Chapter 14 — Natural Resources
Welcome to HSLC Guru! In this lesson we explore Class 9 Science Chapter 14 — Natural Resources for ASSEB students. You will learn about the air, water and soil around us, the cycles that move matter through the biosphere, and the human activities that pollute and disturb these resources. The notes, textbook answers, MCQs and glossary below are designed to help you prepare confidently for your school examinations.
Summary
The Earth supports life because it possesses three vital natural resources — air, water and soil. The thin layer of air around the Earth is called the atmosphere. It is mainly composed of nitrogen (about 78%), oxygen (about 21%), argon, carbon dioxide and water vapour. The atmosphere acts as a blanket that maintains a moderate temperature on Earth, prevents sudden rise of temperature during the day and slows down the escape of heat at night. It also shields living organisms from harmful ultraviolet radiation. Air pollution occurs when harmful gases such as oxides of sulphur, oxides of nitrogen, carbon monoxide and suspended particulate matter are released into the air mainly by burning of fossil fuels and industrial activities. Polluted air causes respiratory diseases, acid rain and global warming.
Water is essential for all forms of life. The continuous movement of water from the oceans to the atmosphere and back to the Earth is called the water cycle. Heat from the Sun causes evaporation of water from oceans, rivers and lakes; water vapour rises, cools, condenses and falls as rain, snow or hail. Water pollution takes place when sewage, industrial wastes, fertilisers, pesticides and hot water from factories enter water bodies. It changes the temperature of water, removes dissolved oxygen and harms aquatic life. Soil is formed by the long process of weathering — the breaking down of rocks by the action of the Sun, water, wind and living organisms. The fertile top layer of soil is called humus. Soil pollution is caused by excessive use of fertilisers and pesticides, dumping of solid wastes and removal of vegetation, leading to soil erosion and loss of fertility.
Substances cycle continuously between the living and non-living parts of the environment through biogeochemical cycles. The nitrogen cycle involves nitrogen fixation (conversion of atmospheric nitrogen into nitrates by lightning and by nitrogen-fixing bacteria like Rhizobium), nitrification (conversion of ammonia into nitrites and then nitrates by nitrifying bacteria), absorption by plants, transfer to animals through food, and finally denitrification (conversion of nitrates back to free nitrogen by denitrifying bacteria). The carbon cycle moves carbon between the atmosphere and living organisms through photosynthesis (plants take in CO2), respiration (organisms release CO2) and combustion (burning of wood and fossil fuels releases CO2). The oxygen cycle keeps the level of oxygen constant — it is consumed during respiration, combustion and the formation of oxides, and is replenished by photosynthesis.
Two major environmental problems linked with the atmosphere are the greenhouse effect and ozone layer depletion. Greenhouse gases such as carbon dioxide, methane and water vapour trap the heat radiated by Earth and keep the planet warm. However, an increase in these gases due to human activities causes global warming, leading to melting of glaciers and rise in sea level. The ozone layer in the upper atmosphere protects living beings from harmful ultraviolet rays. Chemicals like chlorofluorocarbons (CFCs), used in refrigerators and aerosols, destroy ozone and cause an “ozone hole.” Conserving natural resources, reducing pollution and using eco-friendly substances are essential to keep the Earth fit for life.
Textbook Question Answers
Very Short Answer Type Questions (1 Mark)
Q1. What is the thin layer of air surrounding the Earth called?
Answer: The thin layer of air surrounding the Earth is called the atmosphere.
Q2. Name the most abundant gas present in the atmosphere.
Answer: Nitrogen is the most abundant gas, forming about 78% of the atmosphere.
Q3. Which gas is essential for respiration?
Answer: Oxygen is essential for the respiration of living organisms.
Q4. What is humus?
Answer: Humus is the dark, organic material in the topsoil formed from decayed plant and animal remains, which makes the soil fertile.
Q5. Name one nitrogen-fixing bacterium.
Answer: Rhizobium, which lives in the root nodules of leguminous plants, is a nitrogen-fixing bacterium.
Q6. What is meant by weathering of rocks?
Answer: Weathering is the slow process of breaking down of rocks into smaller particles by the action of the Sun, water, wind and living organisms, leading to the formation of soil.
Q7. What is the ozone layer?
Answer: The ozone layer is a layer of ozone (O3) gas in the upper atmosphere (stratosphere) that absorbs harmful ultraviolet rays from the Sun.
