CBSE Class 9 Science Natural Resources Exam Notes

This Class 9 Science exam notes page on Natural Resources covers air and its role in climate control, air pollution, water as a wonder liquid and water pollution, the formation of soil and its components, and the biogeochemical cycles: the water cycle, the carbon cycle and the nitrogen cycle.

Topics Covered

1. Natural resources
2. The breath of life: Air and air pollution
3. Water: a wonder liquid and water pollution
4. Soil and soil formation
5. Biogeochemical cycles: water cycle, carbon cycle and nitrogen cycle

NATURAL RESOURCES

The materials present in the natural environment and useful to living organisms are called natural resources. Natural resources can be classified into two groups:
Physical resources: for example, air, water, soil, minerals, coal etc.
Biological resources: for example, microorganisms, plants and animals.

THE BREATH OF LIFE: AIR

Air is a mixture of gases which is odourless, tasteless and invisible. Air also holds water vapour and dust particles.

Atmosphere
The envelope of air that surrounds the earth is called the atmosphere.

Role of atmosphere in climate control

Air is a bad conductor of heat. It acts as a protective blanket for living organisms to exist in the following ways:
The atmosphere prevents a sudden increase in temperature during the day. During night, it slows down the escape of heat into outer space, thereby preventing excessive cooling at night.
The ozone shield of the atmosphere absorbs most of the harmful UV radiations coming from the Sun. Excessive heat and sun rays are reflected back into outer space by dust particles.

Movement of air

Heating of air occurs due to re-radiation of solar radiations by land and water bodies. When solar radiation falls on the earth, some is absorbed and the majority is reflected back or re-radiated by the land and water bodies. These solar radiations heat up the atmosphere from below. As a result, convection currents are set up in the air. Since land gets heated faster than water, the air over land also gets heated faster than the air over water bodies. Hot air on land rises upwards, thereby producing an area of low air pressure. Air from a region of high pressure will move towards this region of low pressure, producing breeze or wind.

Rain

The air carrying water vapour also gets heated. This hot air rises up in the atmosphere carrying water vapour with it. As the air rises, it expands and cools. This cooling causes the water vapour to condense in the form of tiny droplets. Suspended particles of dust and other materials act as nuclei to facilitate the process of condensation of water around them. A collection of tiny droplets of water appears in the form of clouds. These droplets of water slowly grow bigger by the condensation of more water droplets. When the droplets have grown big and heavy, they fall down in the form of rain.

AIR POLLUTION

An undesirable change in the physical, chemical or biological characteristics of the air, making it harmful for living organisms (including man), is termed air pollution. Common pollutants of air are particulate matter and gaseous pollutants.

Particulate matter is also called suspended particulate matter (SPM) because it remains suspended in air for a reasonable period of time. Common examples of SPM are dust, soot (unburnt carbon particles), smoke and fly ash etc.

Gaseous pollutants include carbon monoxide, excess of carbon dioxide, sulphur dioxide, oxides of nitrogen, hydrocarbons, chlorofluorocarbons (CFCs), hydrogen sulphide, methane and ammonia. Besides, asbestos dust, stone dust, cement dust, pollen grains of plants, radioactive rays etc. are other common air pollutants.

Harmful Effects of Air Pollution

Some common harmful effects of air pollution are:
Respiratory problems, e.g., sneezing, allergy, bronchitis, asthma, tuberculosis and lung cancer.
Carbon monoxide poisoning.
Acid rain.
Depletion of the ozone layer.
Global warming (greenhouse effect).
Serious ailments produced by certain metals and pesticides.
Smog.

WATER: A WONDER LIQUID

Water is one of the basic necessities of life. It is an inexhaustible natural resource which is liquid between 0°C and 100°C. We need water for various activities such as drinking, cooking of food, bathing and washing. It is also needed for irrigation of crops in agriculture, as an essential requirement in industries, and for navigation.

Water plays a vital role in the metabolic reactions taking place within the body of an organism. It acts as a universal solvent, providing a medium for chemical reactions to occur. In fact, all the chemical reactions that occur within body cells involve substances that are dissolved in water. Substances are also transported from one part of the body to the other in dissolved form.

97.5% of the water on the planet earth is found in seas and oceans as saline water and is not available to us for direct use. Only 2.5% of the total water resources of the world consist of fresh water. The majority of it (about 2%) is found frozen in the ice-caps at the two poles and on snow-covered mountains. The remaining 0.6% of the total is available to support terrestrial life. The majority (90%) of this fresh water is found underground as ground water, and only 10% occurs as surface water in lakes, ponds, streams, rivers etc. A limited amount of fresh water is available to us as a renewable source through the water cycle, but its distribution is uneven.

WATER POLLUTION

An undesirable change in the physical, biological or chemical qualities of water (due to the addition of foreign organic, inorganic, biological or radioactive substances) that adversely affects aquatic life and makes the water less fit or unfit for use, is called water pollution.

