CBSE Class 9 Science Natural Resources Notes

CBSE Class 9 Science Natural Resources Notes. There are many more useful educational material which the students can download in pdf format and use them for studies. Study material like concept maps, important and sure shot question banks, quick to learn flash cards, flow charts, mind maps, teacher notes, important formulas, past examinations question bank, important concepts taught by teachers. Students can download these useful educational material free and use them to get better marks in examinations.  Also refer to other worksheets for the same chapter and other subjects too. Use them for better understanding of the subjects.

NATURAL RESOURCES

RESOURCES ON THE EARTH

Biosphere. The whole combination of animals, plants and non-living beings which, by their interaction, make the planet Earth a live and vibrant place is called the biosphere.

Biotic components. Living things constitute the biotic component of the biosphere.

Abiotic components. The air, the water and the soil form the non-living (abiotic) component of the biosphere. The air is called the atmosphere, the water is the hydrosphere and the soil (the rocky crust) is the lithosphere.

Resources on the earth. The natural resources of the earth are air, water, soil, minerals and living organisms. The outer crust of the earth is the lithosphere. The water on the earth is the hydrosphere. The layer of the air around the earth is the atmosphere. Living organisms are found where the atmosphere, hydrosphere and lithosphere interact, and this region is the biosphere.

Cross-section figure labels: Atmosphere (above), Biosphere (where a tree stands at the meeting of land and water), Lithosphere (land) and Hydrosphere (water).

Air is a mixture of many gases such as nitrogen, oxygen, carbon dioxide and water vapour. All living beings need oxygen to break down glucose molecules and get energy for their activities; this produces carbon dioxide. Another process that consumes oxygen and produces carbon dioxide is combustion. This includes not just human activities that burn fuels to get energy, but also forest fires. Despite this, the percentage of carbon dioxide in our atmosphere is only a fraction of a percent because of carbon dioxide fixation.

Air is a mixture of different gases: Nitrogen N₂, Oxygen O₂, Noble gases (Ar), Carbon dioxide CO₂, Water vapour H₂O.

Carbon dioxide fixation

1. Green plants convert carbon dioxide into glucose in the presence of sunlight and water.
2. Many marine animals use carbonates dissolved in sea water to make their shells.

THE ROLE OF THE ATMOSPHERE IN CLIMATE CONTROL

The atmosphere covers the Earth like a blanket. Air is a bad conductor of heat. The atmosphere keeps the average temperature of the Earth fairly steady during the day and even during the whole year. It prevents a sudden increase in temperature during the daylight hours, and during the night it slows down the escape of heat into outer space. The moon, which is about the same distance from the Sun as the Earth but has no atmosphere, has a temperature range of −190 °C to 110 °C.

Climate system diagram (labels):

• Changes in Solar Inputs (Sun)
• Changes in the Atmosphere: Composition, Circulation. Atmosphere: N₂, O₂, Ar, H₂O, CO₂, CH₄, N₂O, O₃, etc.; Aerosols; Terrestrial Radiation
• Changes in the Hydrological Cycle: Clouds, Volcanic Activity, Human Influences
• Atmosphere-Ice Interaction; Precipitation, Evaporation; Heat Exchange; Wind Stress; Sea Ice
• Hydrosphere: Ocean; Hydrosphere: Rivers & Lakes; Ice-Ocean Coupling; Glacier; Ice Sheet
• Atmosphere-Biosphere Interaction; Biosphere; Soil-Biosphere Interaction; Land-Atmosphere Interaction; Land Surface
• Changes in the Cryosphere: Snow, Frozen Ground, Sea Ice, Ice Sheets, Glaciers
• Changes in the Ocean: Circulation, Sea Level, Biogeochemistry
• Changes in/on the Land Surface: Orography, Land Use, Vegetation, Ecosystems

THE MOVEMENT OF AIR: WINDS

Winds, rain and similar phenomena are the result of changes that take place in our atmosphere due to the heating of air and the formation of water vapour. Water vapour is formed by the heating of water bodies and by the activities of living organisms. A rise in temperature creates a low-pressure zone, towards which cooler air from a high-pressure zone moves, pushing up the hot air. The atmosphere is heated from below by the radiation that is reflected back or re-radiated by the land or water bodies. On being heated, convection currents are set up in the air.

