Natural Resources Class 9 Notes Science

CHAPTER NOTES

Introduction :

The materials present in natural environment and useful to living organisms are called natural resources.

A resource satisfies human wants. Water, air, sunshine, land, soil, forests, wildlife, fishes, minerals and power resources all are useful to man. All basic needs of food, shelter and clothing are supplied by natural resources on earth. Natural resource includes total natural environment, that is the entire surface layer of earth, because all parts of earth's surface are of some use to man in they contribute to the production of necessities and comforts of mankind.

The word resource is use for "means of supplying a material generally held in reserve".

Natural Resources

• Substances used by man in bulk for survival are called resources and since they are found in nature, they are called natural resources.
• The resources available on earth include land [Lithosphere], water [Hydrosphere] and Air [Atmosphere].
• Probably, the earth is the only planet on which life exists.
• Life exists on earth because it has all the physical conditions necessary for sustaining it. An ambient temperature, water and food are the basic need of all life forms. These basic needs of the life forms are to be met by the resources available on the earth and the energy from the sun.
• The outer crust of the earth is called lithosphere.
• The water present on the earth's surface and underground constitute hydrosphere.
• The air that covers the whole of the earth like a blanket is called atmosphere.
• About 75% of the earth's surface is covered with water.
• The life sustaining zone of the earth where the lithosphere, the hydrosphere and the atmosphere interact and make life possible is called the biosphere.

Differences between inexhaustible and exhaustible resources

Inexhaustible 

1. These resources have no chance of getting exhausted.

2. These resources are unlimited e.g. wind energy, solar energy,hydropower, tidal energy etc.

Exhaustible

1. These resources have every chance of getting exhausted.

2. These resources are limited, e.g. coal, petroleum, forests etc.

Differences between renewable and non-renewable resources

Renewable resources 

1. Exhaustible resources which can be regenerated or recycled within reasonable time.

2. Can be made to last indefinitely if used judiciously.

3. Availability can be enhanced by increasing their capability of regeneration extraction.

Examples : Ground water, Wildlife, Grasslands, Forests

Non-renewable resources

1. Exhaustible resources which cannot be regenerated or recycled in reasonable time.

2. Will get exhausted whether or not used judiciously.

3. Availability can be enhanced by increased but it will cause early depletion

Examples : Minerals, Fossil fuels like coal & Soil fertility and petroleum.

The Breath of Life : Air

• Air is a mixture of many gases like nitrogen, oxygen, carbon dioxide, water vapour and some others present in the atmosphere.
• Air is an important inexhaustible natural resource.
• Nitrogen and oxygen are the major components of the air.

N₂ - 78.9%             O₂ - 21%                Other gases - 1%

Venus and Mars -

CO₂   95_97% of the atmosphere. That's why no life on these planets.

CO₂ in Earth atmosphere - 0.03%

Small amount of water vapours, dust, salts, smoke is also found.

Importance of air/atmosphere

• It is essential for the survival of life. It supplies oxygen for respiration of all living organisms and carbon dioxide for the photosynthesis in green plants.
• Air (oxygen) is essential for burning and making fire.
• Air acts as a medium for locomotion of flying animals : insects, birds, bats etc.
• Air maintains the temperature of the earth.
• Air helps in the dispersal of spores, pollen grains, seeds and fruits.

The Role of the Atmosphere in the Climate control

• The atmosphere covers the earth like blanket.
• Since, the air is a bad-conductor of heat, the atmosphere keeps the average temperature of the earth fairly constant during the day and also during the course of the whole year.
• The atmosphere acts as a buffer.
• The day and night temperatures show a great difference where atmosphere is absent.

Moon temperature range  -190° C to 110°C.

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Activity

To show the possible directions in which hot and cold air move.

Take a candle. Fix it in a beaker and light it as in figure.

Take one incense stick. Light it and bring it to the mouth of beaker.

Note the result with following actions :

(i) Which way smoke flows, when incense stick is brought near the margin of beaker's mouth ?

(ii) Tell the direction of smoke when incense stick is brought little above the burning candle.

