Where Mindful Learning Begins
Where Mindful Learning Begins
Class 8 Science Curiosity NCERT Solutions Chapter 7 — every Probe and Ponder prompt, all 9 Activities, all 10 "Keep the Curiosity Alive" exercise questions, and all 4 "Discover, Design, and Debate" project prompts, solved and explained on one page.
This chapter builds the idea that all matter — chalk, sugar, water, and air alike — is made of extremely tiny constituent particles, held together by interparticle attraction. It uses this single idea to explain why solids, liquids, and gases behave so differently, and why dissolving, melting, boiling, and the spreading of a fragrance all come down to how particles move and space themselves out.
Chapter 7 opens by asking where pebbles, stones, and sand actually come from, and uses this to ask a deeper question: can matter be broken down forever, or does it eventually reach a smallest unit? Breaking and grinding chalk, and dissolving sugar in water, are used to introduce constituent particles — the tiny building blocks of every substance — and the interparticle spaces and interparticle attraction that exist between them. The chapter then uses this single idea to explain why solids have a fixed shape, why liquids flow but keep a fixed volume, and why gases spread to fill any space available to them, before showing how melting, boiling, evaporation, dissolving, and diffusion (like a fragrance spreading through a room) are all just different ways particles rearrange, move apart, or move faster. Every Activity and exercise is solved here exactly as the textbook presents it.
Every substance is built from huge numbers of tiny constituent particles, far too small to see even under an ordinary microscope.
Attractive forces hold particles together — how strong these forces are decides whether a substance is a solid, liquid, or gas.
Interparticle spacing and freedom of movement differ across the three states — from tightly vibrating to freely flying apart.
Constituent particles, interparticle spaces & interparticle attraction
Comparing the three states of matter
| Solid | Liquid | Gas | |
|---|---|---|---|
| Interparticle spacing | Minimum | A little more than in solids | Maximum |
| Packing of particles | Closely packed | A little loosely packed | Free, spread apart |
| Interparticle attraction | Maximum (strongest) | Slightly weaker than solids | Minimum (negligible) |
| Movement of particles | Negligible — only vibrate in place | Restricted to a limited space | Free, in every available direction |
| Shape & volume | Fixed shape and fixed volume | No fixed shape, but fixed volume | No fixed shape and no fixed volume |
Liquids and gases both flow and take the shape of their container, so together they are classified as fluids — a category that sets them apart from solids.
Melting point & boiling point
The melting point is the minimum temperature at which a solid turns into a liquid at atmospheric pressure. The boiling point is the temperature at which a liquid rapidly turns to vapour, both at the surface and within the liquid (seen as bubbles), at atmospheric pressure.
| Material | Melting point |
|---|---|
| Ice | 0 °C |
| Urea | 133 °C |
| Iron | 1538 °C |
Evaporation is a slower version of the same particle-escape process, but it happens at the surface only and at any temperature — even well below the boiling point. This is why a puddle of spilled water eventually disappears without ever being heated to boiling.
Note: generally, liquid particles are a little farther apart than in the solid state — but ice is an exception, since its particles are actually farther apart than in liquid water.
Sample answer: stones and sand can be piled up because they are solids — each individual grain keeps its own fixed shape, so a heap of grains simply rests on top of one another and holds a mound shape. Water, though made of a huge number of particles too, is a liquid — its particles are free to move past each other, so it cannot hold a heap shape and instead spreads and flows flat, or takes on the shape of whatever is holding it, like a pair of cupped hands, and loses that shape the instant it's released, since nothing is left to contain it.
Even though air is invisible, it is still matter made up of constituent particles, and every particle has some mass. When a balloon is inflated, more and more of these tiny air particles are packed inside it, and their combined mass adds a small but real amount of extra weight to the balloon — we just can't see the particles causing it. Whether today's air is identical to the air from thousands of years ago is something science can actually investigate (for example, using trapped air bubbles in ancient ice) — Activity-based reasoning in this chapter won't answer this directly, but it's a great question to explore as you learn more about gases and the atmosphere in later chapters.
This is an open reflection prompt meant to set up the chapter's central ideas — constituent particles, interparticle attraction, and the three states of matter — all explained in full through the Activities below.