AIR POLLUTION

An air pollutant is a substance in the air that can cause harm to humans and the environment. Pollutants can be solid particles, liquid droplets or gases, and they may be natural or man-made.

Pollutants can be classified as primary or secondary. Primary pollutants are substances directly emitted from a process, such as ash from a volcanic eruption, carbon monoxide gas from a motor vehicle exhaust, or sulphur dioxide released from factories. Secondary pollutants are not emitted directly; they form in the air when primary pollutants react or interact. An important example is ground-level ozone, one of the many secondary pollutants that make up photochemical smog.

Diagram: Primary pollutants come from Natural sources (e.g. volcano), Stationary sources (factories) and Mobile sources (vehicles); they react in the air to form secondary pollutants.

RAIN

When water bodies are heated during the day, a large amount of water evaporates and goes into the air. Some water vapour also gets into the atmosphere through biological activities. This air also gets heated. The hot air rises, carrying the water vapour with it. As the air rises, it expands and cools. This cooling causes the water vapour to condense into tiny droplets. The condensation is helped if some particles act as a “nucleus” for the drops to form around. Once the droplets are formed, they grow bigger by further condensation. When the drops have grown big and heavy, they fall down as rain.

Rainfall patterns are decided by the prevailing wind patterns. In large parts of India, rains are mostly brought by the south-west or north-east monsoons.

Diagram labels (water cycle): Sun; Condensation; Precipitation; Evaporation; Surface water; Ground water; Rock deep percolation.

WATER: A WONDER LIQUID

Water occupies a very large area of the Earth’s surface and is also found underground. Some water exists as water vapour in the atmosphere. Most of the water on the Earth’s surface is found in seas and oceans and is saline. Fresh water is found frozen in the ice-caps at the two poles and on snow-covered mountains. Underground water and the water in rivers, lakes and ponds is also fresh. However, the availability of fresh water varies from place to place. In practically every summer, most places face a shortage of water, and in rural areas where water supply systems have not been installed, people spend considerable time fetching water from faraway sources.

Importance of water. All cellular processes take place in a water medium. All the reactions within our body and within the cells occur between substances dissolved in water. Substances are also transported from one part of the body to another in dissolved form. Hence organisms need to maintain the level of water in their bodies to stay alive. Terrestrial life forms require fresh water because their bodies cannot tolerate or get rid of the high amounts of dissolved salts in saline water. Thus water sources need to be easily accessible for animals and plants to survive on land.

WATER POLLUTION

Water pollution is the contamination of water bodies such as lakes, rivers, oceans and groundwater caused by human activities, which can be harmful to the organisms and plants that live in these water bodies. We use the term water pollution to cover the following effects:

Some of the causes of water pollution are shown in the figure below:

1. The addition of undesirable substances to water bodies. These could be fertilisers and pesticides used in farming, poisonous substances such as mercury salts used by paper industries, or disease-causing organisms such as the bacteria which cause cholera.
2. The removal of desirable substances from water bodies. Dissolved oxygen is used by the animals and plants that live in water; any change that reduces the amount of dissolved oxygen would adversely affect these aquatic organisms. Other nutrients could also be depleted from the water bodies.
3. A change in temperature. Aquatic organisms are used to a certain range of temperature in the water body where they live. A sudden, marked change in this temperature would be dangerous for them or affect their breeding. The eggs and larvae of various animals are particularly susceptible to temperature changes.

SOIL

Soil is an important resource that decides the diversity of life in an area. The outermost layer of our Earth is called the crust, and the minerals found in this layer supply a variety of nutrients to life forms.