(iii) Which way smoke flows when incense stick is taken to other areas

Give the possible reasons.

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Movement of air in coastal regions

• In coastal regions, the air above the land gets heated faster than the air over the sea.
• The heated air over the land rises up and creates a region of low pressure.
• The movement of air from one region to the other creates winds. Thus, the direction of the wind would be from the sea to the land during the day time.
• After the sunset, both land and sea start to cool. The water cools slower than the land. Consequently, the air above water would be warmer than the air above the land. This, would cause blowing of winds from land to the sea.

Air Pollution

The present day industrial growth has polluted air to a greater extent by releasing SO₂, CO₂, CO, oxides of nitrogen, H₂S fumes of acids, dust particles of unburnt carbon, lead asbestos and even cement. For example, the burning of coal and oil to generate electric power, run factories and fuel automobile engines creates oxides of nitrogen and sulphur that acidify the rain.

Air carries many undesirable substances or impurities which are not good for our health. The chief constiuents of the impurities of air include (i) carbon dioxide, (ii) carbon monoxide, (iii) oxides of sulphur, (iv) oxides of nitrogen, (v) fluoride compounds, (vi) metals (e.g. lead nickel, arsenic, cadmum, tin etc.) (vii) hydrocarbons (e.g., benzene), (viii) particulate matter (dust, grit, fly ash) and (ix) toxicants. All these impurities are called pollutants. They cause air pollution.

Definition : The presence of harmful gases and suspended particles in the air which have adverse effect on human beings, animals and vegetation is called air pollution.

Consequences of Air Pollution

Air pollutants can cause respiratory problems, renal problems, high blood pressure, problems in nervous system, eye irritation etc. in the human beings. Many injurious effects such as falling of leaves, reduced growth, degeneration of chlorophyll etc. have been reported in plants. Lichens are found to be very sensitive to the levels of contaminants such as sulphur dioxide present in polluted air.

Acid Rain

Literally acid rain means the presence of excessive acid in rain water : Acid rain is infact cocktail of mainly H₂SO₄ and HNO₃.

H₂SO₄ is the major contributor (6070%) to acid precipitation HNO₃ ranks second (3040%) and HCl third.

Causes of Air Pollution

(i) Increase in human population and rapid industrialization.

(ii) Burning of fossil fuels.

(iii) Oxides of nitrogen and sulphur inhalation adversely affect human health and also causing acid rains.

(iv) During cold weather water vapour get condensed on the suspended particle [Unburnt carbon particles and hydrocarbons] resulting in smog formation.

Water : A Wonder Liquid

Water is a basic human need.

About 3/4 of the earth surface is occupied by water.

Most of the water on earth's surface (about 97.5%) is found in seas and oceans. It is strongly alkaline.

The rest (2.5%) is fresh water.

Importance of water to the living organisms - No life can exists without water because of :-

(i) All cellular processes take place in a water medium.

(ii) Water is essential for the process of digestion.

(iii) Water helps in maintaining body temperatue.

(iv) All the biochemical reactions that take place within our body and within the cells occur between the substances that are dissolved in water.

(v) Substances are also transported from one part of the body to the other in dissolved form. Hence, organisms need to maintain the level of water within their bodies in order to stay alive.

(vi) Water is also required for cooking, cleaning, irrigation, in industries and generating electricity.

Terrestrial life-forms require fresh water because their bodies cannot tolerate or get rid of the high amounts of dissolved salts in saline water.

Therefore, water sources must be easily accessible for animals and plants to survive on land.

Water Pollution

The addition of undesirable substances or removal of desirable substances in/from the water bodies are a change in the temperature of water, which degrades the quality of water so that it either becomes health hazard or unfit for use is called water pollution.

Types of water pollutants

(1) Physical water pollutants

These include heat and oil-spills. Specific industries and thermal / nuclear power plants use water for cooling in various operations and later return this hot water to water bodies. This result in thermal pollution. Another manner in which the temperature of the water in river can be affected is when water is released from dams. The water inside the deep reservoir would be colder than the water at the surface which gets heated by the sun. High temperature of water reduces its dissolved oxygen content.

