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Q1. How is our atmosphere different from the atmospheres on Venus and Mars?
Answer:
Our atmosphere has a much lower percentage of carbon dioxide, with nitrogen and oxygen being the major components, supporting life. Venus and Mars lack life and have atmospheres with 95–97% carbon dioxide.

Q2. How does the atmosphere act as a blanket?
Answer:
The atmosphere acts as a blanket by preventing sudden temperature changes. It stops excessive heating during the day and slows down heat loss at night, keeping Earth’s temperature suitable for life.

Q3. What causes winds?
Answer:
Winds are caused by the uneven heating of the Earth’s surface, resulting in air moving from areas of high pressure (cool) to low pressure (warm).

Q4. How are clouds formed?
Answer:
Clouds form when water vapour rises, cools, and condenses on dust or smoke particles into tiny droplets, gathering to form clouds.

Q5. List any three human activities that you think would lead to air pollution.
Answer: Three human activities that would lead to air pollution are: 

1. Burning fossil fuels (in vehicles, industries)
2. Use of pesticides and chemicals
3. Deforestation and burning forests

📝Page 194

Q1. Why do organisms need water?
Answer:
Organisms need water because all the chemical reactions that occur in their bodies take place in a water medium. Water allows substances to dissolve and be transported throughout the body. It is essential for maintaining cellular processes, proper metabolic function, growth, reproduction, and temperature regulation. Without water, these life processes would not be possible, and organisms would not survive.

Q2. What is the major source of fresh water in the city/town/village where you live?
Answer:
The major source of freshwater in most cities and towns is generally rivers, lakes, or underground water (wells and tube-wells). In villages, people often rely on ponds, nearby rivers, handpumps, or wells depending on local availability. The specific source can vary based on regional water resources and infrastructure.

Q3. Do you know of any activity which may be polluting this water source?
Answer:
Yes, several activities can pollute local water sources such as:

1. Discharging untreated sewage into rivers or lakes.
2. Disposal of industrial waste and chemicals into water bodies.
3. Excessive use of fertilizers and pesticides in agriculture, which get washed into water sources.
4. Bathing, washing clothes, or cleaning domestic animals directly in rivers or lakes.

📝Page 196

Q1. How is soil formed?
Answer:
Soil is formed by the gradual breakdown of rocks through physical, chemical, and biological processes over thousands to millions of years. The sun heats rocks causing them to expand and contract, leading to cracks. Water enters these cracks and, when it freezes, the cracks widen. Flowing water and wind wear down rocks and transport particles. Living organisms like lichens and mosses release chemicals that help in breaking down rocks further. The result is a mixture of mineral particles, humus (decayed organic material), and microorganisms which together form soil.

Q2. What is soil erosion?
Answer:
Soil erosion is the removal of the top fertile layer of soil from the land due to natural agents like wind or flowing water. It reduces soil fertility and can lead to the loss of productive land, making it difficult for plants to grow. Erosion is often accelerated by human activities such as deforestation, overgrazing, and poor agricultural practices.

Q3. What are the methods of preventing or reducing soil erosion?
Answer:
Methods to prevent or reduce soil erosion include:

1. Planting trees and maintaining vegetation cover to hold the soil in place.
2. Practicing contour plowing and terracing on hills to reduce water runoff.
3. Building check dams and other structures to control water flow.
4. Using crop rotation and cover crops to protect the soil surface.

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Q1. What are the different states in which water is found during the water cycle?
Answer:
During the water cycle, water exists in three states. They are:

1. Liquid (water in rivers, lakes, rain)
2. Gas (water vapour in the atmosphere)
3. Solid (ice and snow on mountain tops and at the poles)

Q2. Name two biologically important compounds that contain both oxygen and nitrogen.
Answer: Two biologically important compounds that contain both oxygen and nitrogen are :

1. Proteins (amino acids)
2. Nucleic acids (DNA and RNA)

Q3. List any three human activities which would lead to an increase in the carbon dioxide content of air.
Answer: Three human activities which leadS to an increase in the carbon dioxide content of air are :

1. Burning fossil fuels for energy (in vehicles, industries, households)
2. Deforestation, which reduces the number of trees that absorb CO₂ through photosynthesis
3. Industrial processes such as cement production

Q4. What is the greenhouse effect?
Answer:
The greenhouse effect is a phenomenon where certain gases in the Earth’s atmosphere (like carbon dioxide, methane, water vapour) trap heat, preventing it from escaping into space, and thereby keeping the planet warm enough to support life. An increase in these gases causes more heat to be trapped, leading to global warming.

Q5. What are the two forms of oxygen found in the atmosphere?
Answer: The two forms of oxygen found in the atmosphere are :

1. Diatomic oxygen (O₂), which is necessary for respiration.
2. Ozone (O₃), which forms a layer in the upper atmosphere and protects life from harmful ultraviolet (UV) radiation.