CBSE Class 9 Science Chapter 14: Natural Resources NCERT Solutions

Our atmosphere is significantly different from the atmospheres of Venus and Mars primarily due to its composition. Earth's atmosphere is composed of approximately 79% nitrogen, 20% oxygen, and a small fraction of other gases like carbon dioxide, water vapour, and noble gases. This specific composition is vital for supporting life as we know it.

In contrast, the atmospheres of Venus and Mars are overwhelmingly dominated by carbon dioxide. On Venus, carbon dioxide constitutes about 96.5% of the atmosphere, while on Mars, it makes up around 95%. These compositions create vastly different environmental conditions, making life as we know it impossible on those planets.

The atmosphere acts as a protective blanket for the Earth in several ways, primarily by regulating temperature:

1. Maintains Average Temperature: It helps to keep the Earth's average temperature relatively constant, both throughout the day and across the entire year. This stability is crucial for the survival of living organisms.
2. Prevents Sudden Temperature Increases: During the daytime, the atmosphere absorbs and distributes solar radiation, preventing the Earth's surface from becoming excessively hot very quickly.
3. Slows Heat Escape: At night, the atmosphere traps a significant portion of the heat radiated from the Earth's surface, preventing it from escaping rapidly into outer space. This keeps the nights from becoming extremely cold.

Winds are primarily caused by the uneven heating of the Earth's surface by the sun. Here's how it works:

1. Heating and Rising Air: When a part of the Earth's surface is heated more than another, the air above it also gets heated. Hot air is less dense, making it lighter, so it rises upwards.
2. Low-Pressure Area Formation: As the warm air rises, it leaves behind an area with lower atmospheric pressure.
3. Air Movement: Simultaneously, cooler, denser air from regions with higher atmospheric pressure moves in to fill the space created by the rising warm air. This horizontal movement of air from a high-pressure region to a low-pressure region is what we experience as wind.

Clouds are formed through a process involving evaporation, rising air, cooling, and condensation:

1. Evaporation: During the day, the sun heats up water bodies (like oceans, lakes, and rivers), causing water to evaporate and turn into water vapour, which rises into the atmosphere. Water vapour also enters the atmosphere through biological processes like transpiration from plants and respiration from animals.
2. Rising and Cooling Air: This warm, moist air rises higher into the atmosphere. As it ascends, the atmospheric pressure decreases, causing the air to expand and cool down.
3. Condensation: When the air cools sufficiently, the water vapour within it begins to condense. This condensation process is facilitated by the presence of tiny dust particles, smoke, or salt crystals suspended in the air, which act as nuclei. Water vapour condenses onto these particles, forming microscopic water droplets or ice crystals.
4. Cloud Formation: A large collection of these tiny water droplets or ice crystals suspended in the atmosphere forms a cloud.

Several human activities contribute significantly to air pollution. Here are three major ones:

1. Burning of Fossil Fuels: The combustion of fossil fuels like coal, petroleum, and natural gas in vehicles, power plants, and industries releases harmful pollutants such as carbon monoxide, sulphur dioxide, nitrogen oxides, and particulate matter into the atmosphere.
2. Industrialization: Many industrial processes involve chemical reactions and manufacturing that release various pollutants, including toxic gases, volatile organic compounds (VOCs), and fine particles, into the air.
3. Deforestation: Clearing forests for agriculture, urbanization, or other purposes reduces the number of trees. Trees absorb carbon dioxide and release oxygen, playing a vital role in purifying the air. Their removal diminishes this natural air-cleaning capacity and can also lead to increased burning of biomass, further polluting the air.