CLASS 9 · SOCIAL STUDIES · CHAPTER 3
Simple, step-by-step answers to all 8 end-of-chapter questions and activities.
8 worked answers · short steps first, explanations when you need them
Textbook pages 59–60
Question 1
Air is a mixture of gases.
Air is a mixture of gases. The textbook diagram gives nitrogen about 78%, oxygen 21%, argon 0.93%, carbon dioxide 0.04% and other gases 0.03%. Water vapour varies and tiny dust particles are also present. Small amounts of gases such as helium, neon, krypton, xenon, ozone and hydrogen are found in air.
Question 2
The troposphere contains most water vapour and nearly all weather.
The troposphere contains most water vapour and nearly all weather. The stratosphere contains the ozone layer, which absorbs much harmful ultraviolet radiation. Many meteors burn up in the mesosphere. The thermosphere absorbs energetic solar radiation; auroras occur there and ionospheric conditions help radio communication. The very thin exosphere is the outermost region.
Question 3
Monsoon is a seasonal change in wind direction.
Monsoon is a seasonal change in wind direction. In summer, land heats faster than the sea, creating relatively low pressure over land. Moist winds move from the ocean towards land and bring rain. In winter, land cools faster and winds generally reverse. Winds crossing the Bay of Bengal can bring rain to parts of south-eastern India.
Question 4
We can walk or cycle for short journeys, use public transport or carpool, save electricity, use water carefully, reuse and recycle materials, and avoid unnecessary waste.
We can walk or cycle for short journeys, use public transport or carpool, save electricity, use water carefully, reuse and recycle materials, and avoid unnecessary waste. Choosing durable goods and supporting cleaner energy also helps. A carbon footprint includes emissions from energy, transport and the production of goods.
Question 5
Without the atmosphere, people would not have breathable air, and ordinary winds, clouds and rainfall would not occur.
Without the atmosphere, people would not have breathable air, and ordinary winds, clouds and rainfall would not occur. Earth would lose much of its protection from harmful solar radiation and its usual temperature regulation. The Sun would still shine; it is the source of sunlight, while the atmosphere affects how energy interacts with Earth.
Question 6
Nitrogen is needed to make proteins and other important substances for plant growth.
Nitrogen is needed to make proteins and other important substances for plant growth. Plants cannot use most nitrogen directly from the air. Some bacteria, including bacteria living in nodules on legume roots, change atmospheric nitrogen into forms plants can absorb.
Question 7
Sample response: During a windy day, loose papers in our classroom blew around and a tree branch swayed near the playground.
Sample response: During a windy day, loose papers in our classroom blew around and a tree branch swayed near the playground. This made it harder to keep things in place. Use an example that you have actually experienced; if you have not experienced this, describe a clearly labelled example from the chapter instead.
Question 8
Use the Beaufort scale as a reference.
Use the Beaufort scale as a reference. Light winds rustle leaves; stronger breezes move branches and raise dust; gale-force winds can break branches and make walking difficult; very strong storm-force winds can damage buildings and uproot trees. Make a table with wind category, speed range, and observed effects. Check a reliable weather source and never go outside to observe dangerous winds.