Question 1.
How is our atmosphere different from the atmosphere on Venus and Mars ?
Solution:
Atmosphere on Venus and Mars have high percentage (approx. 95%) of C0₂ which does not support life, while atmosphere of our earth consists of various gases like nitrogen (78%), oxygen (21%), carbon dioxide (0.03%), argon (0.91%), water vapour etc. that help life to exist on earth.

Question 2.
How does the atmosphere act as a blanket?
Solution:
The presence of ozone in the upper layer of atmosphere protects life from harmful ultraviolet radiations. Atmosphere maintains the temperature on the surface of the earth during day and night time. In this way, atmosphere acts as a blanket.

Question 3.
What causes winds ?
Solution:
The uneven heating of the atmosphere in different regions of the earth causes wind. The air above the land gets heated faster and starts rising. As the air rises, a region of low pressure is created and air over the sea moves into this area of low pressure. This movement of air from one region to other causes winds. The rotation of earth and presence of mountain ranges also help in the formation of wind.

Question 4.
How are clouds formed ?
Solution:
During day time water from different sources like river, sea etc. rises in the form of water vapour and goes into the air. At a particular height the air cools and the water vapour condenses to form minute droplets and form clouds. When these watex drops become heavy, they precipitate and fall down as rain.

Question 5.
List any three human activities that you think would lead to air pollution.
Solution:

• Harmful gases released from various industries.
• Burning of fossil fuels like coal and petroleum.
• Burning of diesel and petrol in vehicles.

Question 6.
Why do organisms need water ?
Solution:

• Plants require water for photosynthesis.
• All cellular processes occur in a water medium.
• Transportation of substances in our body takes place in dissolved form.

Question 7.
What is the major source of fresh water in the city/town/village where you live?
Solution:
Major sources of fresh water in the city/town/village are ground water, rivers, streams, lakes, etc.

Question 8.
Do you know of any activity which may be polluting this water source?
Solution:
Discharge of sewage, industrial wastes, hospital wastes, chemical fertilisers etc. into the rivers and streams pollutes these fresh water sources.’ *

Question 9.
How is soil formed ?
Solution:
Soil is formed by breaking down of rocks at or near the surface of earth through various physical, chemical and biological processes by various factors such as sun, water, wind and living organisms. It occurs as follows :

1. Sun : Sun heats up rocks during the day so that they expand. At night these rocks cool down and contract, this causes cracks in the rocks.
2. Water : Rain water enters these cracks. When this water freezes, it exerts pressure on the cracks and the rocks break. Fast flowing water wears the hard rocks and carries small particles of rocks down stream.
3. Wind : Strong winds erode rocks down, which rub with each other and small particles are carried away by wind.
4. Living organisms : Lichens grow on the surface of the rock, they release certain substances that cause the rock surface to powder down and form a thin layer of soil.

Question 10.
What is soil erosion ?
Solution:
The removal of top fertile layer of soil by the action of air, wind, water etc. is called soil erosion.

Question 11.
What are the methods of preventing or reducing soil erosion ?
Solution:

• Intensive cropping;
• planting grasses and xerophytes;
• Terrace farming;
• Proper drainage canals around the fields.

Question 12.
What are the different states in which water is found during the water cycle ?
Solution:
The three different states in which water is found during the water cycle are : solid (snow, ice), liquid (water) and gas (water vapour).

Question 13.
Name two biologically important compounds that contain both oxygen and nitrogen ?
Solution:
Two biologically important compounds that contain both oxygen and nitrogen are proteins and nucleic acids (DNA and RNA).

Question 14.
List any three human activities which would lead to an increase in the carbon dioxide content of air?
Solution:

• Burning of fuels like coal, petrol, diesel etc.
• Deforestation
• Human induced forest fires

Question 15.
What is the greenhouse effect ?
Solution:
Gases like C0₂ and methane trap Sun’s radiation reflected from earth’s surface. This causes warming of atmosphere. This effect is called greenhouse effect.

Question 16.
What are the two forms of oxygen found in the atmosphere ?
Solution:
Oxygen is present in free form like O₂ (oxygen molecule), Ozone (O₃) and combined form like oxides of metals and non-metals e.g., CO₂, NO₂, S0₂, H₂O.

NCERT Exercises

Question 1.
Why is the atmosphere essential for life ?
Solution:
Atmosphere is very important for sustaining life because:

• It contains various gases like O₂ CO₂, N₂ which are required for various life processes by plants and animals.
• C0₂ gas is used by plants to prepare food . by the process called photosynthesis.
• Oxygen is required for combustion and respiration.
• Ozone layer acts as a protective layer which prevents harmful U.V. rays to enter the earth’s atmosphere.
• Atmosphere acts as a temperature buffer.

Question 2.
Why is water essential for life?
Solution:
Water is essential for life because :

• About 70% weight of human beings is due to water.
• All cellular processes take place in water as medium.
• All reactions taking place within our body cells involve substances that are dissolved in water.
• Transportation of substances from one part of .the body to another takes place in dissolved form.
• Terrestrial life forms require fresh water to get rid of high amounts of wastes.

Question 3.
How are living organisms dependent on the soil? Are organisms that live in water totally independent of soil as a resource?
Solution:
All living organisms are directly or indirectly dependent on soil. Plants obtain water, minerals from the soil and prepare food for themselves and for other animals.
Organisms that live in water are not totally independent of soil because the microbes growing on the soil in water are the primary producers which start the food chain and even other microbes that live in soil help in the decomposition of dead plants and animals in water to return the nutrients back to the water.

Question 4.
You have seen weather reports on television and in newspapers. How do you think we are able to predict the weather?
Solution:
We daily see weather reports on televisions and newspapers. These informations are actually recorded by meteorological laboratories present in different cities of the country. Information such as direction and speed of wind, average daily minimum and maximum temperature, relative humidity, patterns of cloud formation, depression zone over an area etc. are recorded with the help of instruments and then displayed on television or published in newspapers. This meteorological information helps us to predict the weather.

Question 5.
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?
Solution:
if the human activities which lead to increasing levels of air, water and soil pollution are spread over wider areas then the pollutants will spread more, but yes, if these activities are limited and confined to a place then the air, soil and water pollution can be controlled to some extent.

Question 6.
Write a note on how forests influence the quality of our air, soil and water resources?
Solution:
Role of forest in influencing the quality of
Air :

• It helps in purifying the air by adding oxygen into it.
• It takes carbon dioxide gas during photosynthesis thus greenhouse effect is controlled.
• Transpiration helps in the formation of clouds and thus, it maintains the temperature of surrounding area, giving cooling effect.

Soil:

• Plants in forest hold the soil thereby preventing soil erosion.
• It holds the soil which helps in increasing the underground water level.
• It adds nutrients to the soil by the decomposition of organic matter present in forest.

Water :

• Forest helps in bringing rain and increasing the level of water in the underground and water bodies.