Keep the Curiosity Alive — Chapter Exercises — Particulate Nature of Matter

1. 13 marksCuriosity Grade 8, Chapter 7, page 113, exercise 1

   Choose the correct option. The primary difference between solids and liquids is that the constituent particles are:

   (i) closely packed in solids, while they are stationary in liquids. (ii) far apart in solids and have fixed position in liquids. (iii) always moving in solids and have fixed position in liquids. (iv) closely packed in solids and move past each other in liquids.

   Hint. Three of the four options describe liquids as more fixed than solids.

   Answer: (iv) closely packed in solids and move past each other in liquids.

   Why it is right. In solids the particles are tightly packed and cannot move past each other. In liquids the particles are free to move, though within a limited space. That single difference — whether particles can change places — is what makes a solid keep its shape and a liquid take the shape of its container.

   Why the others are wrong:

   Option	Error
   (i) stationary in liquids	Liquid particles are not stationary — they move freely within a limited space. If they were still, a liquid could not flow
   (ii) far apart in solids	Reversed. Solids have the minimum interparticle spacing of the three states
   (iii) always moving in solids and have fixed position in liquids	Completely reversed. Solid particles are in fixed positions and only vibrate; liquid particles are the ones that move about

   ✦ Notice the pattern in the wrong answers. Options (i), (ii) and (iii) all make the liquid sound more fixed or more ordered than the solid. If you know only one thing about this comparison — that going solid → liquid → gas means more spacing, weaker attraction and more movement — you can eliminate all three at a glance.

   The one thing solids and liquids share: both have a definite volume. The difference is entirely about shape, and shape comes from whether particles can move past one another.

2. 26 marksCuriosity Grade 8, Chapter 7, pages 113-114, exercise 2

   Which of the following statements are true? Correct the false statements.

   (i) Melting ice into water is an example of the transformation of a solid into a liquid. (ii) Melting process involves a decrease in interparticle attractions during the transformation. (iii) Solids have a fixed shape and a fixed volume. (iv) The interparticle interactions in solids are very strong, and the interparticle spaces are very small. (v) When we heat camphor in one corner of a room, the fragrance reaches all corners of the room. (vi) On heating, we are adding energy to the camphor, and the energy is released as a smell.

   Hint. Five are straightforward. The sixth confuses two quite different things.

   (i) True. Ice is solid water and it melts into liquid water — the standard example of a solid → liquid change, at the melting point of 0 °C (Table 7.1).

   (ii) True. A stage is reached when these vibrations become so vigorous that the particles start leaving their respective positions. The interparticle forces of attraction get weakened and the solid gets converted into the liquid state. Melting is exactly a weakening of interparticle attractions.

   (iii) True. They have a definite shape and volume — because the particles are tightly packed and held in fixed positions.

   (iv) True. In solids, the particles are tightly packed and the interparticle attractions are very strong, and the interparticle spacing is the minimum of the three states.

   (v) True. Exactly what Activity 7.9 shows with an incense stick: the particles spread through the room, helped along by constantly moving air particles.

   (vi) FALSE. ✦ Corrected: On heating, we are adding energy to the camphor, and this energy allows its particles to overcome the interparticle attractions and escape as vapour. The smell is caused by those camphor particles spreading through the air and reaching our nose — not by energy being released as a smell.

   Why (vi) is the interesting one. It confuses energy with matter. A smell is not a form of energy travelling across a room; it is actual particles of a substance arriving at your nose. The heat is absorbed by the camphor, not released as something else. Getting this right matters, because the whole chapter is about matter being made of particles that go places — and a smell is the plainest everyday evidence of that.

   Notice that (v) and (vi) describe the same event. (v) states what happens and is true; (vi) offers an explanation and is wrong.

3. 33 marksCuriosity Grade 8, Chapter 7, page 114, exercise 3

   Choose the correct answer with justification. If we could remove all the constituent particles from a chair, what would happen?

   (i) Nothing will change. (ii) The chair will weigh less due to lost particles. (iii) Nothing of the chair will remain.

   Hint. Ask what the chair is, apart from its constituent particles.

   Answer: (iii) Nothing of the chair will remain.

   Justification. A constituent particle is the basic unit that makes up a larger piece of a substance or material, and the chapter's central claim is that matter is composed of a large number of extremely small particles. The chair is nothing but its constituent particles arranged in a particular way. Take every one of them away and there is no wood, no shape, no weight — nothing at all.