Q8. Name two greenhouse gases.
Answer: Two greenhouse gases are carbon dioxide (CO2) and methane (CH4).
Q9. Which chemicals are mainly responsible for ozone depletion?
Answer: Chlorofluorocarbons (CFCs) are mainly responsible for ozone depletion.
Q10. What is the percentage of oxygen in the atmosphere?
Answer: Oxygen forms about 21% of the atmosphere.
Short Answer Type Questions (2-3 Marks)
Q1. How does the atmosphere act as a blanket?
Answer: The atmosphere acts as a blanket in two important ways:
- Air is a poor conductor of heat. The atmosphere therefore prevents the sudden increase of temperature during the day.
- During the night, it slows down the escape of heat from the Earth’s surface into outer space, preventing it from cooling rapidly.
- This keeps the average temperature of the Earth fairly constant and suitable for life.
Q2. What are the major causes of air pollution?
Answer: The major causes of air pollution are:
- Burning of fossil fuels such as coal, petroleum and diesel in vehicles and industries, which releases oxides of sulphur, nitrogen and carbon.
- Smoke and ash released from factories, thermal power plants and brick kilns.
- Use of CFCs in refrigerators, air conditioners and aerosol sprays.
- Burning of agricultural waste, forest fires and dust from construction work.
Q3. Describe the water cycle in nature.
Answer: The water cycle is the continuous movement of water between the Earth and the atmosphere. The Sun’s heat evaporates water from oceans, rivers, lakes and ponds, turning it into water vapour. Plants also release water vapour through transpiration. The vapour rises, cools at higher altitudes and condenses to form tiny water droplets that make clouds. When the droplets become heavy, they fall back to the Earth as precipitation in the form of rain, snow or hail. This water again flows into rivers and oceans, completing the cycle.
Q4. What is nitrogen fixation? Name two methods by which it occurs in nature.
Answer: The process of converting free atmospheric nitrogen into nitrogen compounds (such as nitrates) that can be used by plants is called nitrogen fixation. In nature it occurs by:
- Biological fixation — by nitrogen-fixing bacteria such as Rhizobium in the root nodules of leguminous plants and free-living bacteria like Azotobacter.
- Atmospheric fixation — during lightning, the high temperature combines nitrogen and oxygen to form oxides of nitrogen, which dissolve in rainwater and reach the soil as nitric acid.
Q5. What is the greenhouse effect? How is it useful and harmful?
Answer: The trapping of the Sun’s heat by gases like carbon dioxide, methane and water vapour in the atmosphere is called the greenhouse effect. It is useful because it keeps the Earth warm enough to support life; without it the average temperature would be very low. It becomes harmful when human activities increase the concentration of greenhouse gases. Excess gases trap more heat and raise the average temperature of the Earth, causing global warming, melting of polar ice and rise in sea level.
Q6. Mention four causes of soil pollution.
Answer: Important causes of soil pollution are:
- Excessive use of chemical fertilisers and pesticides which kill useful soil organisms.
- Dumping of solid waste, plastics and industrial effluents on land.
- Deforestation, which removes the protective vegetation cover and causes soil erosion.
- Disposal of radioactive waste and untreated sewage on land.
Long Answer Type Questions (5-6 Marks)
Q1. Explain the nitrogen cycle in detail with the help of its main steps.
Answer: The nitrogen cycle is the continuous circulation of nitrogen between the atmosphere, soil and living organisms. Although the atmosphere contains 78% nitrogen, plants and animals cannot use it directly. The main steps of the cycle are:
- Nitrogen fixation: Atmospheric nitrogen is converted into nitrates by (a) nitrogen-fixing bacteria such as Rhizobium living in the root nodules of legumes, (b) free-living soil bacteria like Azotobacter, and (c) lightning, which combines nitrogen and oxygen into oxides that dissolve in rainwater.
- Nitrification: Nitrifying bacteria such as Nitrosomonas convert ammonia into nitrites, and Nitrobacter further oxidises nitrites into nitrates.
- Assimilation: Plants absorb nitrates from the soil and use them to make proteins. Animals obtain nitrogen by feeding on plants.
- Ammonification: When plants and animals die, decomposers break down the nitrogenous waste and dead bodies, releasing ammonia back into the soil.
- Denitrification: Denitrifying bacteria like Pseudomonas convert nitrates back into free nitrogen, which returns to the atmosphere.
