NCERT Solutions Curiosity Chapter 9 Chapter exercises — Let Us Enhance Our Learning
Book page 135 & 136 Updated on2026-09-05
Q1.
Complete the journey of food through the alimentary canal by filling up the boxes with appropriate parts— Food → Mouth → ____ → Stomach → ____ → ____ → Anus
Answer
The three empty boxes are the food pipe (oesophagus), the small intestine and the large intestine.
Food → Mouth → Food pipe (Oesophagus) → Stomach → Small intestine → Large intestine → Anus
The complete journey of food through the alimentary canal.
Box
Part
What happens there
1st blank
Food pipe (Oesophagus)
Wave-like contraction and relaxation of the walls pushes the food down into the stomach
2nd blank
Small intestine
Bile, pancreatic juice and its own juice complete the digestion; nutrients are absorbed into the blood
3rd blank
Large intestine
Water and some salts are absorbed; stool is formed and stored in the rectum
Tip: the liver and the pancreas do not go into any box. Food never passes through them — they only pour their juices into the small intestine, so they are called associated parts of the alimentary canal.
Q2.
Sahil placed some pieces of chapati in test tube A. Neha placed chewed chapati in test tube B, and Santushti took boiled and mashed potato in test tube C. All of them added a few drops of iodine solution to their test tubes—A, B, and C, respectively. What would be their observations? Give reasons.
Answer
Iodine turns blue-black wherever starch is present, and keeps its own brownish-orange colour where starch is absent.
Test tube
What it contains
Observation
Reason
A — Sahil
Plain pieces of chapati (not chewed)
Blue-black colour
Chapati is rich in starch and no saliva has acted on it, so all the starch is intact
B — Neha
Chewed chapati
No change in colour, or only a very light blue-black
While chewing, the digestive juice in saliva broke the starch down into simple sugars, so little or no starch is left for the iodine to react with
C — Santushti
Boiled and mashed potato
Blue-black colour
Potato is also rich in starch, and it was only boiled and mashed — no saliva was mixed with it, so its starch is untouched
Why it happens: A and C are alike in the one thing that matters — starch that saliva has never touched — so both give the same blue-black result even though one is wheat and the other a potato. Only B is different, and the only difference is saliva. That single comparison is what proves that saliva digests starch.
Check it yourself: if Neha's tube B still turns dark, she has not chewed long enough. Chewing for a full minute makes the difference clear.
Q3.
What is the role of the diaphragm in breathing? (i) To filter the air (ii) To produce sound (iii) To help in inhalation and exhalation (iv) To absorb oxygen
Answer
The correct answer is (iii) To help in inhalation and exhalation.
Fig. 9.10 of the book: the diaphragm changes the space inside the chest, and air follows.
Inhalation
Exhalation
Ribs
Move up and outwards
Move down and inwards
Diaphragm
Moves downwards
Moves upwards
Space inside the chest
Increases
Decreases
Air
Enters the lungs
Is pushed out of the lungs
Why the other options are wrong: (i) filtering the air is done by the tiny hair and mucus in the nostrils, not by the diaphragm; (ii) sound is produced in the voice box in the windpipe; (iv) oxygen is absorbed into the blood at the alveoli. The diaphragm is a dome-shaped muscle below the lungs — its only job here is to change the size of the chest cavity.
Q4.
Match the following — (i) Nostrils (ii) Nasal passages (iii) Windpipe (iv) Alveoli (v) Ribcage with (a) fresh air from outside enters (b) exchange of gases occurs (c) protects lungs (d) tiny hair and mucus help to trap dust and dirt from the air we breathe (e) air reaches our lungs through this part
Answer
Name of the part
Function
Why
(i) Nostrils
(a) fresh air from outside enters
The pair of nasal openings where the respiratory system begins
(ii) Nasal passages
(d) tiny hair and mucus help to trap dust and dirt from the air we breathe
The air is cleaned here before it goes any further
(iii) Windpipe
(e) air reaches our lungs through this part
It carries air down and divides into two branches, one for each lung
(iv) Alveoli
(b) exchange of gases occurs
Balloon-like sacs with thin walls surrounded by fine blood-carrying tubes
(v) Ribcage
(c) protects lungs
The bony cage around the chest that guards the soft lungs
The human respiratory system (Fig. 9.8 of the book), with the parts named in the question.