(2) Chemical water pollutants

These include organic wastes e.g. sewage, detergents, fertilizers, pesticides (e.g. dieldrin, DDT, DDE, BHC, etc.), [polychloronated biphenyls [PCBs], inorganic chemicals (e.g. arsenic, cadmium, mercury, lead, phosphates, nitrates, fluorides etc.)] and radioactive wastes. Common inorganic impurities in water are compounds of calcium and magnesium.

(3) Biological water pollutants

These include pathogens such as viruses, bacteria, protozoa, fungi, helminths & nematodes etc.

Point and Non-point sources of water pollution

(1) Point sources

These sources have a specific location for the discharge of water pollutants directly into water bodies. Therefore, these sources are located near the water bodies i.e. factories, power plants, breweries, underground coal mines and oil wells. It is always easy to treat the pollution coming out of point sources before their discharge into water bodies.

(2) Non-point sources

These pollution sources are scattered and do not have any specific location for discharging pollutants into particular water bodies. Example of non-point sources are run-off from fields, lawns and gardens, road and streets, construction sites, logging areas etc. Unlike point sources, these sources are difficult to monitor and treat to remove the pollutants.

The term water pollution is used to cover the following effects in water :-

(i) Addition of undesirable substances to water bodies.

Like fertilizers and pesticides used in agriculture or poisonous substances

(ii) Removal of desirable substances from water bodies. i.e. dissolved oxygen.

(iii) Change in temperature.

Eutrophication :- Eutrophication is the process in which dissolved oxygen in water is reduced due to excessive growth of algae as a result of extra loading of nutrients in the water body. In fact, presence of sewage and fertilizers (nitrates and phosphates) in polluted water provide a lot of nutrients to the algae (phytoplankton) present in water body. As a result, excessive growth of algae occurs which is termedalgal bloom. The algae subesquently die and aerobic decomposers become active. They consume rapidly the dissolved oxygen of the water during decomposition of the dead algae. In the absence of dissolved oxygen, all the aquatic life (including fishes) in the water body dies. Thus, eutrophication deprives us of one of our significant sources of food.

Biomagnification (bioaccumulation) :- Pesticides such as DDT, DDE, dieldrin, etc. are harmful chemicals that are regularly being used in agriculture to protect crops from the pests (fungi, insects, etc.). Also heavy metals (e.g. mercury, cadmium, tin, lead, arsenic, etc.) are regularly poured from industries into the water bodies. These chemicals are non-biodegradable, i.e. unlike the organic sewage wastes they are not broken down by the activity of microorganism. These pesticides / heavy metals, therefore, enter the soil or washed out by run off water to surface water bodies such as lakes, streams, rivers. From soil or water thesepesticides / heavy metals then enter the food chain via the producers. At each trophic level, their concentration goes on increasing. This phenomenon of increase in concentration of harmful non-biodegradable chemical substances in the body of living organisms at each trophic level of the food chain is called bioaccumulation, biological accumulation or biomagnification. This makes it more likely that predators such as fish-eating birds such as kingfishers become poisoned. For example, biomagnification of mercury in the fishes through a food chain results in Minimata disease in fish-eating human populations of the regions.

Soil :

Definition :- Soil is a portion of earth's a surface consisting of disintegrated rock and decaying organic material. It provides the support for many plants and animals. Thickness of soil on the earth's surface ranges from a few millimeters to 34 meters. Terrestrial plants depends for their nutrients, water supply and anchorage upon the soil. Even for the aquatic plants, the solid is important as chief storage of all the nutrients which are made available to the water medium.

Mineral Riches in the soil :

Our planet earth has three distinct regions innermost core region (about 2200 miles in thickness), middle mantle region (about 1800 miles in thickness) and theoutermost crust region (about 2025 miles in thickness).The outermost crust region has huge rocks containing variety of bound minerals, some of which become available to us.

About one-fifth of the surface of the earth is exposed solid and distinct from oceans, lakes etc. It is called the land. The top surface layer of this exposed, solid part of crust capable of supporting plant growth is called soil. Over millions of years of long periods of time, the rocks at or near the surface of the earth are broken down by various physical, chemical and some biological processes to form fine soil particles. Soil is a dynamic layer in which many complex physical, chemical and biological activities are going on constantly. It is and important resource that decides the diversity of life in an area.