   Why the other two are wrong:

   Option	Error
   (i) Nothing will change	Treats the particles as an optional extra inside the chair. There is no chair apart from them
   (ii) The chair will weigh less	Imagines the particles as some of the chair, so that removing them leaves a lighter chair behind. But all the particles are being removed, and they are all there is

   ✦ Option (ii) is the revealing wrong answer, because it is the one a student picks who has half-understood the chapter. It treats constituent particles as something matter contains — like raisins in a cake — rather than what matter is. The chapter's chalk argument was designed to rule exactly this out: grind chalk far enough and you do not find particles hidden inside chalk, you find that chalk was those particles all along.

   A way to test your understanding: ask what would be left of a glass of water if all its constituent particles were removed. Not an empty glass of water — nothing whatever.

4. 44 marksCuriosity Grade 8, Chapter 7, page 114, exercise 4

   Why do gases mix easily, while solids do not?

   Hint. Three differences, and all three point the same way.

   Because gas particles are free to move through large spaces, while solid particles are locked in place.

   	Gases	Solids
   Interparticle attraction	Negligible	Maximum — very strong
   Interparticle spacing	Maximum	Minimum
   Movement of particles	In all the available space	Negligible (only vibrations)
   Result	Particles of two gases travel freely into each other's spaces and mix on their own	Particles cannot leave their fixed positions, so they cannot mix

   Gases mix without any help. Activity 7.9 shows it: the fragrance of an incense stick reaches every corner of a room with nobody fanning it, because the particles of air are moving constantly and knock the fragrance particles along. Activity 7.5 shows the same thing with smoke filling a second gas jar.

   Solids cannot do this. Their particles are held in fixed positions, preventing them from moving freely, and can only move to and fro about their positions ... but cannot move past each other. A particle that cannot leave its own position certainly cannot travel into another solid.

   ✦ Which is why we grind, powder and dissolve solids to mix them. Grinding does not make the particles mobile — it just makes the pieces smaller, so more surfaces touch. Genuine mixing at the particle level needs the particles to move, which is why solids are usually melted or dissolved first. Cement, paint and alloys are all made this way.

   Liquids sit in between, which is why potassium permanganate spreads through a tumbler of water in Activity 7.8 — slowly, and only because the water particles are moving.

5. 54 marksCuriosity Grade 8, Chapter 7, page 114, exercise 5

   When spilled on the table, milk in a glass tumbler flows and spreads out, but the glass tumbler stays in the same shape. Justify this statement.

   Hint. One is a liquid and one is a solid — say what that means at particle level.

   The milk is a liquid; the tumbler is a solid. The difference is whether their particles can move past one another.

   The milk flows and spreads. In liquids the interparticle attractions are slightly weaker than in solids, but still strong enough to keep the particles close together, and the particles of liquids are free to move within a limited space. Because they can slide past one another, they rearrange to fit whatever holds them — and on an open table nothing holds them, so the milk spreads out. Liquids have no fixed shape but have a fixed volume: the puddle is a new shape, not a new amount.

   The tumbler keeps its shape. In solids the particles are tightly packed and the interparticle attractions are very strong. These strong forces of attraction hold the particles in fixed positions, preventing them from moving freely — they cannot move past each other. So the arrangement cannot be rearranged, and the tumbler has a definite shape and a definite volume.

   	Milk	Tumbler
   State	Liquid	Solid
   Interparticle attraction	Slightly weaker	Very strong
   Particles move past each other?	Yes	No
   Shape	Takes whatever shape is available	Definite
   Volume	Definite	Definite

   ✦ Both have a definite volume, and that is worth saying explicitly in your answer. The spilled milk has not become more milk. What it has lost is a shape of its own — and that is the only property in which the two differ.

   The everyday test from page 104: you can move a finger through milk and it closes up again; you cannot move a finger through glass at all.

6. 64 marksCuriosity Grade 8, Chapter 7, page 114, exercise 6

   Represent diagrammatically the changes in the arrangement of particles as ice melts and transforms into water vapour.

   Hint. Three boxes, and label what changes between each pair.

