Earth as a System Class 9 CBSE Important Questions with Answers

Answer: (b) cryosphere.

The Earth system has five interacting spheres: the geosphere (rocks and soil), hydrosphere (liquid water), cryosphere (ice and snow), atmosphere (air) and biosphere (living things).

Answer: (c) oxygen.

During photosynthesis plants use carbon dioxide and water and release oxygen, replenishing atmospheric oxygen and driving the oxygen cycle.

Answer: (b) nitrogen fixation.

Rhizobium bacteria in the root nodules of legumes convert atmospheric nitrogen ($\text{N}_2$N₂) into nitrogen compounds (nitrates) that plants can absorb — this is biological nitrogen fixation.

Answer: (b) $500$500 s.

Distance $=1.5\times10^{8}\ \text{km}=1.5\times10^{11}\ \text{m}$=1.5×10⁸ km=1.5×10¹¹ m. Time $=\dfrac{\text{distance}}{\text{speed}}=\dfrac{1.5\times10^{11}}{3\times10^{8}}=500$=distance/speed=1.5×10¹¹3×10⁸=500 s, which is about $8$8 minutes $20$20 seconds. (Option (a) forgets to convert kilometres to metres.)

Answer: (a) Both A and R are true and R is the correct explanation of A. Because the Earth is curved, near the poles the rays arrive at a slant and the same beam spreads over a larger area (and passes through more atmosphere), so each square metre gets less energy. This uneven heating is the starting point of winds.

The five spheres are the geosphere (rocks, soil), hydrosphere (oceans, rivers, ground water), cryosphere (ice and snow), atmosphere (air) and biosphere (all living things).

Example: plants (biosphere) draw water from the soil and release water vapour into the air by transpiration (atmosphere). Rain from the atmosphere also wears away rocks of the geosphere. Any one correct example is enough.

Increasing frequency: radio waves < infrared rays < visible light < ultraviolet rays < X-rays.

All of them are electromagnetic waves: they need no medium and travel through a vacuum at the same speed, the speed of light, about $3\times10^{8}\ \text{m s}^{-1}$3×10⁸ m s^-1. Solar radiation reaches the Earth mainly as visible light and infrared, with some ultraviolet.

The water cycle is the continuous movement of water between the oceans, atmosphere, land and living things.

Heat from the Sun causes evaporation of water from oceans, rivers and lakes, and transpiration from plants, forming water vapour. The vapour rises, cools and condenses into clouds, then falls back as precipitation (rain, snow). Some water flows over the land as run-off into rivers and seas, and some infiltrates the soil to become ground water, which plants absorb and later release by transpiration. Thus water is endlessly recycled.

In the carbon cycle, carbon moves between the atmosphere (as $\text{CO}_2$CO₂), living organisms and fossil fuels. Plants take in $\text{CO}_2$CO₂ for photosynthesis and fix it into carbohydrates; this carbon passes along food chains. Respiration, decay and burning return $\text{CO}_2$CO₂ to the air, keeping the cycle balanced.

Processes that add $\text{CO}_2$CO₂ to the atmosphere:

1. Respiration by plants and animals.
2. Combustion (burning) of fuels, wood and fossil fuels; also decomposition by decomposers.

Processes that remove $\text{CO}_2$CO₂ from the atmosphere:

1. Photosynthesis by green plants.
2. Dissolving of $\text{CO}_2$CO₂ in oceans and its use in forming shells and, over long times, fossil fuels.

Leguminous plants such as gram have root nodules containing nitrogen-fixing bacteria (Rhizobium). These bacteria convert inert atmospheric nitrogen ($\text{N}_2$N₂) into nitrogen compounds (nitrates) in the soil.

When the legume crop is harvested and its roots and residues decay, this fixed nitrogen enriches the soil. The next crop, wheat, absorbs these nitrates to build proteins, so it grows well even without much added nitrogen fertiliser. This is why farmers practise crop rotation with legumes to restore soil fertility naturally.

Nitrogen makes up about 78% of the air as inert $\text{N}_2$N₂, which most organisms cannot use directly. The nitrogen cycle converts it into usable forms and back.

Nitrogen fixation: atmospheric $\text{N}_2$N₂ is converted into nitrates by lightning and by nitrogen-fixing bacteria (Rhizobium in legume nodules, and free-living soil bacteria).

Assimilation: plants absorb nitrates from the soil and use them to make proteins; animals get this nitrogen by eating plants.

Ammonification / Nitrification: when organisms die or excrete waste, decomposers convert the nitrogen to ammonia, and nitrifying bacteria change ammonia to nitrites and then nitrates, returning them to the soil.

Denitrification: denitrifying bacteria convert nitrates back into free $\text{N}_2$N₂ gas, which returns to the atmosphere, completing the cycle.

(a) Land heats up and cools down much faster than water.

By day, the land becomes hotter than the sea. The air above the land warms, expands, becomes lighter and rises, so the pressure over the land drops. Cooler, heavier air from over the sea moves in to take its place: this surface wind from sea to land is the sea breeze. The risen air flows back over the sea at a height and sinks, completing the loop.

At night, the land cools faster than the sea, so the air over the sea is now warmer and rises. Air flows from the cooler land towards the sea: the land breeze.

(b) (i) Shape and latitude: the Earth is curved, so rays fall almost vertically near the equator but at a slant near the poles, where the same energy is spread over a larger area.

(ii) Tilt of the Earth's axis: as the Earth goes round the Sun, the tilt changes the angle of the rays at a place through the year, which gives seasons.

(iii) Different surfaces: ice and snow reflect most sunlight, while forests, soil and oceans absorb more of it; land and water also warm at different rates.

(i) Plants release oxygen during photosynthesis.

(ii) (a) Burning of fossil fuels (coal, petrol, diesel) and (b) large-scale cutting/burning of forests; increased industrial and vehicular activity is also acceptable.

(iii) Deforestation removes trees that take in $\text{CO}_2$CO₂ and give out $\text{O}_2$O₂. So less carbon dioxide is absorbed and less oxygen is released, causing $\text{CO}_2$CO₂ to build up and disturbing both the carbon and oxygen cycles, which enhances global warming.

(iv) Planting more trees (afforestation) and reducing the burning of fossil fuels by using cleaner or renewable energy sources helps restore the balance.