The soil is a complex mixture. It consists of five components :

(i) Mineral matter = 45%

(ii) Organic matter = 5%

(iii) Water = 25%

(iv) Air = 25%

(v) Living organisms.

All these components are essential for proper plant growth. 

Soil profile :

Soil profile shows four distinct layers, called horizons. Horizon A is the topsoil. It is darker and of a looser texture than the underlying horizon B. Plant and animal matter collects at the surface of this horizon, forming the litter. Below the litter is the humus, i.e., organic matter undergoing decay by microbial action. The rest of horizon is rich in organic and mineral contents. The horizon B has soil particles smaller and usually more compacted than in the horizon C. Minerals brought by rain water from the upper horizon accumulate in this horizon. The horizon C consists of the weathered material derived from the intact parent rock. The parent rock forms the horizon D.

Humus is partially decayed organic matter. It makes the soil porous, there by increasing its air and water holding capacity. Humus is rich in nutrients that promote plant growth. Being black, it absorbs heat to warm up the soil.

Composition of top soil

Soil particles differ in their size, look and texture. Mainly four types of soil particles are commonly found in top soil. These are :

(i) Gravels :- These are large particles having size of greater than 2 mm in diameter. These can be easily picked up by hands.

(ii) Sand particles :- These are still smaller in size ranging between 0.005 mm to 2 mm. These are rough to touch and can be seen with naked eye.

(iii) Silt particles :- These are still smaller in size ranging between 0.005 mm to 0.05 mm in diameter.

(iv) Clay particles :- These are the smallest soil particles having size less than 0.005 mm.
 

Types of soils

Depending upon the presence of relative amounts of soil particles, soils are classified into following three types:

1. Sanday soil :- It contains very large proportion of sand particles and very small quantity of silt and clay. Since sand particles are relatively larger in size, this soil can not hold much water. It is found in deserts and is unfit for plant growth.

2. Clayey soil :- It contains large proportion of clay particles (40% or more) and small amounts of humus and silt. Clayey soil, being compact, can hold water. However, it is poorly aerated as it can not trap air. It is also not suitable for plant growth.

3. Loamy soil :- Loamy soil contains relatively larger quantities of clay, silt, sand particles and humus. In fact, it contains about one part clay, two parts silt and two parts sand.

Therefore, it is porous has very good water holding capacity and also allows aeration of roots.

Major types of soils in India

The soil is classified on the basis of its composition and nature. The major types of soils found in our country and their composition are presented in table.

Soil pollution

The contamination of soil (or land) with solid waste, chemicals (through industrial wastes or acid rain), fertilizers and pesticides, reducing its fertility is called soil pollution (or land pollution).

Soil pollution : Soil pollution can be defined as decrease in soil fertility because of addition of some foreign elements. Soil is polluted with dumping of solid wastes generated in house hold and manufacturing units. Domestic wastes include kitchen garbage, broken bottles, cloth rags, ash, etc. Industrial wastes include flyash, metal scraps, dyes, plastics, etc. Agricultural chemical and fertilizers are also the cause of land pollution. The dumping of human excreta and waste from cow-sheds and slaughter houses befouls the land. Most important causative pollutant of woil isplastics.

Source of Soil Pollution

The main sources of soil pollution include :

1. Solid wastes

2. Chemical (directly through industrial wastes or indirectly through acid rain)

3. Excess of fertilizers and pesticides

1. Solid wastes :- These are considered the main source of soil pollution. Solid waste generally comes from residences, cattle sheds, industries, agricultural fields, and many other places. It includes peelings of fruits and vegetables, cow dung, human excreta, ash, paper, glass, plastics, leather and rubber articles, brick, sand worn out clothes, and metal objects. These heaps of solid waste make the surroundings dirty,and pollute the soil.