The factors or processes that make soil

• The Sun: the Sun heats up rocks during the day so that they expand. At night the rocks cool down and contract. Since all parts of a rock do not expand and contract at the same rate, cracks form and ultimately huge rocks break up into smaller pieces.
• Water: water helps in the formation of soil in two ways. First, water gets into the cracks in the rocks formed by uneven heating; if this water later freezes, it widens the cracks. Second, flowing water wears away even hard rock over long periods of time. Fast-flowing water carries big and small particles of rock downstream. These rocks rub against other rocks and the resulting abrasion wears them down into smaller and smaller particles. The water then carries these particles and deposits them further down its path. Soil is thus found in places far away from its parent rock.
• Wind: in a process similar to the way water rubs against rocks and wears them down, strong winds also erode rocks. The wind also carries sand from one place to another like water does.

BIOGEOCHEMICAL CYCLES

A constant interaction between the biotic and abiotic components of the biosphere makes it a dynamic but stable system. These interactions consist of a transfer of matter and energy between the different components of the biosphere.

THE WATER CYCLE

The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above and below the surface of the Earth.

Water Cycle diagram labels: Sun; evaporation (from the sea); transpiration (from trees); condensation (clouds); precipitation (rain); percolation; collection (sea); underground water.

Water can change states among liquid, vapour and ice at various places in the water cycle. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go. The Sun, which drives the water cycle, heats water in the oceans, and the water evaporates as vapour into the air. Ice and snow can sublimate directly into water vapour. Rising air currents take the vapour up into the atmosphere, where cooler temperatures cause it to condense into clouds. Air currents move clouds around the globe; cloud particles collide, grow and fall out of the sky as precipitation. Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Snow packs can thaw and melt, and the melted water flows overland as snowmelt. Most precipitation falls back into the oceans or onto land, where it flows over the ground as surface runoff. A portion of the runoff enters rivers in valleys, with stream flow moving water towards the oceans. Runoff and groundwater are stored as fresh water in lakes.

Not all runoff flows into rivers. Much of it soaks into the ground as infiltration. Some water infiltrates deep into the ground and replenishes aquifers, which store huge amounts of fresh water for long periods of time. Some infiltration stays close to the land surface and can seep back into surface water bodies (and the ocean) as groundwater discharge. Some groundwater finds openings in the land surface and emerges as fresh water springs. Over time, the water re-enters the ocean, where the water cycle started.

THE NITROGEN CYCLE

The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. It includes gaseous components.

Nitrogen cycle in nature (diagram):

• Nitrogen in atmosphere → Protoplasm (green plants) by Nitrogen fixation
• Nitrates → Nitrogen in atmosphere by Denitrification
• Nitrates → Protoplasm (green plants)
• Protoplasm (green plants) → Protoplasm (animals)
• Protoplasm (green plants) → Ammonia by Ammonification; Protoplasm (animals) → Ammonia
• Ammonia → Nitrites → Nitrates (Nitrification)
• Atmospheric and industrial fixation: Nitrogen in atmosphere → Nitrates (outer arc, with Nitrification)

The Earth’s atmosphere is about 78% nitrogen, making it the largest pool of nitrogen. Nitrogen is essential for many biological processes and is crucial for life on Earth. It is in all amino acids, is incorporated into proteins, and is present in the bases that make up nucleic acids such as DNA and RNA. In plants, much of the nitrogen is used in chlorophyll molecules, which are essential for photosynthesis and growth.

Fixation is necessary to convert gaseous nitrogen into forms usable by living organisms. Some fixation occurs in lightning strikes, but most is done by free-living or symbiotic bacteria. These bacteria have the enzyme nitrogenase, which combines gaseous nitrogen with hydrogen to produce ammonia, which is then converted by the bacteria into their own organic compounds. Some nitrogen-fixing bacteria, such as Rhizobium, live in the root nodules of legumes (such as peas or beans). Here they form a mutually beneficial (mutualistic) relationship with the plant, producing ammonia in exchange for carbohydrates. Nutrient-poor soils can be planted with legumes to enrich them with nitrogen. A few other plants can form such symbioses. Nowadays a considerable portion of nitrogen is also fixed in ammonia chemical plants.

THE CARBON CYCLE

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere and atmosphere of the Earth. The cycle is usually thought of as four major reservoirs of carbon interconnected by pathways of exchange. These reservoirs are:

1. The atmosphere.
2. The terrestrial biosphere, usually defined to include fresh water systems and non-living organic material such as soil carbon.
3. The oceans, including dissolved inorganic carbon and living and non-living marine life.
4. The sediments, including fossil fuels.

