1. How is our atmosphere different from the atmospheres on Venus and Mars? Earth’s atmosphere is different from those of Venus and Mars. This difference lies essentially in their compositions. Earth’s atmosphere is a mixture of nitrogen (79%), oxygen (20%), and a small fraction of carbon dioxide, water vapours and other gases. This makes the existence of life possible on Earth. However, the atmospheres on Venus and Mars mainly consist of carbon dioxide. The amount of carbon dioxide on these planets can range from 95% to 97%. 2. How does the atmosphere act as a blanket?
The atmosphere acts as a blanket by performing the following functions: (a) It keeps the average temperature of the Earth fairly constant during day time and even during the course of whole year. (b) It prevents a sudden increase in the temperature during day time. (c) It slows down the escape of heat from the surface of the Earth into outer space during night time. 3. What causes winds?
An uneven heating of the Earth’s surface causes winds. On being heated, air becomes lighter and rises up. As a result, a region of low pressure is created. Then, air from a high pressure region moves to a low pressure region, causing wind. 4. How are clouds formed?
During day time, on being heated, a large amount of water evaporates from various water bodies and goes into the air. A part of this water vapour also reaches the atmosphere through biological activities such as transpiration and respiration. This causes the air in the atmosphere to heat up. When this heated air rises, it expands and cools, which results in the condensation of water vapour forming water droplets. The presence of dust and other suspended particles in air also facilitates the process of condensation. The formation of water droplets leads to the formation of clouds. 5. List any three human activities that you think would lead to air pollution. The following three human activities would lead to air pollution: (i) Burning of fossil fuels such as coal and petroleum
6. Why do organisms need water?
Organisms need water for the following reasons:
(i) All cellular processes need water as a medium. Usually, the reactions that take place in our body or within the cells occur between substances that are dissolved in water. (ii) Since most of the substances are transported in a dissolved form, water is necessary. 7. What is the major source of fresh water in the city/town/village where you live? River is a major source of fresh water.
8. Do you know of any activity which may be polluting this water source? The discharge of waste water from homes, industries, hospitals, etc. into the river pollutes this fresh water source. 9. How is soil formed?
Soil is formed by breaking down of rocks at or near the surface of the Earth through various physical, chemical, and biological processes by various factors such as the sun, water, wind, and living organisms. (i) Sun:
During day time, the rocks are heated. This causes the rocks to expand. During night time, these rocks cool down and contract. Since all parts of the rock do not undergo expansion and contraction at the same rate, this causes the formation of cracks in these rocks. These cracks lead to the breaking up of huge rocks into smaller pieces. (ii) Water:
Water catalyses the process of formation of soil in two ways. (a) Water goes into the cracks and crevices formed in the rocks. When this water freezes, its volume increases. As a result, the size of the cracks also increases. This helps in the weathering of rocks. (b) Running water wears away hard rocks over long periods of time. Water moving in fast speed carries big and small particles of rock downstream. These rocks rub against each other, resulting in breaking down of rocks. These smaller particles are carried away by running water and deposited down its path. (iii) Wind:
Strong winds carry away rocks, which causes rubbing of...