EN हिं

NCERT Solutions Curiosity Chapter 9 –180Exercise questions 1 to 10 — Let us enhance our learning

Book page 178 Updated on2026-09-05

Q1.
What purpose does handpicking serve in the process of separation? (i) Filtration (ii) Sorting (iii) Evaporation (iv) Decantation
Answer

Answer: (ii) Sorting

Why: handpicking means picking pieces out of a mixture with the fingers, on the basis of size, colour and shape. That is exactly what sorting means — arranging or separating things by looking at them. Removing stones from rice, or chhole from rajma, is sorting by hand.

Why the others are wrong:

  • (i) Filtration uses a filter with pores to separate an insoluble solid from a liquid — no hands involved.
  • (iii) Evaporation turns a liquid into vapour to recover a dissolved solid.
  • (iv) Decantation is pouring off a liquid after the solid has settled.
Remember: handpicking is convenient only when the unwanted component is present in a small quantity and the pieces are big enough to pick up.
Q2.
Which of the following substances are commonly separated using the churning method? (i) Oil from water (ii) Sand from water (iii) Cream from milk (iv) Oxygen from air
Answer

Answer: (iii) Cream from milk

Why: churning beats the liquid so that the tiny fat globules join into bigger lumps. Since fat is lighter (less dense) than the watery part, the lumps float up and can be taken off — this is how cream and butter are separated from milk or curd.

Why the others are wrong:

  • (i) Oil from water — the two form separate layers on their own; decantation is used, not churning.
  • (ii) Sand from water — sand is an insoluble solid; sedimentation and decantation, or filtration, are used.
  • (iv) Oxygen from air — air is a mixture of gases; you cannot churn a gas apart.
Q3.
Which factor is usually essential for the filtration? (i) Apparatus size (ii) Presence of air (iii) Pore size (iv) Temperature of the mixture
Answer

Answer: (iii) Pore size

Particle bigger than the pore → held back as residue
Particle smaller than the pore → goes through with the liquid, into the filtrate
Why: a filter is simply a barrier full of holes. Whether it separates anything depends entirely on how the size of its pores compares with the size of the particles. That is why a tea strainer cannot clean muddy water but a filter paper can — the chapter shows this with the tea strainer, the cloth and the filter paper.

Why the others are wrong: the size of the apparatus only decides how much you can filter at a time; air plays no part; and although warm liquid flows a little faster, temperature does not decide whether the particles are stopped.

Q4.
State with reason(s) whether the following statements are True [T] or False [F]. Also, correct the False statement(s). (i) Salt can be separated from salt solution by keeping it under the Sun. (ii) Handpicking should be used only when the quantity of one component is less. (iii) A mixture of puffed rice and rice grains can be separated by threshing. (iv) A mixture of mustard oil and lemon water can be separated by decantation. (v) Sieving is used to separate a mixture of rice flour and water.
Answer
StatementT / FReason
(i) Salt can be separated from salt solution by keeping it under the Sun.TrueThe Sun’s heat makes the water evaporate and the salt is left behind. This is how salt is made from seawater in salt pans.
(ii) Handpicking should be used only when the quantity of one component is less.TrueIf there is a lot of it, picking piece by piece takes far too long — as Valli realises about the husk.
(iii) A mixture of puffed rice and rice grains can be separated by threshing.FalseThreshing only beats grain off stalks. Puffed rice and rice grains are already loose.
(iv) A mixture of mustard oil and lemon water can be separated by decantation.TrueOil does not mix with water and is lighter, so it forms a clear upper layer that can be poured off.
(v) Sieving is used to separate a mixture of rice flour and water.FalseSieving works only on a solid–solid mixture. Here one component is a liquid.