   Carbon cycle diagram: Carbon in the atmosphere (mainly CO₂) → Photosynthesis → Plants → Animals → Decomposers; Decomposers → Plants. Plants, Animals and Decomposers → Respiration → Carbon in the atmosphere. Volcanic eruptions → Carbon in the atmosphere. Plants → Burial and compaction over time → Carbon in fossil fuels (coal, oil, gas) → Combustion → Carbon in the atmosphere. Wood combustion → Combustion.

THE GREENHOUSE EFFECT

The greenhouse effect refers to the change in the steady-state temperature of a planet or moon caused by an atmosphere containing gases that absorb and emit infrared radiation. Greenhouse gases, which include water vapour, carbon dioxide and methane, warm the atmosphere by efficiently absorbing thermal infrared radiation emitted by the Earth’s surface, by the atmosphere itself and by clouds. As a result of its warmth, the atmosphere also radiates thermal infrared in all directions, including downward to the Earth’s surface. Thus greenhouse gases trap heat within the surface–troposphere system. The greenhouse effect is one of several factors that affect the temperature of the Earth.

Greenhouse effect diagram: Incoming solar radiation (0.2 to 4 µm) from the Sun passes through the atmosphere; part of the solar radiation is reflected; infrared radiation is emitted by the Earth's surface (4 to 100 µm); some outgoing infrared radiation escapes, and some infrared radiation is absorbed by greenhouse gases in the atmosphere and re-radiated back to the Earth.

THE OXYGEN CYCLE

The oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within and between its three main reservoirs: the atmosphere (air), the biosphere (living things) and the lithosphere (the Earth’s crust). The main driving factor of the oxygen cycle is photosynthesis, which is responsible for the modern Earth’s atmosphere and life.

Oxygen cycle diagram: Atmospheric oxygen (O₂) → Respiration → CO₂ and H₂O → Photosynthesis → Atmospheric oxygen (O₂). Photosynthesis → Organic molecule (C₆H₁₂O₆) → Respiration.

ENERGY CYCLE

All the above cycles can be grouped together as the energy cycle on the Earth. The Sun is the main source of energy for every activity on Earth. This energy facilitates the everlasting cycle of all resources in the biosphere, and this system ensures that whatever we take from the Earth and its atmosphere we return in some way or other. A living organism is made of carbon, oxygen, nitrogen and other elements, and all living organisms need a regular supply of these elements to continue life. During life, these things are returned to the atmosphere in some way; for example, we return carbon dioxide in our breath and return water as sweat or urine.

Ultimately, when a living being dies, its body is decomposed by decomposers such as bacteria, which break it down into the basic elements out of which it was originally made. That is how the everlasting cycle of life goes on.

OZONE LAYER

The ozone layer is a layer in the Earth’s atmosphere which contains relatively high concentrations of ozone. This layer absorbs 93–99% of the Sun’s high-frequency ultraviolet light, which is potentially damaging to life on Earth. Over 91% of the ozone in the Earth’s atmosphere is present here. It is mainly located in the lower portion of the stratosphere, from approximately 10 km to 50 km above the Earth’s surface, though the thickness varies seasonally and geographically. Because of the heavy use of CFCs (chlorofluorocarbons) in refrigerators and pressurised cans, the ozone layer has been depleted at some places. This has caused an alarming rise in ultraviolet radiation, leading to increased cases of skin cancer.

Ozone layer depletion diagram: The ozone layer shields Earth from UV rays (UV-A, UV-B). Stratosphere 10-50 km; Troposphere 0-10 km. (1) Ultra-violet rays split a chlorine atom away from the CFC (chlorofluorocarbon) molecule (chlorine, fluorine, carbon). (2) The chlorine atom breaks up an ozone molecule (O₃), making a hole in the ozone layer. (3) The molecules left behind are chlorine monoxide and oxygen (O₂). Ozone-depleting chemicals: CFCs (aerosols, refrigerants, solvents); HCFCs (aerosols, refrigerants, solvents); Halons (fire extinguishers); Methyl bromide (pesticides).