In this way the amount of nitrogen in the atmosphere remains balanced.
Q2. Describe the carbon cycle and explain the role of photosynthesis, respiration and combustion in it.
Answer: The carbon cycle describes the circulation of carbon among the atmosphere, hydrosphere, lithosphere and living organisms. Carbon is mainly present in the atmosphere as carbon dioxide (CO2) and in living bodies as carbohydrates, proteins and fats. The main processes are:
- Photosynthesis: Green plants absorb CO2 from the atmosphere and, using sunlight and water, convert it into glucose and oxygen. Carbon thus enters the food chain.
- Respiration: Plants and animals respire, oxidising food to release energy along with CO2 and water, which return carbon to the atmosphere.
- Decomposition: When organisms die, decomposers break down their bodies, releasing carbon as CO2.
- Combustion: Burning of wood, coal, petroleum and natural gas releases large amounts of CO2 into the air.
- Dissolution in water: CO2 dissolves in oceans where it is used by aquatic plants and forms carbonates.
Through these processes the level of carbon dioxide in the atmosphere is maintained, although excessive combustion has now disturbed this balance.
Q3. What is air pollution? Explain its main causes and harmful effects on the environment.
Answer: The contamination of air by harmful gases, smoke and dust particles that affect the health of living organisms is called air pollution.
Main causes:
- Burning of fossil fuels in vehicles, factories and power plants, releasing CO, CO2, SO2 and NO2.
- Industrial smoke containing soot and toxic chemicals.
- Use of chlorofluorocarbons in refrigerators and aerosols.
- Burning of agricultural waste, forest fires and volcanic eruptions.
Harmful effects:
- Causes respiratory diseases such as asthma, bronchitis and lung cancer.
- Oxides of sulphur and nitrogen produce acid rain, which damages buildings, soil and aquatic life.
- Increase in CO2 intensifies the greenhouse effect, leading to global warming.
- CFCs cause depletion of the ozone layer.
- Reduces visibility and creates smog over cities.
Q4. How is soil formed? Discuss the factors involved and the importance of soil.
Answer: Soil is formed by the slow process of weathering of rocks over thousands of years. Several factors take part in this process:
- Sun: The Sun heats rocks during the day, causing them to expand. At night they cool and contract. Repeated expansion and contraction crack the rocks into smaller pieces.
- Water: Water enters cracks in rocks. When it freezes, it expands and widens the cracks. Flowing water also wears down rocks into fine particles.
- Wind: Strong winds carry sand and dust which strike against rocks and break them into smaller fragments.
- Living organisms: Lichens, mosses and roots of trees grow on rocks and slowly break them. Earthworms and microorganisms mix decayed matter with rock particles to form humus.
Importance of soil: Soil supplies water and minerals to plants; it supports microorganisms that recycle nutrients; it forms the base of agriculture and food production; it stores groundwater; and it provides a habitat for many living organisms. Therefore, conservation of soil is essential for life on Earth.
Q5. What is ozone layer depletion? Discuss its causes and effects, and suggest measures to prevent it.
Answer: The thinning of the ozone layer in the stratosphere, especially over the polar regions, is called ozone layer depletion. In some areas the layer becomes so thin that it forms an “ozone hole.”
Causes:
- Release of chlorofluorocarbons (CFCs) used in refrigerators, air conditioners, aerosol sprays and foam.
- Use of halons in fire extinguishers.
- Emission of nitrogen oxides from supersonic jet planes and industrial activity.
Effects:
- Increased ultraviolet radiation reaches the Earth, causing skin cancer, cataracts and weakening of the immune system.
- Damage to crops and reduction in agricultural yield.
- Killing of phytoplankton in oceans, disturbing the food chain.
- Contributes to climate change.
Preventive measures: Stop the use of CFCs and replace them with eco-friendly substances; properly service air conditioners and refrigerators; avoid aerosol sprays; support international agreements such as the Montreal Protocol; spread awareness about ozone protection.
Additional Practice Questions
Multiple Choice Questions (MCQ)
Q1. The percentage of nitrogen in the atmosphere is about:
(a) 21% (b) 78% (c) 0.03% (d) 1%
Answer: (b) 78%
Q2. Which of the following is a greenhouse gas?