Tip: remember the order in which air travels — nostrils, nasal passages, windpipe, its two branches, finer branches, alveoli. Four of the five matches simply follow that order; the rib cage is the one part that does not carry air at all, so it takes the odd option, "protects lungs".
Q5.
Anil claims to his friend Sanvi that respiration and breathing are the same process. What question(s) can Sanvi ask him to make him understand that he is not correct?
Answer
Sanvi should ask questions whose honest answers force Anil to admit that the two words stand for two different things.
"If breathing and respiration are the same, then where does the energy in our body come from? Does simply pulling air in and pushing it out release energy?"
"Write the word equation of respiration. Now write one for breathing. Why can you write Glucose + Oxygen → Carbon dioxide + Water + Energy for one and not for the other?"
"Breathing takes place in the nose, windpipe and lungs. In which part of the body does respiration take place?"
"An earthworm has no lungs and a fish has no nose to breathe with — yet both are alive and active. If breathing and respiration were the same word, how would you explain that?"
"Is a physical change the same as a chemical change? Breathing is a physical process; respiration is a chemical one. Can two processes of different kinds be the same process?"
"When you hold your breath for half a minute, does respiration in your body also stop?"
Breathing
Respiration
Taking air in and pushing it out
Breaking down glucose with oxygen to release energy
A physical process
A chemical process
Happens in the respiratory system
Happens inside the body, in all its parts
No energy is released
Energy is released
Only supplies oxygen and removes carbon dioxide
Actually uses that oxygen to do useful work
Why it happens: breathing is the delivery; respiration is the use. Comparing them is like comparing a milkman bringing milk to the house with the family drinking it — related, needed one after the other, but not the same event.
Q6.
Which of the following statements is correct and why? Anu: We inhale air. Shanu: We inhale oxygen. Tanu: We inhale air rich in oxygen.
Answer
Tanu's statement is correct — "We inhale air rich in oxygen."
Fig. 9.13 of the book, drawn as bars: inhaled air is rich in oxygen but is not oxygen.
Statement
Verdict
Reason
Anu: We inhale air.
True, but incomplete
We do inhale air, but this says nothing about the oxygen that makes the air useful to us
Shanu: We inhale oxygen.
Incorrect
We never inhale pure oxygen. Air is a mixture; only about 21% of it is oxygen, and even that is not fully used — exhaled air still has 16–17% oxygen
Tanu: We inhale air rich in oxygen.
Correct
It says both things that matter — what we take in is air, and that air is rich in oxygen compared with the air we breathe out
Why it happens: the lungs simply take in whatever mixture is around them. Oxygen is picked up from that mixture at the alveoli and carbon dioxide is added to it, so the air that comes out has less oxygen and much more carbon dioxide — but it is still air, not a single gas.
Q7.
We often sneeze when we inhale a lot of dust-laden air. What can be possible explanations for this?
Answer
Sneezing is the body's way of throwing the dust out again. Possible explanations:
The filter is overloaded. The tiny hair and the mucus in the nostrils can trap only a limited amount of dust. When the air is very dusty, more particles get through than they can hold.
The lining is irritated. Dust particles settling on the delicate lining of the nasal passage irritate it, and this irritation sets off a sneeze automatically.
A sneeze is a protective reflex. It is not something we decide to do. A sudden, forceful rush of air is pushed out through the nose and mouth, and it carries the dust particles out with it.
It keeps the lungs safe. If the dust travelled further down, it would reach the windpipe and the delicate alveoli, where there is no easy way to remove it. Sneezing stops the dust while it is still near the entrance.
Extra mucus is produced along with the sneeze, so more dust is trapped and washed out; this is why the nose runs in a dusty place.
Why it happens: the whole respiratory system is built to protect one thing — the thin, moist walls of the alveoli. Nose hair, mucus, coughing and sneezing are all layers of the same defence.
Tip: always cover your nose and mouth with a handkerchief or your bent elbow while sneezing. A sneeze throws out tiny droplets that can carry infection to people standing nearby. In a dusty place, cover your face with a cloth or a mask.
Q8.
Paridhi and Anusha of Grade 7 started running for their morning workout. After they completed their running, they counted their breaths per minute. Anusha was breathing faster than Paridhi. Provide at least two possible explanations for why Anusha was breathing faster than Paridhi.
