Get notes, summary, questions and answers, MCQs, extras, and PDFs of Chapter 14 “Ozone Layer Depletion and Greenhouse Effect” which is part of NBSE Class 11 Environment Education. However, the notes should only be treated as references and changes should be made according to the needs of the students.
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Summary
The chapter discusses the ozone layer depletion and the greenhouse effect, two significant environmental issues affecting our planet. The ozone layer, located in the stratosphere, plays a crucial role in protecting life on Earth by absorbing harmful ultraviolet (UV) radiation from the sun. Ozone is formed by the action of UV radiation on oxygen molecules, resulting in a protective layer that shields the planet. However, human activities, particularly the release of chlorofluorocarbons (CFCs) and other ozone-depleting substances, have caused significant damage to this layer.
CFCs, once considered a breakthrough in refrigeration technology, were later discovered to be highly destructive to ozone. When CFCs reach the stratosphere, they release chlorine atoms under the influence of UV radiation, which then break down ozone molecules. This process has led to the formation of the “ozone hole,” especially over Antarctica, where up to 95% of the ozone can be destroyed during certain periods. The depletion of ozone increases the amount of UV radiation reaching the Earth’s surface, which can cause skin cancer, eye diseases, and other health issues, as well as impact ecosystems.
The chapter also explains the greenhouse effect, which is the warming of the Earth’s surface due to the presence of greenhouse gases like carbon dioxide, methane, and water vapour. These gases trap heat from the sun, preventing it from escaping back into space, much like a greenhouse traps heat to protect plants. While the greenhouse effect is a natural process essential for life, human activities have significantly increased the concentration of greenhouse gases, leading to global warming. This warming causes climate change, resulting in melting polar ice, rising sea levels, and more extreme weather patterns. These changes have serious implications for human society, agriculture, and natural ecosystems. The chapter underscores the urgency of addressing these environmental challenges to protect the planet and its inhabitants.
Video tutorial
Textbook solutions
Multiple-Choice Questions
1. Which of the following layers of the atmosphere contains 90% of all ozone?
A. Troposphere
B. Exosphere
C. Ionosphere
D. Stratosphere
Answer: D. Stratosphere
2. When is “World Ozone Day” celebrated every year?
A. 5th June
B. 17th November
C. 16th September
D. 9th December
Answer: C. 16th September
3. The reduction in the amount of ozone in the upper atmosphere is called
A. Ozone reduction
B. Ozone bypass
C. Ozone thinning
D. Ozone depletion
Answer: D. Ozone depletion
4. Which of the following is not a greenhouse gas?
A. Water vapour
B. Carbon-dioxide
C. Oxygen
D. Ozone
Answer: C. Oxygen
5. Which of the following is not a potential adverse effect of global warming?
A. Extreme weather patterns
B. Retreat of glaciers
C. An increase of UV radiation
D. Sea-level rise
Answer: C. An increase of UV radiation
Short Answer Questions
1. What is ozone?
Answer: Ozone is a bluish gas with a pungent smell, formed by three atoms of oxygen. It is a three-atom isotope of oxygen or merely a triatomic form of oxygen. Ozone gas is unstable because it is created as well as destroyed.
2. How is the ozone layer formed?
Answer: The ozone layer is formed when oxygen molecules are photolysed, yielding two oxygen atoms. These oxygen atoms then either recombine with an O₂ molecule to form ozone or combine with other substances.
3. What causes ozone to break down?
Answer: Ozone is lost by a reaction of the oxygen atom or the ozone molecule with each other or with some other trace gases such as chlorine.
4. Name the gases causing ozone depletion.
Answer: The gases causing ozone depletion include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), freons, and halons. Nitrogen oxides emitted from supersonic jets also deplete ozone.
5. Define “greenhouse effect”.
Answer: Greenhouse effect means the progressive warming up of the Earth’s surface due to the blanketing effect of man-made carbon dioxide in the atmosphere.
6. Where was the ozone hole noticed for the first time?
Answer: The ozone hole was noticed for the first time over Antarctica.
7. Name the greenhouse gases.
Answer: The greenhouse gases include water vapour, methane, carbon dioxide, nitrous oxide, and chlorofluorocarbons (CFCs).
