Class 9 Science, Chapter 13 (NCERT). 8 questions by concept. Open “Show answer” for the answer and why.
How to teach this chapter · Common misconceptions & worked visual for this chapter
Which region of Earth receives the most solar energy on each square metre of its surface?
(b) Near the equator
Near the equator the Sun's rays fall almost straight down, so the same energy is concentrated on a smaller area. Nearer the poles the rays slant, spreading that energy over a larger area and passing through more air.
At sunrise the sand on a beach and the sea water beside it are both at 25 °C. The same sunlight falls on both until noon. By noon the sand is at 45 °C. Which temperature is the sea water most likely to show at noon?
(d) 28 °C
Water needs far more heat than sand for the same rise in temperature, and the sea also mixes its heat down into deeper water. So the same sunlight gives the sea only a small rise, to about 28 °C, while the sand rises 20 °C to reach 45 °C.
Why does a sea breeze commonly blow from sea to land during the day?
(b) Land warms faster, air rises over it, and cooler sea air moves toward the lower pressure over land.
Differential heating establishes a local pressure circulation.
A warm ocean current flows along a coast. What effect does this current have on the climate of that coast?
(a) Warmer air along the coast than inland places at the same latitude
The current carries warm water past the coast. This water gives out heat to the air above it. Wind blows that warmer air over the land, so the coast stays warmer than inland places at the same latitude.
Warm air carrying water vapour rises and cools. What happens to that water vapour?
(b) It condenses into liquid droplets
Rising air cools the water vapour. On cooling, the vapour changes from gas to liquid and forms the tiny droplets that build a cloud. The water is not used up; it keeps moving through the water cycle.
Nitrogen gas makes up most of the air. Nitrogen fixation acts on this nitrogen gas. What does nitrogen fixation change it into?
(b) Nitrogen compounds that plants can take up from the soil
Most organisms cannot take in nitrogen gas from the air. Nitrogen fixation changes it into nitrogen compounds that plants take up from the soil, and these compounds then pass along food webs.
How does burning fossil fuel change the carbon cycle?
(b) Carbon stored underground for millions of years moves into the air as carbon dioxide.
Coal, oil and gas hold carbon that was stored underground for millions of years. Burning them sends that carbon into the air as carbon dioxide. The carbon is not destroyed, only moved to another store.
A large area of forest is cleared and the wood is burnt. Over the next few years, what happens to the amount of carbon dioxide in the air?
(b) It rises.
Burning the wood sends the carbon stored in the trees into the air as carbon dioxide. The cleared trees are also no longer there to take carbon dioxide in during photosynthesis. Both changes leave more carbon dioxide in the air.
Wrong
Energy comes back to producers as dead material breaks down.
Why it feels right. Nutrients return, so energy seems to return too.
Right
Matter can cycle. Ecosystems receive energy and release much of it as heat.
Worked example