POINTS TO REMEMBER: OXYGEN CYCLE

Water cycle diagram: Sun; Clouds; Cloud formation; Precipitation; Snow; Surface runoff; Lake; Transpiration; Condensing water vapour; Evaporation; Ground water; Ocean.

• The oxygen in the atmosphere is freed by the process of photolysis. The energy in sunlight breaks the oxygen-bearing molecules to produce free oxygen. Oxygen molecules are broken down by UV rays from the sun. This cycle shields the Earth from harmful UV rays.
• In the biosphere, oxygen goes through cycles of respiration and photosynthesis. Humans and animals breathe in oxygen, which is used in metabolic processes, and carbon dioxide is given out. Plants and phytoplankton carry out photosynthesis, in which carbon dioxide is used in the presence of sunlight to form carbohydrates and oxygen.
• In the lithosphere, oxygen is fixed in minerals such as silicates and oxides. This oxygen is freed by chemical weathering: when the oxygen-bearing mineral is exposed to chemical reactions, it wears down and free oxygen is produced.

  The Oxygen Cycle (diagram): Most available oxygen comes from photosynthesis by plants on land and phytoplankton on the ocean's surface. Some oxygen is made in the atmosphere when sunlight breaks down water. Oxygen is used by animals, plants, bacteria, fire, decomposition, and even rusting metal (oxidation). Most oxygen is stored in the oxide minerals of the Earth's crust and mantle, called the lithosphere, but is bound to rocks and unavailable for use. Labels: plants on land; the ocean; phytoplankton floating on the surface of the ocean.

INTEXT QUESTIONS (PAGE 193)

How is our atmosphere different from the atmospheres on Venus and Mars?

Answer: The Earth’s atmosphere contains a mixture of many gases: nitrogen (78.08%), oxygen (20.95%), carbon dioxide (0.03%) and water vapour (in varying proportion). The atmospheres of Venus and Mars mainly contain carbon dioxide, about 95–97%. This may be the reason why no life is known to exist on either Venus or Mars.

How does the atmosphere act as a blanket?

Answer: The atmosphere mainly contains air, which is a bad conductor of heat. Because of this, the atmosphere keeps the average temperature of the Earth fairly steady during the day and throughout the year. It prevents a sudden increase in temperature during the daylight hours, and during the night it slows down the escape of heat into outer space. In this way the atmosphere acts as a blanket.

What causes winds?

Answer: Winds occur due to unequal heating of atmospheric air. The heat causes rising up of air along with water vapour. As the air rises, it expands and cools. This cooling causes the water vapour in the air to condense. The condensation of water occurs if some particles (like dust particles) act as the 'nucleus' for these drops to stick around. These tiny droplets grow bigger by more and more condensation of other water droplets and finally form the clouds. When the air over land (or any region) is heated, it rises and a low-pressure zone is created. Cooler air from the surrounding high-pressure region moves in to take its place. This movement of air from a high-pressure region to a low-pressure region is wind.

How are clouds formed?

Answer: Water evaporates from water bodies and goes into the atmosphere. The air also becomes hot due to sunlight and rises, carrying the water vapour with it. As the air rises, it expands and cools. This cooling makes the water vapour condense. The condensation happens if some particles (like dust) act as a “nucleus” for the drops to form around. These tiny droplets grow bigger by more and more condensation of other water droplets. In this way clouds are formed.

List any three human activities that you think would lead to air pollution.

Answer: (i) Excessive burning of fossil fuels, i.e. coal and petroleum, produces large amounts of oxides of nitrogen and sulphur. These oxides mix with air and cause acid rain, which has many harmful effects. (ii) Many industries release large amounts of poisonous gases into the atmosphere. (iii) Forest fires, excessive use of chlorofluorocarbons (CFCs) in refrigerators, and excessive mining and ore refining release harmful gases into the air.

INTEXT QUESTIONS (PAGE 194)

Why do organisms need water?

Answer: (i) Cellular processes need water for their functioning. (ii) Substances dissolve in water so that reactions can take place within the cells. (iii) Transportation of substances within the body needs water. (iv) Water helps in the digestion of food and its absorption into the blood. (v) It helps to maintain body temperature.