Observation: even after being broken again and again and ground into a very fine powder, every tiny grain seen under the magnifying glass is still recognisably chalk.
Answer (recalling physical vs chemical change): grinding chalk is a physical change, not a chemical change — the chalk never turns into a new substance; only the size of each individual speck keeps getting smaller, while its identity as chalk stays exactly the same.
Embedded question — are these the smallest units of chalk? If we imagine continuing this grinding process indefinitely, we would eventually reach tiny units that simply cannot be broken down any further by this method. These smallest units are the basic building blocks that the whole piece of chalk was made up of, called its constituent particles — the basic unit that makes up a larger piece of a substance. Just like chalk, the grains of sand and clay are also not the smallest units of bigger rocks; they too are made up of huge numbers of their own constituent particles.
Observation: before stirring, the top layer of water does not taste sweet, since the sugar simply sits undissolved at the bottom. After stirring until the sugar dissolves completely, the top layer of water does taste sweet — even though no sugar can be seen anywhere in the glass.
Answer: since the water tastes sweet throughout but no visible sugar grains remain, the sugar must still be present — broken up into constituent particles far too tiny to see, though their presence can still be sensed by taste. Each original grain of sugar is itself made up of millions and millions of such constituent particles.
Embedded question — where did the sugar go? The sugar's constituent particles haven't disappeared; they've simply spread out and settled into the empty gaps that exist between the water's own particles. These gaps are called interparticle spaces, and it's here that the dissolved sugar particles occupy space, spreading evenly enough to make the entire glass of water taste uniformly sweet.
Observation: all six objects are solids with a definite shape and volume of their own, and hammering them does not easily change that shape.
Answer: this shows that in solids, particles are tightly packed together with very strong interparticle attractions holding them in fixed positions — they can only vibrate (move to and fro) about those positions, never sliding past one another. Among the six, the metal objects (the iron nail and the piece of aluminium) generally have among the strongest interparticle attractions of common solids, which is reflected in metals typically having high melting points.
Embedded question — can these particles ever be moved apart? Yes — heating a solid makes its particles vibrate more and more vigorously. Eventually the vibrations become strong enough that particles break free of their fixed positions, weakening the interparticle attraction until the solid turns into a liquid. The specific temperature at which this happens (at atmospheric pressure) is called the solid's melting point — for example, ice melts at 0 °C, urea at 133 °C, and iron only at a much higher 1538 °C, showing just how much the strength of interparticle attraction can vary between substances.
Observation: the water takes on the exact shape of whichever container it is poured into, yet its level lines up with the same 200 mL mark in each of the three differently shaped containers (allowing for a negligible amount that may stick to the walls of an unclean container).
Answer: liquids have no fixed shape of their own — they take the shape of whatever container holds them — but they do have a fixed (definite) volume, since the same amount of water measures out to the same volume no matter the container's shape. This happens because the particles of a liquid are free to move around, but only within a limited, bounded space.
Going further — comparing to solids (moving a finger through water): unlike with a solid, you can push a finger straight through standing water without permanently damaging it — the water is simply and temporarily displaced, settling back into place the instant the finger is removed. This shows that interparticle attraction in liquids, while still strong enough to keep particles fairly close together, is somewhat weaker than in solids, letting particles slip past each other.
Going further — boiling point and evaporation: heating a liquid enough makes its particles move so vigorously that they overcome this (already weaker) interparticle attraction and escape the liquid altogether as vapour — this is called boiling, and the specific temperature at which it happens (at atmospheric pressure) is the liquid's boiling point, marked by bubbles forming throughout the liquid, not just at its surface. Below the boiling point, a slower version of the same escape happens only at the surface, called evaporation — which is exactly why spilled water on a table eventually disappears even without being heated at all.
Observation: the trapped smoke does not stay confined to Gas Jar A — as soon as the gap between the two jars is opened, it spreads out and fills the entire available space of Gas Jar B as well.
Answer: gases have neither a fixed volume nor a fixed shape of their own — they expand to occupy all the space made available to them, and (like liquids) they take on the shape of whatever container holds them. This happens because gas particles move freely in every direction, and the interparticle attraction between them is negligible — far weaker than in liquids or solids. (This activity works equally well using iodine vapour released from solid iodine kept in a closed jar, instead of incense smoke.)