8. Name the regions where ozone hole appeared.
Answer: The regions where the ozone hole appeared include Antarctica, the Arctic, Tibet, Xinjiang, Qinghai, and Hindukush.
9. Write the expanded form of HCFC.
Answer: The expanded form of HCFC is Hydrochlorofluorocarbon.
Long Answer Questions
1. What are the causes of ozone layer depletion?
Answer: The ozone layer’s depletion began in 1928 with the discovery of chlorofluorocarbons (CFCs), which replaced sulphur dioxide and ammonia in refrigerators. CFCs, developed in 1930, were later found to be destroying the ozone layer. Atomic halogens, particularly chlorine from CFCs, are responsible for the catalytic destruction of ozone in the stratosphere. These substances are transported into the stratosphere, where ultraviolet radiation decomposes them, releasing chlorine radicals. Each chlorine atom can react with one lakh ozone molecules, converting O₃ into O₂, causing significant ozone depletion. The amount of chlorine released by CFCs and hydro-chlorofluoro-carbons (HCFCs) shows how dangerous these substances are to the environment.
2. List the sources of greenhouse gases.
Answer: The best known greenhouse gas is carbon dioxide. Besides this, halogenated gases such as methane, chlorofluorocarbons, water vapour, nitrous oxides ozone, etc., also act as greenhouse gases.
Greenhouse gases are released in the atmosphere in various ways. For example,
(i) Electric power stations based on fossil fuels, mainly coal and mineral oil emit huge amounts of carbon dioxide which reaches the atmosphere.
(ii) Numerous factories spread all over the world, burn immense quantity of coal, mineral oil and natural gas and spew huge amount of carbon dioxide together with other undesirable gases through their chimneys into the atmosphere.
(iii) The third major source is the transport sector which includes various types of vehicles run on coal and petroleum. Similarly, large fleet of automobiles (trucks, buses, cars and two-wheelers, motorcycles, scooters, etc.), agricultural implements like tractors, combines, etc., and aircraft all over the world burn immense quantity of diesel and petroleum each year.
(iv) The fourth major source of the production of carbon dioxide is deforestation and burning of firewood. Amount of CFCs in the atmosphere is small but they are 10,000 times more effective than CO₂ in trapping heat.
3. How do CFCs deplete the ozone layer?
Answer: In the upper atmosphere, UV rays break off a chlorine atom from a CFC molecule. Free chlorine atom then attacks an ozone molecule, splitting it apart and thereby destroying ozone. This results in an oxygen molecule and chlorine monoxide. A free oxygen atom from the atmosphere attacks this chlorine monoxide, releasing a free chlorine atom and forming an oxygen molecule. The chlorine is free to repeat the process of destroying more oxygen molecules for the next hundred years.
4. What is the effect of ozone layer depletion on human beings?
Answer: The effects of ozone depletion on human beings include:
(i) Due to ozone depletion, the increased ultraviolet solar radiations in turn would raise the temperature of the Earth and trigger a chain of effects in the life forms as well as in the environment.
(ii) Due to ozone depletion, more ultraviolet radiation is coming on the Earth.
(iii) It will cause skin cancer and diseases of the eye.
5. What is the effect of greenhouse gases on the biotic community?
Answer: Greenhouse gases trap the Sun’s heat and help keep the surface of the Earth warm. The net result of the greenhouse gases is to increase the temperature of the Earth’s surface and lower atmosphere.
As the Earth heats up, climatic patterns will shift, and in some places, local weather conditions will become much more violent. Due to global warming, the polar ice will melt and sea level will rise by more than 6 metres. There will be thermal expansion of the upper layers of the oceans with the sea level rise by 6 cm. Due to global warming by 5°C, sea water will submerge coastal areas in Asia and Islands like Maldives may disappear totally!
6. Explain “global warming.”
Answer: Greenhouse effect means the progressive warming up of the Earth’s surface due to the blanketing effect of man-made carbon dioxide in the atmosphere. Greenhouse is a glasshouse with plants inside and which allows short wave, incoming solar radiation to come in but does not allow the long wave, outgoing infrared radiation to escape from inside the structure. Some of the gases in the Earth’s atmosphere act like the glass in a greenhouse. They trap the Sun’s heat and help keep the surface of the Earth warm. Without them, our world would be a much colder place. Carbon dioxide and water vapour act as a greenhouse in the sense that these allow the visible light of the sun to reach the surface of the Earth but absorb and reflect the long wave, outgoing terrestrial radiation mainly infrared rays and thus help in keeping the Earth’s surface warmer. The gases with the properties of greenhouse are called greenhouse gases. The net result of the greenhouse gases is to increase the temperature of the Earth’s surface and lower atmosphere.