   Draw three boxes in a row, each containing circles for particles:

   Box	State	How to draw the particles	Label
   1	Ice (solid)	Many circles in a regular, closely packed arrangement, touching or nearly touching, in neat rows	Minimum spacing · strongest attraction · particles only vibrate
   2	Water (liquid)	The same number of circles, close together but irregular, no fixed pattern	A little more spacing · attraction slightly weaker · particles move within a limited space
   3	Water vapour (gas)	The same number of circles, far apart and scattered, filling the whole box, with arrows to show motion	Maximum spacing · negligible attraction · particles move freely in all directions

   Between the boxes, mark the changes:

   Box 1 → Box 2: melting, at the melting point (0 °C for ice)

   Box 2 → Box 3: boiling, at the boiling point

   ✦ Three things must be true of a good diagram, and all three are marked:

   1. The same number of particles in every box. Melting and boiling rearrange particles; they do not create or destroy them. Drawing fewer particles in the gas box is the commonest mistake.
   2. The particles are the same size in every box. They move further apart — they do not swell.
   3. The spacing increases from left to right, and the arrangement becomes less ordered.

   One honest complication about ice, worth adding as a note: the chapter says ice is an exception — its particles are farther apart than those in water. So strictly, ice's particles are more spread out than liquid water's, even though ice is the solid. Mention it beside your diagram; the general pattern still holds for the liquid → gas step, which is by far the bigger change.

7. 74 marksCuriosity Grade 8, Chapter 7, page 114, exercise 7

   Draw a picture representing the particles present in the following: (i) Aluminium foil, (ii) Glycerin, (iii) Methane gas.

   Hint. First decide which state each one is, then draw the standard picture for that state.

   Step 1 — identify the state of each.

   	Substance	State	How you can tell
   (i)	Aluminium foil	Solid	It has a definite shape and volume; aluminium appears among the solid objects of Activity 7.3
   (ii)	Glycerin	Liquid	It flows and takes the shape of its container, but has a definite volume
   (iii)	Methane gas	Gas	Named as a gas; it has neither fixed shape nor fixed volume

   Step 2 — draw the standard picture for each state (following Fig. 7.12):

   • (i) Aluminium foil: circles packed closely and regularly in neat rows, touching or nearly touching, filling the box. Minimum spacing.
   • (ii) Glycerin: circles close together but irregularly arranged, with slightly more space between them than in the solid, no fixed pattern. Fills the lower part of a container drawn around them.
   • (iii) Methane gas: only a few widely separated circles, scattered right across the box with large empty spaces, arrows showing motion in all directions.

   ✦ The question is really testing whether you can name the state, not whether you can draw. Once you have decided solid, liquid, gas, the three pictures are the ones the chapter has already given you in Fig. 7.12. Aluminium foil is the one that catches people out — foil is thin and flexible, so it feels different from a block of metal, but it is just as much a solid, with a definite shape and volume.

   Keep the particles the same size in all three drawings, and change only their spacing and arrangement. That is what distinguishes the states.

8. 84 marksCuriosity Grade 8, Chapter 7, page 114, exercise 8

   Fig. 7.16a shows a candle just extinguished after burning for some time. Identify the different states of wax in the figure and match them with the particle arrangements in Fig. 7.16b.

   Hint. One candle, three states of the same substance, all visible at once.

   The three states of wax in the photograph:

   State	Where you see it in Fig. 7.16a
   Solid wax	The body of the candle, and the hardened drips that have run down and set on the sides
   Liquid wax	The molten pool around the wick at the top, and any drips still running
   Wax vapour	The wisp rising from the wick, visible because the candle was just extinguished

   Matching with the three particle boxes of Fig. 7.16b:

   Wax	Particle picture	Why
   Solid	The box with particles regularly and closely packed in neat rows	Tightly packed, minimum spacing, held in fixed positions
   Liquid	The box with particles close together but irregularly arranged	Attractions slightly weaker; a little more spacing; particles move within a limited space
   Vapour	The box with only a few widely scattered particles	Negligible attraction, maximum spacing, free movement

   ✦ The point of the question is that all three states are the same substance. Nothing has been added to the candle and nothing chemical has changed the wax — the flame simply supplied thermal energy, and it is the thermal energy of the particles that determines the physical state of matter. Wax near the flame got enough energy to melt and then to vaporise; wax further down the candle never did.

   Why the candle must be just extinguished. Only then is the vapour still visible rising from the wick. A moment later it has spread through the room — exactly as the incense fragrance does in Activity 7.9 — and you would see only two states.

9. 94 marksCuriosity Grade 8, Chapter 7, page 114, exercise 9

   Why does the water in the ocean taste salty, even though the salt is not visible? Explain.

   Hint. Activity 7.2 answered this question with sugar in a tumbler.