Fluids: since both liquids and gases flow and don't hold a fixed shape of their own, they are together classified as fluids, setting them apart from solids.
With air: pushing the plunger noticeably decreases the volume of the trapped air inside the syringe, and letting go allows the plunger to spring back close to its original position.
Answer: this shows that gas particles have a lot of empty space between them in their natural state, and pressing the plunger forces these particles closer together, compressing that space — in other words, gases are easily compressible.
With water: repeating the same push with water inside the syringe instead of air shows that the plunger barely moves at all — water is practically incompressible, since liquid particles already sit fairly close together, leaving very little space left to compress any further.
Observation: adding sugar (before dissolving) raises the water level from mark A to mark B, since the undissolved sugar grains take up their own separate space. After stirring until the sugar fully dissolves, the water level (mark C) settles slightly lower than mark B, though usually still a little above the original mark A.
Answer: since the final volume of the sugar solution is less than the combined volume of the water and solid sugar added, this shows that water particles are not perfectly packed together — there is some space between them (interparticle space) — and the dissolved sugar's constituent particles slot into these empty spaces rather than simply adding on top of the water's existing volume.
Repeating with common salt or glucose (soluble solids): gives a similar result — the level rises less after dissolving than it did right after adding the solid, since these particles also settle into the water's interparticle spaces.
Repeating with sand or stone pieces (insoluble solids): gives a different result — since sand does not dissolve in water at all, its particles remain undissolved and simply settle at the bottom, continuing to occupy their own separate space. So the total volume (and water level) stays raised at the higher level it reached right after the sand was added, and does not decrease afterwards the way it does with a true solution.
Embedded question — why does sugar dissolve but not sand? This comes down to the relative strength of interparticle forces. In sugar, the water particles are able to overcome the interparticle attraction holding sugar's constituent particles together, pulling them apart and letting them mix throughout the water. In sand, the constituent particles are held together by much stronger interparticle forces that water particles simply cannot overcome, so sand cannot be pulled apart into the water and remains undissolved.
Observation: pink streaks initially spread out from the grains, and after some time, without any stirring at all, the entire glass of water turns a uniform pink colour throughout.
Answer: water particles are in constant, random motion. They first pull the potassium permanganate's constituent particles away from the solid grain, and then keep striking and jostling these particles, spreading them evenly throughout the entire volume of water over time.
Going further — Think Like a Scientist (hot, room-temperature, and ice-cold water): repeating this experiment with a grain of potassium permanganate dropped into hot water, water at room temperature, and ice-cold water shows that the colour spreads fastest in hot water, more slowly at room temperature, and slowest of all in ice-cold water. This shows that heating a liquid makes its particles move faster (spreading dissolved particles more quickly), while cooling slows their movement down — in short, particle movement increases as more heat (thermal energy) is supplied.
Observation: at first, the fragrance can only be smelled right around the burning incense stick, but within a few minutes it can be smelled throughout the entire room, even far from the stick itself.
Answer: this shows that the invisible particles of air in the room are constantly moving. These moving air particles repeatedly strike the fragrance particles released by the incense stick, helping carry and spread them throughout the whole room, even though no one is blowing air around deliberately.
Embedded question — other real-life examples of gas particles moving: the smell of food cooking reaching another room, the aroma of a freshly cut onion or a sprayed perfume spreading through a space, the scent of flowers drifting across a garden, and (as a safety example) the smell of a cooking-gas (LPG) leak spreading through a kitchen — all of these are everyday examples of gas particles diffusing and spreading through the air on their own.
Since ancient times, people have wondered exactly how far a piece of matter could be broken down, and what it is ultimately made up of. Acharya Kanad, an ancient Indian philosopher, was among the very first thinkers on record to propose an answer: he believed that all matter is built from tiny, indivisible, eternal particles, which he called Parmanu.
Kanad set out this idea in his philosophical work known as the Vaisheshika Sutras. His central claim — that matter has a smallest, unbreakable building block — anticipated, by many centuries, the very idea of the "atom" that modern science later developed, tested, and refined into the atomic theory we study today.
When we wash oil-stained clothes with soap, countless soap particles surround each tiny oil particle sitting on the fabric. One end of a soap particle attaches to the oil, while the other end mixes freely with water — together, this lets the oil be lifted off the fabric and washed away.