2. Chemicals :- Industrial wastes are generally dumped in vacant sites along the roads, railway tracks orelsewhere. These industrial wastes contain a lot of chemicals that pollute the soil. Chemicals discharged into the air in the form of fumes such as compounds of sulphur and lead, as well as gases (e.g., SO₂ and NO_X) eventually come down and settle as dry deposition or as acid rain on the soil and pollute it.

3. Excess of fertilizers and pesticides :- Fertilizers are used in the agricultural fields to increase the crop production. Also, different kinds of chemicals (pesticides) are sprayed on the crops to kill the pests, weeds, etc. All these chemicals, when used in excess, get mixed with soil and pollute it. From the soil, many non-biodegradable chemicals (e.g. DDT) even enter the food chains and biomagnify.

Effects of soil pollution

1. The industrial pollutants increase the toxicity levels of the soil.

2. Soil pollution due to domestic sewage may cause diseases like giardiasis, tetanus, etc. in human beings.

3. Land pollution may also cause several plant diseases.

4. Weedicides act as metabolic inhibitors or reduce the plant yield.

5. Mine dust causes many types of deformities in animals and human beings. It also destroys the vegetation of that area.

6. Excess of fluorides in land cause fluorosis.

Soil Erosion

Soil is a complex mixture of non-living materials and living organisms. It provides anchorage (firm support) to plants, and is also a source of nutrients and water to the plants. Majority of the plants, thus, grow in the soil. The top layer of soil (commonly called top soil) is very fertile. It is often carried away by environmental agencies i.e., strong winds and fast flowing water.

Cazri : is actively engaged in research to suggest measures for controlling wind erosion.

The removal and transportation of top layer of soil from its original position to another place with the help of certain agents such as strong winds and fast running rain water, is called soil erosion.

Soil erosion normally occurs in bare areas i.e. areas without plant cover. It is so because the bare top-soil is loose and thus can easily be carried away by strong winds or fast moving water of heavy rains or rivers.

Causes of soil erosion

1. Strong winds 

2. Heavy rains 

3. Improper farming and suspended cultivation

4. Human actions 

5. Dust storms

Effects of Soil Erosion

1. Loss of fertility and desertification 

2. Landslides in hilly areas

3. Flash floods 

4. Famines

Prevention of Soil Erosion

1. Intensive cropping 

2. Sowing grasses and planting xerophytes

3. Terrace farming 

4. Proper drainage canals around the field

5. Making strong embankments along the river banks

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Activity

To show the effect of vegetative cover on ground and soil errosion.

• Take two trays 'A', 'B' of same size and fill them with soil.
• Plant seeds of gram or paddy in tray 'A'.
• Regular pour water in both the trays.
• Seedings will appear in tray 'A' and shows coverage of plant growth.
• Place both the trays at tilted position.
• Pour equal amount of water in both the trays.
• Observe the amount of soil emitted out of trays.
• Repeat the process 35 times.
• Now find out the quantity of soil, carried out of trays.

Now answer the following :

(i) How much soil is carried out of trays i.e. 'A' and 'B' ?

(a) When water is poured gently ? Give the possible reasons.

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Bio-Geochemical Cycle

Both non-living (abiotic) and living (biotic) components of the biosphere constantly interact with each other to form a dynamic, but stable system. The intersection include transfer of matter and energy between the different components of the biosphere.

As far as nutrients are concerned, all living organisms require eight elements as nutrients in relatively larger amounts. These include carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium and calcium. The living organisms get the nutrient elements from lithosphere (mainly soil), hydrosphere and atmosphere.

Bio-refers to living, geo-refers to the rocks, soil, air and water of the earth.

The cyclic flow of elements or compounds between non-living environment [Soil, rock, air, water] and living organisms is known as 'bio-geochemical cycle.

Water, oxygen, carbon and nitrogen cycles are very important in maintaining the balance in our environment

1. Water Cycle

(i) Water is a compound and include two vital elements

hydrogen and oxygen. It forms a very important component of the environment and survival of all living organisms depend on it.

(ii) Ocean is the biggest store house of water which on evaporation form clouds and which after condensation fall down as rain or snow.

(iii) After rain it passes through puddles, ponds and rivers and get collected again in the ocean. The circulation of water in this manner is known as hydrologic cycle or water cycle.

