Answer key

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Last edited 25 days ago by Learn LoopHQ.

Chapter: 06. Air And Atmosphere

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Similarities: Both respiration and burning require oxygen and both processes produce carbon dioxide and water vapour. Differences:

Energy Release: Respiration releases a small amount of energy as heat only, whereas burning releases a large amount of energy in the form of both heat and light.

Speed: Respiration is a slow process, while burning is a fast and rapid process.

Nature: Respiration is a natural and continuous biochemical process occurring in living cells, whereas burning is an artificial and discontinuous chemical process.

Temperature: Respiration takes place at body temperature, but burning occurs at much higher temperatures.

Acid Rain: This occurs when harmful gases like sulphur dioxide and nitrogen dioxide, released from burning fossil fuels, react with water vapour and oxygen in the atmosphere to form sulphuric and nitric acids. These acids then fall to Earth with rain. Acid rain damages crops, harms aquatic life, erodes buildings (especially marble), and corrodes metallic objects. Global Warming: This is the increase in the average temperature of the Earth’s atmosphere. It is caused by an increase in greenhouse gases (like carbon dioxide, methane, CFCs) due to human activities such as burning fossil fuels and deforestation. These gases trap more heat from the Sun, intensifying the natural greenhouse effect and leading to a rise in global temperatures.

Air is considered a mixture because its composition is not fixed; it varies depending on location and time, unlike a compound which has a definite composition. For example, water vapour levels differ in rainy versus summer seasons. Secondly, when the components of air mix, there is no change in energy, and each gas retains its individual properties (e.g., oxygen still supports burning), which is not the case when a compound is formed. A compound also has a chemical formula, which air does not.

To show the presence of oxygen and nitrogen, light a candle and fix it in a trough partially filled with water. Immediately cover the burning candle with an inverted gas jar. You will observe that the candle burns for some time and then extinguishes, while the water level inside the gas jar rises. This occurs because the limited oxygen inside the jar is consumed during burning, causing the candle to blow out. The water rises to fill the space left by the consumed oxygen. The remaining major part of the air inside the jar, which does not support burning, is nitrogen.

The atmosphere is a thin layer of air that envelops the Earth, extending many kilometers above its surface. Gravity, the force by which Earth attracts everything towards itself, is crucial as it binds this layer of air to the planet’s surface, preventing it from escaping into space. The atmosphere is vital for life because it maintains a stable temperature on Earth, preventing extreme hot or cold conditions. It also acts as a protective shield, absorbing and blocking harmful rays from the Sun.

To demonstrate air’s presence, take an inverted glass tumbler and dip it into a vessel filled with water. Initially, water will not enter the glass, even though it appears empty. This shows that the glass is filled with air, preventing water entry. When the glass is then tilted slightly, air bubbles will be seen escaping from the mouth of the glass, and water will begin to enter. This observation confirms that the space inside the glass was indeed occupied by air, which escaped as bubbles to make way for the water.

Rusting is a process where iron objects develop a reddish-brown, flaky substance called rust when exposed to moist air. Chemically, rust is hydrated iron oxide formed when iron reacts with oxygen and water. The necessary conditions for rusting are the presence of both oxygen (from the air) and water vapour (moisture). Rusting is considered harmful because the rust layer flakes off, continuously exposing fresh iron surfaces to the environment. This ongoing process weakens the iron object over time, making it brittle, structurally unsound, and ultimately useless.

Carbon dioxide is profoundly significant for plants because it is a key raw material for photosynthesis. Green plants use carbon dioxide from the air, along with water and sunlight in the presence of chlorophyll, to produce their own food (glucose) and release oxygen. Besides this vital role, carbon dioxide has other practical uses: it helps maintain Earth’s temperature by trapping heat, it is used in fire extinguishers to put out fires, solid carbon dioxide (dry ice) serves as a refrigerant for cooling, and it is dissolved in water to make fizzy soft drinks.

Air pollution is the contamination of air with unwanted substances that have harmful effects on both living organisms and non-living things. Three common air pollutants are sulphur dioxide, oxides of nitrogen, and carbon monoxide. Two main causes of air pollution include:

Burning of fossil fuels: Industries, power plants, and vehicles release harmful gases and particulate matter into the atmosphere when coal, oil, and natural gas are burned.

Deforestation and increased human population: The cutting of forests reduces the number of trees that absorb carbon dioxide, while an increasing human population leads to more resource consumption, waste generation, and energy demand, all contributing to higher pollutant emissions.

The balance of oxygen and carbon dioxide in nature is primarily maintained through a continuous cycle of respiration, combustion, and photosynthesis. Living organisms inhale oxygen and exhale carbon dioxide during respiration, while the burning of fuels also consumes oxygen and releases carbon dioxide. Counteracting this, green plants absorb carbon dioxide from the air for photosynthesis and release oxygen. This natural interplay keeps the gas levels stable. However, human activities like air pollution (due to increased industrial emissions and vehicle exhaust) and deforestation (cutting down forests) can significantly upset this delicate balance by adding excess carbon dioxide or reducing oxygen production.

Three effective measures to prevent air pollution are:

Afforestation and Green Spaces: Growing more trees is vital because trees absorb carbon dioxide, a major greenhouse gas and pollutant, and release oxygen, thus naturally cleaning the air.

Industrial Emission Control: Factories and power houses should install tall chimneys equipped with filters. These filters can capture pollutants before they are released into the atmosphere, and tall chimneys help disperse any remaining gases high into the air, reducing local concentrations.

Promotion of Cleaner Fuels and Energy Sources: Encouraging the use of cleaner fuels like CNG in vehicles instead of petrol or diesel, and promoting alternative sources of energy such as solar or wind power, can significantly reduce the burning of fossil fuels and the release of harmful emissions.

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