Answer
Any two of the following are acceptable explanations.
Anusha may have run faster or covered a longer distance. Harder work means her muscles needed more energy, so more glucose had to be broken down, so more oxygen was needed and more carbon dioxide had to be thrown out — and the only way to do that is to breathe faster.
Anusha may be less used to running than Paridhi. Someone who exercises regularly has stronger breathing muscles and takes in more air with each breath, so she needs fewer breaths for the same work. A person who runs rarely gets out of breath sooner.
Their bodies are different. Chest size, lung capacity and body weight vary from person to person. A smaller lung capacity means less air per breath, so more breaths per minute.
Anusha may have counted her breaths sooner after stopping. Breathing rate falls gradually after exercise, so counting right away gives a higher number than counting three minutes later.
Anusha may have been carrying a bag, running uphill, or running on soft sand, so the same distance cost her more effort.
Anusha may not have been well — a cold or a blocked nose makes each breath shallow, so more breaths are needed.
More muscle work → more glucose to be broken down
→ more oxygen needed and more carbon dioxide to remove
→ breathing rate goes up
Check it yourself: count your breaths for one minute while sitting quietly, then skip for two minutes and count again. Note both numbers, and repeat it for a week — you will see your resting rate slowly drop as you become fitter.
Q9.
Yadu conducted an experiment to test his idea. He took two test tubes, A and B, and added a pinch of rice flour to the test tubes, half-filled with water and stirred them properly. To test tube B, he added a few drops of saliva. He left the two test tubes for 35–45 min. After that, he added iodine solution into both the test tubes. Experimental results are as shown in Fig. 9.15. What do you think he wants to test?
Answer
Yadu wants to test whether saliva breaks down starch — that is, whether the digestive juice in saliva changes the starch of rice flour into something that is no longer starch.
In Fig. 9.15 the two tubes look completely different:
Test tube
Contents
Colour after iodine (Fig. 9.15)
What it means
A
Rice flour + water only
Dark blue-black
Starch is still present — nothing acted on it
B
Rice flour + water + a few drops of saliva
Stays brownish-orange, the colour of iodine itself
The starch has been broken down; there is nothing for iodine to react with
Only one thing is different between A and B — the saliva. So the difference in colour can only be the work of the saliva.
Why this design is good: tube A is the control. Without it, Yadu could not claim anything, because someone might say the rice flour never had starch in the first place. He also keeps everything else the same — the same pinch of flour, the same amount of water, the same stirring, the same 35–45 minutes and the same iodine — so that saliva is the only variable.
Did you know? This experiment also shows something extra: saliva goes on working even outside the mouth, in a test tube. Digestion is a chemical action of the juice, not a magic that needs the body.
Q10.
Rakshita designed an experiment taking two clean test tubes, A and B and filled them with lime water as shown in the figure. In test tube A, the surrounding air that we inhale was passed on by sucking air from the pipe, and in test tube B, the exhaled air was blown through the pipe (Fig. 9.16). What do you think she is trying to investigate? How can she confirm her findings?
Answer
Rakshita is trying to find out whether the air we breathe out contains more carbon dioxide than the air we breathe in.
Lime water turns milky only with carbon dioxide
Tube A (inhaled air) → stays clear
Tube B (exhaled air) → turns milky
∴ exhaled air has more carbon dioxide than inhaled air
How she can confirm her findings:
Compare the two tubes side by side. Tube A is the control. Both tubes get the same freshly prepared lime water, the same amount, and air is passed for the same length of time. If only B turns milky, the difference must come from the exhaled air.
Repeat the experiment two or three times, and let two or three classmates blow into fresh tubes. A result that comes back every time is trustworthy.
Blow for longer. The milkiness in tube B should become more and more marked as more exhaled air is passed — showing that the effect grows with the amount of carbon dioxide.
Test in the reverse direction. Pass carbon dioxide from a known source into fresh lime water; it too turns milky. This proves that the milkiness really is the test for carbon dioxide.
Use fresh lime water each time. Lime water left standing in the open has already met the carbon dioxide of the air and may be cloudy before the test begins.
Why it happens: carbon dioxide is a waste product of respiration. It is made all over the body when glucose is broken down, is carried by the blood to the alveoli, and is released into the air we exhale. So the air coming out must carry far more of it than the air going in — nearly 4–5% against nearly 0.04%.