This cycle is driven by solar power and also performed through living organisms such as absorption and transpiration of water by plants and drinking by animals. The formation of soil, too depend on water.

2. Nitrogen Cycle

(i) Nitrogen is an important chemical on the earth and present in all the living organisms in the form of protein, amino acids and nucleic acid.

(ii) In atmosphere it exists in molecular form (N₂) and in form of some oxides [N₂O, NO, NO₂ , NO₃^_ ]. Nitrogen is the most abundant component of air (78 percent). Atmospheric nitrogen directly cannot be used by living organisms.

(iii) During lightning nitrogen of atmosphere reacts with oxygen and ultimately form dilute nitric acid. This acid comes down to earth with rainwater. Nitrates are absorbed by plants and utilized for making organic matter (proteins), etc.

(iv) When animals consume plant matter, they break down the plant's nitrogenous compounds and use them to form new animal proteins and other cell components.

Biological Nitrogen fixation

Nitrogen fixation [conversion of atmospheric nitrogen gas into N₂-compounds]

(i) Rhizobium [in root nodules of leguminous plants]

(ii) Azotobacter [in soil]

(iii) Blue Green Algae

Ammonification [conversion of nitrogen containing proteins of dead plants and animals into ammonia]

(i) Putrefying bacteria

(ii) Fungi

Nitrification [conversion of ammonia into nitrites and then into nitrates]

(i) Nitrosomonas bacteria - Convert NH₃ into nitrites ()

(ii) Nitrobacter bacteria - Convert nitrites into nitrates ()

Denitrification [Conversion of nitrate salts into free nitrogen gas] e.g. Pseudomonas.
 

OXYGEN CYCLE

Importance of Oxygen :- Oxygen is also an essential component of biomolecules. It is needed for respiration also.

Main sources :- Oxygen is availiable in molecular form (O₂) in the air, forming about 21% of it. Some oxygen is found dissolved in water. Oxygen also occurs as a component of water and carbon dioxide.

Use and Release :- The oxygen of the atmosphere is in a state of dynamic equilibrium. It is taken by animals and plants from the air or as dissolved in water for use in oxidative reactions (respiration). It is returned to the environment, either in combination with carbon as carbon dioxide or with hydrogen as water. The carbon dioxide and water are used by plants in photosynthesis, which liberates molecular oxygen into the envirnoment for reuse in respiration. Thus, the cycle is completed.

The concentrations of oxygen in the air and water are maintained by equal rates of its use in respiration and release in photosynthesis.

Oxygen is also released as a part of CO₂ by decay of dead organic matter.

Some oxygen is added to the air as CO₂, H₂O, sulphur dioxide and nitrogen oxides during burning of fuel (wood, coal, petroleum and natural gas).

Some oxides are formed by microbial oxidation. These oxides release O₂ when reduced by chemical and biological processes.

Effect of Human Activity :- Oxygen was not present when the earth was formed. It was added to the atmosphere later when photosynthesis started with the evolution of photoautotrophs. Human activity has not affected the oxygen content of the atmosphere because it is replenished by photosynthesis.

Oxygen is essential element required for respiration. It forms about 21% of the air in the atmosphere.

CARBON CYCLE

The cyclic flow of carbon between non-living envirnoment and living organisms of biosphere is called carbon cycle in nature.

In abiotic envirnoment, carbon is present in the following forms :

• as carbon dioxide in the air or atmosphere.
• as dissolved carbon dioxide or carbonic acid and bicarbonates in water bodies or hydrosphere.
• as fossil furels, like coal, petroleum and natural gas, and
• as carbontates and graphite in rocks.

In biotic environment, Carbon forms the backbone of complex organic molecules like carbohydrates, lipids, proteins, nucleic acids, enzymes, hormones etc.

The availability of carbon in the environment is, therefore, a crucial factor in the maintenance of living beings. It is essential that the carbon 'locked up' within the organisms be returned to the environment for reuse.

Then main reservoir of carbon is the atmosphere.