The word "particle" changes meaning depending on the context. When discussing air pollution, "Suspended Particulate Matter (SPM)" refers to tiny dust particles floating in the air — these are much bigger than the constituent particles that build up matter itself. In fact, even a single speck of dust is made up of enormous numbers of these smaller constituent particles.
The tiny constituent particles that build up all matter are called atoms and molecules. Iron is made of iron atoms and gold of gold atoms. Some atoms, like those of hydrogen, oxygen, and sulfur, can't exist independently — instead, a fixed number of atoms bond together to form a molecule, such as two hydrogen atoms forming a hydrogen molecule, or two hydrogen atoms plus one oxygen atom forming a water molecule.
Answer: (iv) closely packed in solids and move past each other in liquids.
Reason: in solids, particles are closely packed and held in fixed positions by strong interparticle attraction, so they can only vibrate in place — they are not "stationary" in an absolute sense (option i is wrong), nor "always moving" freely (option iii is wrong), and they are certainly not "far apart" (option ii is wrong, since solids are the most tightly packed state). In liquids, particles are also fairly close together, but the slightly weaker interparticle attraction lets them slide and move past one another within a limited space — matching option (iv).
(i) True. Melting ice into water is indeed an example of a solid transforming into a liquid.
(ii) True. The melting process does involve a decrease in interparticle attraction, as particles gain enough thermal energy to break free of their fixed positions and move apart slightly.
(iii) True. Solids do have both a fixed shape and a fixed volume, thanks to their strong interparticle attraction and tightly packed, fixed particle positions.
(iv) True. Interparticle interactions in solids are indeed very strong, and the interparticle spaces between their particles are correspondingly very small.
(v) True. When camphor is heated in one corner of a room, its vapour particles do spread out and eventually reach every corner of the room, carried by the constant motion of air particles.
(vi) False — needs correcting. The statement claims that "the energy is released as a smell," which isn't quite accurate. Corrected: on heating, we are adding thermal energy to the camphor, which makes its particles move vigorously enough to escape into the air as vapour (camphor sublimes directly from solid to gas). It is these spreading vapour particles reaching our nose that we perceive as a smell — the energy itself is not "released as a smell."
Answer: (iii) Nothing of the chair will remain.
Justification: as this chapter establishes, all matter — including every part of a chair — is entirely made up of constituent particles. If every single one of these particles were removed, there would be absolutely nothing left to make up the chair; it isn't a case of the chair simply weighing less (option ii) or staying unchanged (option i), since the particles are the chair's entire substance, with nothing besides them.
Answer: gas particles have negligible interparticle attraction and a lot of empty space between them, so they can move freely in every direction and intermix rapidly and thoroughly with the particles of another gas. Solid particles, on the other hand, are held tightly in fixed positions by very strong interparticle attraction and cannot move past one another at all — so two solids placed next to each other simply cannot mix on their own, the way gases (or even liquids) can.
Answer: milk is a liquid, so its particles are held together by a moderate interparticle attraction that still allows them to move past one another within a limited space — this is why, once spilled, milk has no fixed shape of its own and simply flows and spreads out to cover whatever surface it lands on.
The glass tumbler, on the other hand, is a solid — its constituent particles are packed tightly together and held firmly in fixed positions by very strong interparticle attraction, allowing only tiny vibrations rather than any real movement. This is exactly why the tumbler keeps its own definite, fixed shape no matter what liquid is poured into or spilled from it.
Description of the diagram (three panels, left to right):
Panel 1 — Ice (solid): draw small circles (particles) arranged in a neat, tightly packed, evenly spaced grid pattern, touching close together in fixed rows and columns — representing strong interparticle attraction and minimum interparticle spacing.
Panel 2 — Water (liquid): draw the same circles a little more spread out and arranged irregularly rather than in a neat grid, still fairly close together and clustered, but with small random gaps between them — representing slightly weaker attraction and slightly greater spacing than ice.
Panel 3 — Water vapour (gas): draw the same circles scattered far apart from each other across a much larger empty space, with large gaps and no set pattern — representing negligible interparticle attraction and maximum interparticle spacing, with particles free to move in every direction.