Corrected false statements:

  • (iii) A mixture of puffed rice and rice grains can be separated by winnowing (the light puffed rice is blown away) or by sieving, since the two differ in weight and in size.
  • (v) Filtration is used to separate a mixture of rice flour and water, because rice flour is insoluble in water and is held back on the filter.
Tip: before choosing a method, ask two questions — are both components solid? and does one of them dissolve? The answers rule out most of the wrong methods at once.
Q5.
Match the mixtures in Column I with their method of separation in Column II. Column I: (i) Gram flour mixed with black gram (ii) Chalk powder mixed with water (iii) Corn mixed with potatoes (iv) Iron powder mixed with sawdust (v) Oil mixed with water. Column II: (a) Handpicking (b) Magnetic separation (c) Decantation (d) Sieving (e) Filtration
Answer
Column IColumn IIProperty used
(i) Gram flour mixed with black gram(d) SievingFlour particles are tiny, the gram is large — difference in particle size
(ii) Chalk powder mixed with water(e) FiltrationChalk is insoluble in water and is bigger than the pores of the filter
(iii) Corn mixed with potatoes(a) HandpickingBoth are large and easily told apart by size, colour and shape
(iv) Iron powder mixed with sawdust(b) Magnetic separationIron is attracted to a magnet, sawdust is not — difference in magnetic property
(v) Oil mixed with water(c) DecantationThe two do not mix; oil is lighter and floats as a separate layer
(i) → (d)  ·  (ii) → (e)  ·  (iii) → (a)  ·  (iv) → (b)  ·  (v) → (c)
Why chalk powder needs filtration and not decantation: chalk does settle if left alone, so you could decant most of the water. But very fine chalk stays suspended and clouds the water. Filtration removes it completely — decantation never does.
Q6.
In what situations would you use decantation instead of filtration to separate solids from liquids?
Answer

Decantation is chosen when the solid settles by itself and complete separation is not needed.

  • When the solid is heavy and insoluble and sinks quickly — mud in pond water, tea leaves in a pan, sand in a bucket.
  • When there is a large quantity of liquid — filtering a whole drum of muddy water through paper would take hours; letting it settle and pouring it off takes minutes.
  • When no filter is available — Dada has no strainer, so he lets the leaves settle and pours the tea out gently.
  • When a filter would get choked — a lot of thick mud would clog a filter paper at once, so the mud is settled and decanted off first, and only then is the water filtered.
  • When the two components are liquids that do not mix — oil and water, or cream floating on milk. A filter cannot separate two liquids at all.
  • When we only want to rinse something — washing rice or dal: the dirt and husk float or settle and the water is poured away.
The honest limitation: decantation is quick and free, but it is never complete. Fine particles stay behind in the liquid. When you need the liquid really clear — drinking water, a chemical in the laboratory — you must filter.
Best practice: in real life the two are used together — sedimentation → decantation → filtration. That is what Leela does in Question 10, and what a water treatment plant does on a huge scale.
Q7.
Can you relate the presence of nasal hair to any separation process?
Answer

Yes — nasal hair works exactly like a filter, so the process is filtration.

Air breathed in = clean air + dust, pollen, smoke particles, tiny insects
The hairs in the nose act as a filter with fine gaps
→ big particles are trapped on the hairs (the residue)
→ cleaner air goes down to the lungs (the filtrate)
Why the body needs it: the lungs are extremely delicate. Dust reaching them would irritate and damage them. The nasal hairs, helped by the sticky mucus lining, catch most of the dust right at the entrance. The nose also warms and moistens the air on the way in.
Do not pluck nasal hair. It is doing a job for you. This is also why we are told to breathe through the nose, not the mouth — the mouth has no such filter, which is why dust reaches the throat faster when we run with our mouth open.
Q8.
During the COVID-19 pandemic, all of us wore masks. Generally, what material are they made of? What is the role of these masks?
Answer

What they are made of:

Type of maskMaterial
Home-made cloth maskTwo or three layers of tightly woven cotton or muslin
Surgical (three-ply) maskThree layers of non-woven polypropylene fabric
N95 respiratorSeveral layers of very fine synthetic fibre mesh

The role of the mask — it acts as a filter, in both directions:

  • Going out: when an infected person coughs, sneezes, speaks or breathes, tiny droplets carrying the virus are thrown out. The mask traps these droplets so they do not reach other people.
  • Coming in: the mask stops droplets, dust and germs from the air outside from reaching the wearer’s nose and mouth.
  • It also stops us from touching the nose and mouth with unwashed hands.
Why more layers work better: the pores of one layer do not line up with the pores of the next, so a droplet that slips past the first is caught by the second or third — the same reason several layers of cloth filter muddy water better than one, as Malli learns on page 171.
Did you know? Masks are our own version of nasal hair. Both are filters: they use the fact that the harmful particles are bigger than the gaps in the material.
Q9.
A mixture containing potatoes, salt and sawdust has been given to you. Outline a stepwise procedure for separating each component from this mixture.
Answer

The three components differ in three ways — size, whether they float, and whether they dissolve. Use one property at each step.

StepWhat to doMethodWhat you get
1Pick out the potatoes with your hand — they are much bigger than everything else.HandpickingPotatoes separated
2Add plenty of water to the remaining salt + sawdust and stir. The salt dissolves; the sawdust floats on top.DissolvingSalt solution with sawdust floating
3Skim off the floating sawdust, or pour the mixture through a filter paper.Decantation / FiltrationSawdust separated (dry it in the sun)
4Heat the clear salt solution, or keep it in the sun, until all the water has gone.EvaporationSalt left behind as a white solid
Potatoes + salt + sawdust
—[handpicking]→ potatoes + (salt + sawdust)
—[add water, filter]→ sawdust + salt solution
—[evaporation]→ salt
Why this order: always remove the largest and easiest component first, and add water only after that. If you added water at the start, the potatoes would get soggy and the sawdust would stick to them. Also note that salt must be taken out last, because evaporation destroys nothing but does need a clear solution.
Remember: a mixture with more than two components almost always needs a combination of methods — that is the hint the book gives at the end of the chapter.
Q10.
Read the following story titled ‘Intelligent Leela’ and tick the most appropriate options. Provide a suitable title of your choice for the paragraph. — Leela was working in the farm with her father when she realised that they left their drinking water at home. Before her father felt thirsty/hungry, she went to the nearby pond to fetch some water/grains. After obtaining some water in the container, she noticed that the water was muddy and fit/unfit for drinking. To purify the water, she kept it for some time and then she filtered/churned the muddy water using a piece of paper/muslin cloth. Leela, then, cooled/boiled the water for about 10 minutes in a covered pan. After cooling/boiling, she filtered/churned it again and made it fit/unfit for drinking. She served this water to her father while having food, who blessed her and appreciated her efforts.
Answer

The completed paragraph (the correct choice is shown in bold):

Leela was working in the farm with her father when she realised that they left their drinking water at home. Before her father felt thirsty, she went to the nearby pond to fetch some water. After obtaining some water in the container, she noticed that the water was muddy and unfit for drinking. To purify the water, she kept it for some time and then she filtered the muddy water using a piece of muslin cloth. Leela, then, boiled the water for about 10 minutes in a covered pan. After boiling, she filtered it again and made it fit for drinking.

ChoiceCorrect optionReason
thirsty / hungrythirstyThe problem was the forgotten drinking water
water / grainswaterA pond gives water, not grain
fit / unfitunfitMuddy water is not safe to drink
filtered / churnedfilteredChurning separates butter from curd; here mud must be strained out
paper / muslin clothmuslin clothOrdinary paper tears and would not let water through; muslin has fine pores
cooled / boiledboiledBoiling for about 10 minutes kills the germs
cooling / boilingboilingIt follows the previous step
filtered / churnedfilteredTo remove anything settled after boiling
fit / unfitfitThe water is now clean and germ-free

Suitable titles you could give: “Leela Makes the Water Safe”, “From a Muddy Pond to a Clean Glass”, “A Daughter’s Clever Idea”, or “Sedimentation, Filtration, Boiling”.

Why Leela is called intelligent: she used three ideas in the right order — she first let the water stand so the mud settled (sedimentation), then strained it through cloth (filtration), and finally boiled it, because filtering removes only what is big enough to be trapped and germs are far smaller than that.
Tip: the pan was kept covered while boiling. That saves water — the steam condenses on the lid and falls back instead of escaping.
Was this helpful? Report an error