Processes by which carbon dioxide of atmosphere is consumed :-

The plants use carbon dioxide as one of the raw materials for the process of photosynthesis and prepare carbohydrates.

When animals feed on the plant products, plant carbohydrates change into animal carbohydrates.

Some of the dead plants and animals get buried deep under the earth and change into fossil fuels (coal and petroleum) through slow chemical change.

Some of the dissolved carbon dioxide in oceans and other water bodies gets converted into limestone and other carbonate rocks.

Processes by which carbon is released from biotic world to abiotic world

Both plants and animals release carbon dioxide in the atmosphere as a product of respiration.

When plants and animals die, their bodies are decomposed by decomposers and carbon dioxide is released into the atmosphere.

Combustion of fossil fuels like coal, petroleum gas, kerosene, petrol, diesel, etc gives carbon dioxide which goes into atmosphere.

Weathering of carbonate rocks by the action of microoganisms or when acid rain falls on these rocks liberates carbon dioxide.

Volcanic eruptions and hot springs also release carbon dioxide into the atmosphere.

Thus, there is continuous exchange of carbon dioxide between living and non-living world.

Greenhouse effect :

In greenhouse, the heat is trapped by glass, hence the temperature in a glass enclosure is much higher than the surroundings. Due to higher temperature in glasshouses, the tropical plants can be kept warm and protected from cold temperature during winter.

Some gases, like carbon dioxide, methane and nitrous oxides also prevent the escape of heat from the earth. With industrialization, widespread deforestation and burning of more fossil fuels, the concentration of carbon dioxide in the air is increasing with an alarming rate. Carbon dioxide gas traps heat within the atmosphere preventing its escape into the space.

Therefore increased CO₂ concentration in air is the major cause of rise in atmospheric temperature. It causes melting of polar ice. It also causes submerging of coastal land under water.

CFC is the major compound responsible for ozone depletion.

Green House effect and global warming :-

Earth temperature is maintained by re-radiated infra-red (heat) radiation by greenhouse gases which prevent heat from escaping to outer space. This is comparable to glass panels of a greenhouse which keep CO₂ concentration higher and so higher temperature inside i.e. greenhouse. This effect is called greenhouse effect.

Green house gases - CO₂, CH₄, NOx. They prevent the escape of heat from the earth.

Increased CO₂ concentration in air is the major cause of rise in atmospheric temperature. It causes melting of polar ice. It also causes submerging of coastal land under water.

Due to higher temperature in glasshouses, the tropical plants can be kept warm and protected from cold temperature during winter.

Depletion of Ozone layer :

Ozone Gas :

• Ozone is poisonous in nature.
• Ozone is formed in atmosphere by the action of ultraviolet radiation on oxygen gas.
• The high energy ultraviolet radiation (UV radiation) coming from the sun splits oxygen gas into free oxygen atoms

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• The free oxygen atoms are highly reactive. One oxygen atom reacts with an oxygen molecule to form an ozone molecule.

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Ozone Layer : It is a layer of the earth's atmosphere ozone is concentrated. The Ozone layer is very important for the existence of life on earth because it absorbs most of the harmful ultraviolet radiation coming from the sun and prevents them from reaching the earth.

The thining of ozone layer is commonly called ozone depletion. Ozone is being depleted by air pollutants. Chlorofluorocarbons (CFCs) are air pollutants that are mainly responsible for the depletion of ozone layer in the stratosphere. Besides, methane (CH₄) and oxides of nitrogen (NO_x) also cause destruction of ozone.

Ozone hole : Decline in thickness of ozone layer a restricted area is called ozone hole. Ozone hole was first discovered over Antarctica in 1985. Amount of atmospheric ozone is measure by Dobson spectrometer and is expressed in Dobson units (DU).

Harmful effects

• Skin cancer and skin disorder.
• Cornea layer will slowly become opaque which will cause slow blindness.
• Change at genetic level may lead to genetic disorder.
• Ultraviolet rays weaken the immune system.
• An excess of ultraviolet light kills phytoplankton, the minute aquatic plant life, resulting in the disturbance of the food chain.
• These rays also disrupt global rainfall and can cause ecological imbalance.