Agents or substances that pollute the water are called water pollutants. These can be classified into three categories:
Physical pollutants: These include heat and oil spills.
Chemical pollutants: These include organic wastes, detergents, pesticides (e.g. DDT, BHC), polychlorinated biphenyls (PCBs), inorganic chemicals (e.g., arsenic, cadmium, mercury, lead, nickel, phosphates, nitrates, fluorides) and radioactive wastes. Common inorganic impurities in water are compounds of calcium and magnesium.
Biological pollutants: These include pathogens such as viruses, bacteria, protozoa, helminths, algae, fungi, etc.

Main sources of water pollution are:

Sewage: It is the filthy matter carried in large underground drains called sewers. It is the main water pollutant in towns and cities. It mainly includes organic wastes.
Industrial wastes: Industrial wastes contain large quantities of harmful chemicals, including acids and alkalies, that are discharged into water bodies. These include both organic and inorganic chemicals.
Synthetic soaps and detergents: Water containing soaps and detergents is called grey water. These pollutants are discharged from houses and certain factories. Their excessive use during washing produces a lot of foam.
Fertilisers and pesticides: Fertilisers and pesticides are used excessively in fields to increase crop production. These are washed by rain water into water bodies and pollute them.
Petroleum oil: Drilling and shipping operations are common in the oceans. Leakage of petroleum oil during such operations or due to accidents results in water pollution.
Heat is another source of water pollution, as high temperature of water reduces its dissolved oxygen content.

SOIL

The top surface layer of the exposed, solid part of the earth's crust, containing weathered minerals and humus and capable of supporting plant growth, is called soil.

Soil formation

The process of soil formation is so slow that soil is regarded as a non-renewable resource.

Pedogenesis
Pedogenesis is the process of formation of soil from the rocky earth's crust. It involves the following two processes:
Weathering
Decomposition of organic matter and subsequent humification and mineralisation.

Main factors that influence the formation of soil from rocks are:
Temperature variations due to radiations of the sun.
Rain water.
Winds.
Living organisms.

Weathering
Weathering is the process of breaking down of rocks into small, fine mineral particles. It may occur due to physical, chemical or biological means.

Physical weathering involves pulverisation of rocky matter caused by physical (climatic changes such as heating, cooling, wetting-drying, frost action) and mechanical forces (abrasion by rain and hail, rolling stones, wave action, wind action).
Sun: Under the influence of solar radiations, rocks heat up and expand. At night, they cool down and contract. Since all parts of the rocks do not expand and contract at the same rate, cracks appear in the rocks and ultimately the large rocks break up into smaller pieces.
Water: Water influences the formation of soil in two ways. It gets into the cracks in rocks formed due to uneven heating of different parts of the rocks by the sun. On freezing, the water expands in rock crevices and breaks the rocks. Flowing water wears away even hard rocks over long periods of time. Fast flowing water generally carries various-sized particles of rock downstream. On the way, these moving rock particles rub against other rocks. The resultant abrasion forms still smaller particles. The water takes these particles of rock and deposits them down its path. In this way, soil is found in places far away from its parent rock.
Wind: Strong winds influence the formation of soil by continuously rubbing against rocks and eroding them. They also carry sand from one place to another.

Living organisms (biological weathering)
Lichens, mosses (bryophytes) and other plants also influence the formation of soil. Lichens live on rocks and produce acids. These acids corrode the surface of rocks to form a thin layer of soil. Other small plants, e.g., mosses, later grow on such surfaces and cause the rocks to break up further. Roots of trees sometimes enter the cracks and provide anchorage. As the trees grow, the roots also grow bigger and force the cracks to widen. The whole process of weathering of rocks involving living organisms is called biological weathering.

Chemical weathering of rocks involves a number of chemical processes such as hydrolysis, hydration, oxidation and reduction. For instance, complex compounds present in rocks are broken down by the action of carbonic acid present in water, or by acidic substances derived from the decomposition of organic matter in soil. The end products of chemical weathering are silica, hydrated oxides, inorganic salts etc.

Decomposition of Organic Matter and Subsequent Humification and Mineralisation
This involves stepwise degradation of organic materials by bacteria and fungi of decay, and subsequent humification and mineralisation. Dead remains of plants and animals are called detritus. The decomposition involves stepwise degradation of detritus. The process of decomposition of detritus is vital in any ecosystem because it involves the release of nutrients from dead organic matter. Without this process, all the nutrients would remain locked in the dead remains of plants and animals and would not be available for the formation of living matter.

Humification: It is the process by which simplified detritus (partially decomposed organic matter) gets converted into a dark coloured amorphous substance called humus in the soil. Humus is the reservoir of nutrients.
Mineralisation: It is the process which results in the release of inorganic substances such as CO₂, H₂O and nutrients (e.g., NH₄⁺, Ca⁺⁺, Mg⁺⁺, K⁺ etc.) in the soil.