What is the major source of fresh water in the city/town/village where you live?

Answer: In a city, town or village the major source of water is usually underground water, which is drawn with the help of hand pumps and tube wells. Other nearby sources are rivers, lakes and ponds. (The answer depends on where the student lives.)

Do you know of any activity which may be polluting this water source?

Answer: (i) Disposal of garbage or sewage from cities and towns and from factories into the water. (ii) Hot water released from industries, which may disturb the temperature of the water body and lead to the death of many aquatic organisms.

INTEXT QUESTIONS (PAGE 196)

How is soil formed?

Answer: (i) Rocks near the surface of the Earth are broken down by various physical, chemical and some biological processes. This takes millions of years. (ii) This weathering leads to the formation of fine particles called soil. (iii) Some other factors also help in the formation of soil: (a) the Sun heats rocks so that they crack and break into small particles; (b) water breaks up rocks by freezing in cracks and wears them away by fast flow; (c) wind erodes rocks by blowing fast; (d) lichens and mosses grow on rock surfaces and break them down into powder, forming a thin layer of soil, and big trees sometimes enter cracks in rocks and force them to break further as they grow.

What is soil erosion?

Answer: Soil erosion is the removal of the top layer of soil, which is rich in humus and nutrients. The agents of soil erosion are mainly flowing water and wind. If soil erosion continues for a long time, the land becomes infertile and barren because of the loss of its valuable nutrients.

What are the methods of preventing or reducing soil erosion?

Answer: Methods of preventing or reducing soil erosion:

1. Afforestation: planting more trees reduces soil erosion.
2. Contour ploughing: ploughing the land in furrows across the natural slope helps trap water and prevents the washing away of top soil.
3. Step (terrace) farming: farmers form a series of steps by making horizontal strips supported by walls to catch the descending water. This gives the water enough time to percolate into the soil and nourish the crop.
4. Soil cover: after harvesting a crop, the soil is covered with dried vegetation to prevent erosion.
5. Preventing overgrazing: grasses bind soil particles and prevent erosion. If overgrazing is allowed, the grasses are uprooted and the soil gets eroded.

INTEXT QUESTIONS (PAGE 201)

What are the different states in which water is found during the water cycle?

Answer: Water is found in all three states in the water cycle:

1. Gaseous state: water vapour, formed when water evaporates from the surface of water bodies and mixes with the air.
2. Liquid state: water vapour condenses high up in the atmosphere and falls on the Earth as rain.
3. Solid state: formed by the freezing of liquid droplets in the upper layers of the atmosphere; it falls on the Earth as snow, hail or sleet.

Name two biologically important compounds that contain both oxygen and nitrogen.

Answer: Nitrites (NO₂⁻) and nitrates (NO₃⁻). These are important forms of nitrogen that plants use to make biomolecules such as proteins.

(The source named the first compound “nitrates (NO₂⁻)”; the ion NO₂⁻ is nitrite.)

List any three human activities which would lead to an increase in the carbon dioxide content of air.

Answer: Human activities that increase the carbon dioxide content of the air:

1. Deforestation: trees carry out photosynthesis and convert CO₂ into organic compounds such as glucose and starch. When forests are cut, less CO₂ is used up and the CO₂ level in the air rises.
2. Combustion of fuels: fuels are burnt for cooking, transportation and industrial processes, which releases CO₂ into the air.
3. Respiration: burning plant material releases large amounts of CO₂.

(Respiration also releases CO₂, and it is balanced by the release of oxygen by plants, so it is not harmful for the environment.)

Respiration (listed as the first point in the PDF) is the natural way to release CO₂ by both plants and animals. It is balanced by the release of oxygen by plants. So, it is not harmful for the environment.

What is the greenhouse effect?

Answer: Some gases called greenhouse gases, e.g. carbon dioxide, prevent the escape of heat from the Earth. When the amount of such gases increases beyond their normal levels, the average temperature of the Earth increases. This is called the greenhouse effect.

What are the two forms of oxygen found in the atmosphere?