7. How is agriculture affected by global warming?
Answer: Adverse effects of Global warming on agriculture, plants and animals:
(i) Increased temperature due to Global warming would affect plant communities.
(ii) Due to higher temperature, the evaporation rate of surface water and soil moisture would be greatly accelerated resulting in a phenomenal decrease in soil moisture content.
(iii) The photosynthesis by phytoplankton in the aquatic ecosystem would be markedly reduced.
(iv) Mortality rate of larvae of zooplankton will also increase.
8. What would be the change in sea level if global warming continues?
Answer: Due to global warming, the polar ice will melt and sea level will rise by more than 6 metres. There will be thermal expansion of the upper layers of the oceans with the sea level rise by 6 cm. Due to global warming by 5°C, sea water will submerge coastal areas in Asia and Islands like Maldives may disappear totally!
9. Draw a diagram to show greenhouse effect.
Answer: Please refer to the textbook.
Think and Answer
1. Greenhouse effect is a well known global issue nowadays. Write down your role as an individual to minimise its adverse effect.
Answer:
- Reduce, reuse, and recycle materials to minimize waste.
- Use energy-efficient appliances and light bulbs.
- Reduce the use of vehicles by carpooling, using public transport, walking, or cycling.
- Conserve water and electricity by turning off taps and switches when not in use.
- Support and use renewable energy sources like solar and wind power.
- Plant trees and support afforestation efforts to increase carbon absorption.
- Educate others about the importance of reducing greenhouse gas emissions.
2. How can you minimise the sources (from your everyday use) which cause ozone layer depletion?
Answer:
- Avoid using products that contain chlorofluorocarbons (CFCs), such as certain aerosol sprays and refrigerants.
- Properly maintain air conditioning units and refrigerators to prevent leaks of CFCs and HCFCs.
- Use products that are labeled as “ozone-friendly” or “CFC-free.”
- Reduce the use of private vehicles to lower emissions from fossil fuels, which can indirectly contribute to ozone layer depletion.
- Support and use eco-friendly alternatives to harmful chemicals in cleaning agents and solvents.
Extras MCQs
1. When is World Ozone Day celebrated?
A. September 16
B. April 22
C. June 5
D. November 7
Answer: A. September 16
26. Which greenhouse gas is primarily emitted by electric power stations based on fossil fuels?
A. Methane
B. Nitrous oxide
C. Carbon dioxide
D. Ozone
Answer: C. Carbon dioxide
Extra Questions and Answers
1. What are the two most immediate global environmental threats?
Answer: The two most immediate global environmental threats mentioned are climatic changes due to the artificial introduction of large amounts of greenhouse gases into the atmosphere and the abnormally high incidence of ultraviolet radiation on the Earth’s surface due to the hole in the ozone layer.
26. What are the potential effects of global warming on the Earth’s climate and ecosystems?
Answer: The potential effects of global warming on the Earth’s climate and ecosystems are profound and far-reaching. As global temperatures continue to rise due to increased levels of greenhouse gases in the atmosphere, several changes are expected to occur. One of the most significant effects is the alteration of climatic patterns, which may lead to more extreme and unpredictable weather events, such as severe storms, droughts, and floods. These changes in weather patterns can have devastating effects on ecosystems, disrupting the habitats of many species and leading to a loss of biodiversity. In addition, global warming is causing the polar ice caps to melt, leading to a rise in sea levels. This rise in sea levels threatens to submerge coastal areas, displacing millions of people and affecting agriculture and infrastructure. The warming of the Earth’s surface also affects the oceans, leading to changes in ocean currents and the disruption of marine ecosystems. Coral reefs, for example, are particularly vulnerable to rising temperatures and acidification caused by increased CO₂ levels, which can lead to widespread coral bleaching and the loss of marine life that depends on these ecosystems. Overall, global warming poses a serious threat to the stability of the Earth’s climate and the health of its ecosystems, with significant implications for human societies.
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