   Because the salt has dissolved into its constituent particles, which are far too small to see but are spread throughout the water.

   The explanation, following Activity 7.2:

   1. When salt dissolves, it breaks up into its constituent particles which cannot be broken down further.
   2. These particles separate and occupy the available spaces between the water particles — the interparticle spaces.
   3. The particles are so small that they cannot be seen even through an ordinary microscope, so nothing is visible.
   4. They are nevertheless present everywhere in the water, so their presence can be sensed by taste.

   ✦ This is exactly the sugar experiment, at the scale of an ocean. In Activity 7.2 you tasted the top layer of a tumbler and found it sweet although no sugar could be seen. The ocean is the same result, and the same reasoning: invisible is not the same as absent.

   Why the salt is spread evenly rather than sitting at the bottom. Activity 7.8 supplies the answer: the water particles are in constant motion, and they first pull out the particles ... from its grain, and later hit these particles so that they get spread throughout the liquid. Ocean currents help, but the particle-level spreading happens even in a still tumbler.

   And a contrast worth adding: sand thrown into water settles to the bottom and stays visible, because the constituent particles are held together strongly that the water particles are unable to pull these out. Salt behaves differently because its particles can be pulled apart by water.

10. 105 marksCuriosity Grade 8, Chapter 7, page 114, exercise 10

    Grains of rice and rice flour take the shape of the container when placed in different jars. Are they solids or liquids? Explain.

    Hint. Ask what is taking the container's shape — the grains, or the heap of them?

    They are solids.

    The mistake the question invites. Liquids take the shape of their container, and rice poured into a jar does seem to take the jar's shape. But look at what is actually changing shape.

    Each individual grain of rice keeps its own shape exactly. Pour rice from a tall jar into a wide bowl and every single grain is the same shape and size it was before. Nothing about any grain has changed — the grains have simply rearranged themselves, sliding past one another and settling into the new container.

    	A rice grain	A liquid
    Does the individual unit change shape?	No	Yes — a liquid has no shape of its own
    What moves?	Whole grains slide past each other	The constituent particles move past each other
    Definite shape?	Each grain, yes	No
    Definite volume?	Yes	Yes

    The same for rice flour, only more convincingly — the particles are finer, so the heap flows more easily and looks even more liquid-like. But each speck of flour is still a tiny solid with its own definite shape, exactly as each speck of ground chalk in Activity 7.1 was still a speck of chalk.

    ✦ The confusion is between a grain and a heap. The heap takes the container's shape; the grains do not. In a real liquid there is no such distinction — the constituent particles themselves are what rearrange, and there is nothing smaller that keeps a shape.

    A test that settles it. Try to move a finger through rice: the grains part and stay parted. Do the same in water and as soon as you remove your finger, the position of the water is restored. Pick out a single grain and it has a definite shape you can examine. None of this can be done with a liquid.

    This is the same confusion as the very first Probe-and-ponder question, about why sand can be piled but water cannot.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity, Textbook of Science for Grade 8, Chapter 7 'Particulate Nature of Matter', book pages 98-115 (hecu107.pdf, 18 pages, Reprint 2026-27), downloaded from ncert.nic.in and read page by page. Every activity number, figure number, quantity and quoted sentence below was checked against that PDF. Fig. 7.16 (exercise 8) was rendered and read: 7.16a shows a just-extinguished candle with solid wax in the body and set drips, liquid wax pooled round the wick, and vapour rising from the wick; 7.16b gives three particle boxes - regular close-packed (solid), close but irregular (liquid), and a few widely scattered particles (gas). Table 7.1's melting points were transcribed exactly: ice 0 C, urea 133 C, iron 1538 C. Three deliberate restraints. (1) The chapter covers ONLY melting point and boiling point, with evaporation mentioned in two sentences as prior learning. Sublimation, deposition, condensation, freezing, the factors affecting evaporation, evaporative cooling and the effect of pressure on state changes appear NOWHERE in this book and appear nowhere here. (2) The Acharya Kanad box gives no date - the chapter says only 'ancient Indian philosopher' and names the Vaisheshika Sutras - so no century is supplied, and the solution notes explicitly that dating estimates differ and would need a cited source. (3) Atoms and molecules are introduced only as the chapter's final 'A step further' box introduces them, with its own closing limit quoted: 'You will learn about atoms and molecules in higher grades.' Nothing about atomic structure or chemical formulae is added.. Questions are referenced from the NCERT textbook for identification.