CBSE Class 9 Science Exemplar Chapter 13: Natural Resources NCERT Solutions

The Earth's atmosphere is primarily heated indirectly. While the sun radiates energy towards the Earth, it is the surfaces of the land and water bodies that absorb this solar radiation. These surfaces then re-radiate the absorbed heat energy back into the atmosphere. This re-radiated heat is the main source that warms the air surrounding our planet.

Answer: (d) re-radiated by land and water

The atmosphere plays a crucial role in regulating Earth's temperature. During the day, it allows some solar radiation to pass through, warming the surface. However, it also reflects and absorbs some radiation, preventing extreme overheating. At night, the atmosphere acts like a blanket, trapping the heat radiated from the Earth's surface and preventing it from escaping rapidly into space. Without this atmospheric blanket, the Earth's surface would become extremely hot during the day due to direct solar exposure and then cool down drastically at night as heat dissipates quickly.

Answer: (c) increase during day and decrease during night

Oxygen (O2) is essential for the respiration of most living organisms. Ozone (O3) is a different molecule, and while the ozone layer in the stratosphere is vital for absorbing harmful ultraviolet radiation, high concentrations of ozone at ground level are toxic. If all atmospheric oxygen were converted to ozone, organisms that rely on O2 for breathing would suffocate. Furthermore, ground-level ozone is a harmful pollutant that can damage lung tissue and ecosystems.

Answer: (b) It will become poisonous and kill living forms

Soil formation, or pedogenesis, is a natural process involving the weathering of rocks and the accumulation of organic matter over long periods. Natural factors like the sun (causing temperature fluctuations that break rocks), water (through erosion, chemical weathering, and transport), and wind (through erosion and deposition) are key agents in this process. Polythene bags, however, are synthetic materials and a form of pollution. They do not contribute to soil formation and can degrade soil quality and health.

Answer: (d) polythene bags

Oxygen exists in the atmosphere primarily in two molecular forms. The most abundant form is molecular oxygen (O2), which is essential for the respiration of most living organisms. The other significant form is ozone (O3), which is concentrated in the stratosphere and plays a critical role in absorbing harmful ultraviolet radiation from the sun. Water is H2O and carbon dioxide is CO2, neither of which are forms of oxygen itself.

Answer: (c) ozone and oxygen

Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted into ammonia or other nitrogenous compounds that can be used by plants. Many bacteria that perform biological nitrogen fixation are sensitive to oxygen. Some are anaerobic, meaning they cannot survive in the presence of oxygen, while others have mechanisms to protect their nitrogenase enzyme from oxygen damage. Therefore, the presence of elemental oxygen can inhibit or prevent the process of nitrogen fixation by these bacteria.

Answer: (b) elemental form of oxygen

Rainfall patterns are complex and influenced by several factors. However, the presence of significant water bodies, such as oceans, seas, and large lakes, is fundamental because they are the primary source of evaporation, providing the moisture necessary for cloud formation and precipitation. While seasons and atmospheric conditions are critical for the timing and type of precipitation, the availability of water vapor largely depends on nearby water sources. Underground water tables and human population density have indirect or negligible effects on large-scale rainfall patterns.

Answer: (b) the number of water bodies in an area