Answer: The two forms of oxygen found in the atmosphere are:

1. Elemental (molecular) oxygen, found as the diatomic molecule O₂ in the lower part of the atmosphere. It makes up about 21% of the air and is not poisonous.
2. Ozone, found in the stratosphere. It contains three atoms of oxygen (O₃) and is the poisonous form of oxygen.

Oxygen is also found in combined forms: in the Earth’s crust as the oxides of most metals and silicon and as carbonates, sulphates, nitrates and other minerals, and in biological molecules such as carbohydrates, proteins, fats and nucleic acids.

EXERCISE QUESTIONS (PAGES 201–202)

Why is the atmosphere essential for life?

Answer: The atmosphere is essential for life for the following reasons:

1. It keeps the average temperature of the Earth steady during the day and throughout the year.
2. It prevents sudden increases in temperature during the daylight hours.
3. The gases it contains are required to sustain life: oxygen is required for respiration by all living organisms; carbon dioxide is used in photosynthesis by plants to make food; nitrogen forms an inert part of the atmosphere and is an important component of proteins.
4. A layer of ozone in the stratosphere filters the harmful ultraviolet radiation reaching the Earth. UV rays produce harmful effects on all living organisms.

Why is water essential for life?

Answer: Water is essential for life because:

1. It provides the medium for all cellular processes.
2. All the reactions in our body and within cells take place between substances dissolved in water.
3. It is required for the transport of materials from one part of the body to another.
4. It helps to maintain body temperature.
5. Water makes up about 70% of the body weight of living organisms.

How are living organisms dependent on soil? Are organisms that live in water totally independent of soil as a resource?

Answer: Living organisms depend on soil in these ways:

1. It is the natural habitat of various living organisms such as bacteria, fungi, algae and earthworms, which help to maintain the fertility of the soil.
2. Earthworms carry out all their activities in the soil and maintain its fertility by releasing nitrogen-rich excreta.
3. Many animals such as rats and rabbits make their homes in the soil.
4. Soil anchors the roots of plants, and the nutrients in soil are absorbed by plants for their growth and development.

Organisms that live in water are not totally independent of soil. The mineral nutrients in water are in dissolved form, and their recycling depends on the decomposers present in the soil beds. All water bodies have soil (sediment) beds containing decomposers which recycle the nutrients, so aquatic organisms also depend on soil indirectly.

You have seen weather reports on television and in newspapers. How do you think we are able to predict the weather?

Answer: Meteorologists collect information about the pattern of temperature, speed of wind, air pressure and all other features that influence weather. This information is collected by remote-sensing and weather-forecasting satellites and by weather stations. It is compiled in meteorological departments, which prepare the weather report displayed on maps. The report is then transmitted through radio, television and newspapers.

We know that many human activities lead to increasing levels of pollution of the air, water bodies and soil. Do you think that isolating these activities to specific and limited areas would help in reducing pollution?

Answer: Isolating human activities to specific and limited areas would definitely help to reduce pollution to some extent. For example:

1. If sewage and garbage generated by homes and industries are treated properly before being discharged into water sources, water pollution will be reduced and less harm will be caused to aquatic life.
2. If hot water from industries is collected at a common place, allowed to cool and then discharged into water bodies, it will not affect the breeding of aquatic organisms.
3. If commercial areas, factories and industries are shifted to isolated areas far away from residential areas, the effect of air pollution on people will be reduced.

Write a note on how forests influence the quality of our air, soil and water resources.

Answer: Forests influence the quality of our air, soil and water resources as follows:

1. Air: (a) forests help to maintain the balance of oxygen and carbon dioxide in the air; they reduce the level of CO₂ and so help to prevent the greenhouse effect; (b) they help to maintain the temperature of the environment; (c) they increase the rate of photosynthesis in the surrounding region.
2. Soil: (a) trees spread their roots deep into the earth and bind the soil particles firmly, which reduces soil erosion; (b) forests help to maintain the nutrient cycles (biogeochemical cycles).
3. Water: (a) trees help to maintain the water cycle; (b) forests conserve water and make it available on the surface of the Earth as water sources.

Please click the link below to download CBSE Class 9 Science Natural Resources Notes.