(a) Oxygen (b) Nitrogen (c) Carbon dioxide (d) Argon
Answer: (c) Carbon dioxide
Q3. The fertile top layer of soil rich in decayed matter is called:
(a) Bedrock (b) Humus (c) Subsoil (d) Gravel
Answer: (b) Humus
Q4. Which bacterium is found in root nodules of leguminous plants?
(a) Nitrobacter (b) Pseudomonas (c) Rhizobium (d) Lactobacillus
Answer: (c) Rhizobium
Q5. Conversion of nitrates back into free nitrogen is known as:
(a) Nitrogen fixation (b) Nitrification (c) Ammonification (d) Denitrification
Answer: (d) Denitrification
Q6. The ozone layer is located in the:
(a) Troposphere (b) Stratosphere (c) Mesosphere (d) Thermosphere
Answer: (b) Stratosphere
Q7. Which gas is released during photosynthesis?
(a) Carbon dioxide (b) Nitrogen (c) Oxygen (d) Hydrogen
Answer: (c) Oxygen
Q8. Acid rain is mainly caused by oxides of:
(a) Carbon and oxygen (b) Sulphur and nitrogen (c) Hydrogen and oxygen (d) Argon and helium
Answer: (b) Sulphur and nitrogen
Q9. Which of the following is not a natural resource?
(a) Air (b) Water (c) Plastic (d) Soil
Answer: (c) Plastic
Q10. CFCs mainly damage:
(a) Soil (b) Ozone layer (c) Water bodies (d) Forests
Answer: (b) Ozone layer
Fill in the Blanks
Q1. The thin layer of air around the Earth is called the __________.
Answer: atmosphere
Q2. The process by which water vapour changes into water droplets is called __________.
Answer: condensation
Q3. __________ bacteria convert ammonia into nitrites.
Answer: Nitrifying (e.g., Nitrosomonas)
Q4. Plants take in carbon dioxide and release oxygen during __________.
Answer: photosynthesis
Q5. The increase in average temperature of the Earth due to greenhouse gases is called __________.
Answer: global warming
True or False
Q1. Air is a mixture of gases.
Answer: True
Q2. Soil is formed in a few days by the action of wind.
Answer: False — soil is formed over hundreds and thousands of years.
Q3. Ozone protects living beings from harmful ultraviolet rays.
Answer: True
Q4. Carbon dioxide is the most abundant gas in the atmosphere.
Answer: False — nitrogen is the most abundant gas.
Q5. Denitrifying bacteria convert nitrates back to free nitrogen.
Answer: True
Glossary
| Term | Meaning |
|---|---|
| Atmosphere | The thin layer of gases surrounding the Earth. |
| Biosphere | The part of the Earth where life exists, including land, water and air. |
| Weathering | The breaking down of rocks into smaller particles by physical, chemical and biological agents. |
| Humus | Dark organic matter in soil formed from decayed plant and animal remains. |
| Water cycle | The continuous movement of water between Earth and atmosphere through evaporation, condensation and precipitation. |
| Transpiration | Loss of water from the aerial parts of plants in the form of vapour. |
| Nitrogen fixation | Conversion of free atmospheric nitrogen into nitrogen compounds usable by plants. |
| Nitrification | Conversion of ammonia into nitrites and then nitrates by nitrifying bacteria. |
| Denitrification | Conversion of nitrates back into free nitrogen by denitrifying bacteria. |
| Photosynthesis | Process by which green plants prepare food using CO2, water and sunlight, releasing oxygen. |
| Respiration | Oxidation of food in living cells to release energy, producing CO2 and water. |
| Combustion | Burning of a substance in the presence of oxygen, releasing heat and CO2. |
| Greenhouse effect | Trapping of the Sun’s heat in the atmosphere by greenhouse gases. |
| Global warming | Rise in the average temperature of the Earth due to increased greenhouse gases. |
| Ozone layer | A layer of ozone in the stratosphere that absorbs harmful UV radiation. |
| CFCs | Chlorofluorocarbons — chemicals that destroy ozone molecules. |
| Acid rain | Rain containing dissolved oxides of sulphur and nitrogen, which is harmful to life and structures. |
| Air pollution | Contamination of air by harmful gases, smoke and particulate matter. |
| Water pollution | Addition of harmful substances to water bodies, making the water unfit for use. |
| Soil pollution | Degradation of soil quality due to chemicals, wastes and poor agricultural practices. |
Keep revising these questions and concepts regularly. For more Class 9 Science notes and ASSEB question answers in English Medium, stay connected with HSLC Guru.