skin cancer, cataracts (eye damage), damage to the immune system, harm to crops, and disruption of marine food chains (phytoplankton). 4. Ozone depletion: (a) Starting in the 1980s, atmospheric ozone levels began dropping sharply. (b) The cause was traced to synthetic chemicals — primarily chlorofluorocarbons (CFCs) — used as refrigerants (in air conditioners and refrigerators), propellants in aerosol sprays, and in fire extinguishers. (c) CFCs are extremely stable compounds that rise into the stratosphere, where UV radiation breaks them apart, releasing chlorine atoms. Each chlorine atom can catalytically destroy thousands of ozone molecules. 5. Global response: (a) In 1987, the United Nations Environment Programme (UNEP) forged the Montreal Protocol, an international agreement that froze CFC production at 1986 levels and mandated a phased reduction. (b) Today, it is mandatory for manufacturing companies worldwide to produce CFC-free refrigerators and air conditioners. (c) Activity 13.4 in the textbook asks students to research whether regulations have succeeded in reducing ozone layer damage and whether the size of the 'ozone hole' has changed in recent years. The Montreal Protocol is widely regarded as the most successful international environmental agreement — the ozone layer is slowly recovering.

Key Points

Ozone ( $\text{O}_3$ ): molecule of three oxygen atoms; deadly poison at ground level, protective shield in the stratosphere
Formation: $\text{O}_2 \xrightarrow{\text{UV}} \text{O} + \text{O}$ , then $\text{O} + \text{O}_2 \rightarrow \text{O}_3$
Role: absorbs harmful ultraviolet (UV) radiation from the Sun, protecting life from skin cancer, cataracts, and ecosystem damage
Ozone depletion: caused by CFCs (chlorofluorocarbons) used as refrigerants and in fire extinguishers
CFCs release chlorine atoms in the stratosphere that destroy ozone molecules catalytically
1987 Montreal Protocol (UNEP): international agreement to freeze and phase out CFC production
CFC-free refrigerators are now mandatory worldwide; the ozone layer is gradually recovering

Common Mistakes

Confusing the protective role of ozone in the stratosphere with its harmful nature at ground level — it is location-dependent
Saying ozone depletion is caused by CO₂ or general air pollution — it is specifically caused by CFCs and related chemicals

What is ozone? How is it formed, and what is its role in protecting life on Earth? Explain how CFCs cause ozone depletion and what steps have been taken to address this problem.