Humus binds the weathered rock particles into aggregates called crumbs. These crumbs absorb water and hold air in the spaces.

Detritivores such as nematodes, earthworms, centipedes, millipedes, mites and ants consume organic matter and add excretory nitrogen to it. It is, thus, the final step in the formation of soil.

BIOGEOCHEMICAL CYCLE

Biogenic elements (macro-, micro- and other elements) flow from the environment into and out of the plant in a cyclic manner. This flow of nutrients from abiotic to biotic components of the ecosystem, and vice versa, constitutes the biogeochemical cycles.

The Water Cycle

[Figure: The Water Cycle flow diagram. Evaporation from Ponds and Lakes, Rivers, Plants, Sea and Animals goes up to Clouds. Clouds give Snow, Hail, Sleet, Rain, which falls to Run off, Ponds and Lakes, Local Rain and the Ocean. Transpiration from Plants goes to Clouds. Evaporation from the Sea and Ocean goes to Clouds. Soil and Gravitational Water lie below the surface.]

Hydrological or water cycle: Water on earth is cycled by two processes, evaporation and precipitation.
The atmospheric precipitation occurs in the form of snow, hail or sleet etc. The run off water is finally collected in the ocean through rivers.
Some water remains solid in the form of snow, which gradually melts and reaches the sea.
Soil water is used by plants, and most of it again reaches the atmosphere through transpiration.
Animals consume water directly from water bodies and also from gravitational water.
By evaporation, the water returns to the atmosphere and the cycle is repeated.

The Carbon Cycle

[Figure: The Carbon Cycle flow diagram. CO₂ in Atmosphere is linked to Producers by Photosynthesis, and to Decomposers by Decomposition. Combustion, Weathering and Volcanic activity return CO₂ to the atmosphere from the Fossil fuel (Peat, Coal, Oil) and Rock box. CO₂ in water is linked to Producers, and Lime Rock is linked to CO₂ in water. Producers pass to Consumers by Chemosynthesis; Consumers pass to Decomposers by Death. Respiration of Decomposers returns CO₂ to the atmosphere. Death and Decay of Producers, Consumers and Decomposers leads to Fossil fuel, Peat, Coal, Oil CO₂, Rock.]

CO₂ is 0.03% in the atmosphere, which is utilised by producers in photosynthesis for making food.
From producers, it goes to consumers and then through decomposers into the atmosphere.
The producers, consumers and decomposers may be converted into fossil fuel (petrol, coal etc.) or form carbonate rock after death.
By way of respiration, the biotic component returns CO₂ to the atmosphere.
CO₂ may get dissolved in water. Lime rocks also contribute to CO₂ in water. The aquatic producers use this CO₂ for photosynthesis and return it by respiration.
By combustion of fossil fuel, and also by volcanic activity, CO₂ is returned to the atmosphere.

The Nitrogen Cycle

[Figure: The Nitrogen Cycle flow diagram. N₂ in Atmosphere is converted to Nitrate in Soil by non-biological N₂ fixation, and to NH₃ in Plants by biological N₂ fixation. Nitrate in Soil leads to Nitrate in Plants, then NH₃ in Plants, Amino acid in Plants, Protein in Plants and Animals. Death of Plants and Animals gives Organic Nitrogenous Compound in Soil. Ammonification converts it to NH₃ in Soil. Nitrification converts NH₃ in Soil to Nitrate and Nitrite in Soil. Denitrification returns Nitrate and Nitrite in Soil to N₂ in Atmosphere. Combustion also returns nitrogen to the atmosphere.]

The atmosphere is the source of N₂, where it is about 79%. Plants cannot use N₂ directly.
In living organisms, nitrogen is an important constituent of protein and nucleic acid.
The N₂ cycle has five important steps:

Nitrogen fixation
Conversion of N₂ gas into its compounds, like nitrates and nitrites, is called N₂ fixation. It is done either non-biologically by lightning or biologically by symbiotic or free-living bacteria. O₂ is harmful for N₂-fixing bacteria.

Assimilation of nitrogen
N₂ cannot be used by plants directly. They absorb it in the form of nitrate. Nitrate is later reduced to ammonia, which provides the amino (-NH₂) group. It is an important part of proteins.

Ammonification
Dead plant and animal protein and their waste like urea and uric acid are converted to ammonia by some ammonifying bacteria in soil, e.g., Bacillus mycoides, B. vulgaris and B. ramosus etc.

Nitrification
Ammonia is converted into nitrite by Nitrosomonas bacteria, and Nitrobacter converts nitrite into nitrate. This nitrate can again be absorbed by plants and thus cycled back.

Denitrification
Some denitrifying bacteria, like Pseudomonas, reduce nitrate into nitrogen gas in soil. This gas goes back to the environment.

Main sources of water pollution are - 

• Weathering 

• Decomposition of organic matter and subsequent humification and mineralization.

• Decomposition of Organic Matter and Subsequent Humification and Mineralization :