Chapter-14, NCERT (CBSE), Science (Biology)
Q.1: How is our atmosphere different from the atmosphere on Venus and Mars?
Ans: On the planets Venus and Mars carbon dioxide forms the major component constituting up to 95-97% of the atmosphere. On the contrary the major component of the earth’s atmosphere is air which consists of nitrogen, oxygen, carbon dioxide, argon, water vapour and trace components. Moreover, unlike Venus and Mars earth has life on it.
Q.2: How does the atmosphere act as a blanket?
Ans: The mass or body of gases that surrounds the earth or any heavenly body is called atmosphere. Earth’s atmosphere extends from the surface to a height of more than 1000 km, beyond which it merges gradually with solar atmosphere. Air is a bad conductor of heat and so, earth’s atmosphere acts as a protective blanket. It keeps the average temperature of the earth fairly steady during the day and the whole year. The atmosphere prevents the sudden increase in temperature during daytime and also helps in retaining heat of the earth by slowing down the escape of heat during night.
Q.3: What causes winds?
Ans: The movement of air from one region to another creates winds. During the daytime, the air above the land gets heated faster and starts rising above creating a region of low pressure below. As a result, the air from the surrounding areas and from the top of the water bodies (oceans, seas) rushes into this region of low pressure and forms the wind.
Q.1: Why do organisms need water?
Ans: Organisms need water because it plays a vital role in the reactions taking place within organism’s cells and body. Water acts as a universal solvent, providing a medium for the chemical reactions to occur. Substances are also transported from one part of the body to the other in the dissolved state. Therefore, it is necessary for the organisms to maintain a certain level of water within their bodies in order to stay alive.
Q.2: What is the major source of fresh water in the city / town / village where you live?
Ans: Major sources of fresh water where we live are underground water.
Q.3: Do you know of any activity which may be polluting this water source?
Ans: The dissolved fertilizers, chemicals, pesticides percolated with rain water from the fields, wastes from industries, breeding of decomposers (micro-organisms) in water are the major sources or causes of pollution of underground water.
Q.1: How is soil formed?
Ans: Formation of Soil
Soil is formed from the rocks by undergoing the following two processes: (1) Weathering and (2) Paedogenesis
Breakdown of bigger rocks into fine smaller mineral particles is called Weathering. Weathering occurs by following three means:
Physical Weathering - This is done by various climatic factors such as temperature, wind, rain water, ice, snow, glaciers and running waters. Water and High temperature cause corrosive humidity and bring about unequal expansion and contraction of rocks, facilitating their breakdown. Rock-pulverizing glaciers, low temperature and water grind the rocks. The freezing water expands in rock crevices and breaks the rocks. Wind action also causes the weathering of rocks. River water causes erosion, weathering and transportation. In this process the bigger rocks are broken into smaller particles. The roots of the plants also have a role in weathering process. They penetrate into the cracks of the rocks and enhance rock-breaking process.
Chemical Weathering - It involves a variety of chemical processes such as hydrolysis, hydration, oxidation and reduction. Chemical Weathering, for example, involves the breaking down of complex compounds by the carbonic acid present in the water and by acidic substances derived from the decomposition process of organic matter in soil. The main end-products of the Chemical Weathering are silica, clay, inorganic salts and hydrated oxides.
Biological Weathering - Lichens, bryophytes (mosses) and other plants live on rocks and produce acids, which accelerate the process of rock weathering.
(2) Paedogenesis (Soil Development)
This process involves the Decomposition process by bacteria and fungi by which organic materials are broken down and leads to Humification and Mineralization. Thus, addition of organic matter (humus) from dead and decomposed plants and animals is the final stage in the formation of soil. A mature soil has minerals, stored energy in the form of organic matter (such as starch, sugars, cellulose, lipids, and proteins, oxides of nitrogen, water and air).
Q.2: What is soil erosion?
Ans: The top most layer of the soil that contains humus and living organisms in addition to the minerals is called Topsoil. The removal and transportation of topsoil from its original place to another place with the help of certain agents such as strong winds, running water etc. is called Soil Erosion.
Q.3: What are the methods of preventing or reducing soil erosion?
Ans: The following are some of the methods which can help in preventing or reducing soil erosion:
· To maintain vegetation cover on the land (soil) or forbidding cutting of trees.
· Avoid overgrazing.
· By afforestation by planting xerophytes, sowing grasses, other plants etc.
· Intensive cropping and maintaining soil fertility.
· Terrace farming.
· Constructing proper drainage canals around the fields.
· Making strong embankments along with riverbanks.
Q.1: What are the different states in which water is found during the water cycle?
Ans: Water is found during the water cycle in three states, namely solid (ice or snow), liquid and vapour.
Q.2: Name two biologically important compounds that contain both oxygen and nitrogen.
Ans: Proteins and Nucleic acids (DNA and RNA)
Q.3: List any three human activities which would lead to an increase in the carbon dioxide content of air.
Ans: The following activities of human beings may increase carbon dioxide content of air:
a) Burning of fossil fuels such as patrol, diesel and coal in various activities.
b) Burning associated with agricultural practices like burning of wood and charcoal.
c) Deforestation that reduces green plant and tree population, consequently utilization of atmospheric carbon dioxide during photosynthesis.
Q.4: What is greenhouse effect?
Ans: A part of the sunlight incident on the earth is reflected back in the form of infrared light. Some gases such as carbon dioxide, methane, chlorofluorocarbons (CFCs), also known as greenhouse gases, present in the earth’s atmosphere prevent the escape of heat from the earth by absorbing this infrared light which is reflected back by the earth. An increase in the percentage of this greenhouse gases in the atmosphere would cause an increase in the average temperature world-wide and this phenomena is known ‘Greenhouse Effect’.
Thus, the heating of the atmosphere due to the absorption of infrared radiations by carbon dioxide molecules and other greenhouse gases is called the ‘Greenhouse Effect’.
Q.5: What are the two forms of oxygen found in the atmosphere?
Ans: Oxygen is found in the atmosphere in the form of
(i) A diatomic molecule having two atoms of oxygen, with chemical formula O2.
(ii) A tri-atomic molecule having three atoms of oxygen, with chemical formula O3 called Ozone.
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