Label an arrow from Panel 1 to Panel 2 as "melting (at melting point)" and from Panel 2 to Panel 3 as "boiling/evaporation (at boiling point, or slowly at any temperature)."
(i) Aluminium foil (a solid): draw particles as small circles packed tightly together in a neat, ordered, closely spaced grid pattern with almost no gaps — representing very strong interparticle attraction, minimum spacing, and particles that are fixed in position (only vibrating slightly).
(ii) Glycerin (a liquid): draw particles as circles that are still fairly close together but arranged irregularly rather than in a neat grid, touching in small clusters with a few visible gaps — representing moderately strong attraction that still allows particles to move past one another within a bounded space.
(iii) Methane gas (a gas): draw particles as circles scattered widely and randomly apart across a large empty area, with big gaps between each one and no fixed pattern — representing negligible interparticle attraction and particles moving freely in all directions to fill the entire available space.
States of wax visible in a just-extinguished candle:
Matching to the particle diagrams in Fig. 7.16b:
Answer: just like sugar dissolving in a glass of water in Activity 7.2, the salt present in ocean water has dissolved completely — breaking up into constituent particles far too tiny to see with the naked eye. These dissolved salt particles spread out and occupy the interparticle spaces of the water, distributing themselves evenly throughout the ocean.
Even though no visible grains of salt remain to be seen, the presence of these dissolved salt particles can still be sensed by taste, which is exactly why ocean water tastes salty throughout, despite there being no visible salt anywhere in it.
Answer: both rice grains and rice flour are still solids, not liquids.
Explanation: although a bulk collection of loose rice grains or flour appears to take on the shape of whichever jar it's poured into, this happens because the many separate, tiny solid particles slide and rearrange around one another and settle into the container's shape as a group — it is not the same as true liquid flow. Each individual grain of rice, or each tiny particle of flour, still keeps its own fixed shape and does not deform or flow the way the actual constituent particles of a true liquid do. This is different from a genuine liquid, where the individual particles themselves are free to move past one another continuously — here, it's simply a heap of separate solid particles rearranging in bulk, not molecular-level flow.
These four prompts are hands-on investigations, creative projects, and a class discussion rather than fixed-answer questions. Here's guidance on how to approach each one.
Expected observation: the balloon gradually inflates and expands as the bottle sits in the hot water.
Reasoning: the bottle traps a fixed amount of air inside it. Placing the bottle in hot water heats this trapped air, giving its particles more thermal energy so they move more vigorously and spread further apart, needing more space than before. Since the air can't escape anywhere else, it pushes into the balloon fixed over the neck, inflating it — a direct, hands-on demonstration of how heating a gas makes its particles move faster and spread into a larger volume.
Guidance for building the models:
Labelling each model with its interparticle spacing, attraction, and movement (matching the summary table in this chapter) makes the models useful as a revision aid too.
Guidance for the role-play:
Showing temperature: to represent heating, have students in each group move faster and more energetically (bigger sways for the "solid" group, or quicker shuffling for the "liquid" group); to represent cooling, have them slow down and move less.
Case that it is beneficial: this same spreading property lets fresh air and oxygen circulate and reach every corner of a room, is essential for us to breathe and for gas exchange inside our lungs, allows the pleasant aroma of food, flowers, or perfume to reach us from a distance, and lets fuel gases like LPG mix evenly with air for clean, even combustion when used properly in a stove.
Case that it is harmful: the very same property means that toxic gases, smoke, or an accidentally leaked poisonous or flammable gas can spread rapidly and fill an entire room or building, making gas leaks and air pollution genuinely dangerous and hard to contain — for example, a leaking LPG cylinder can let gas spread quickly through a kitchen, creating a serious fire or explosion risk, and tiny Suspended Particulate Matter can spread widely through the air we breathe.
A balanced conclusion for the debate: this property of gases is neither purely good nor purely bad — it is genuinely useful when it lets beneficial gases (like oxygen or fragrance) circulate and reach us, but hazardous when it lets harmful gases spread just as freely, which is exactly why safety measures like ventilation, exhaust fans, and gas-leak detectors matter in homes and labs.
Now that constituent particles and the three states of matter are covered, move on to how matter is classified into elements, compounds, and mixtures, revisit Chapter 6, or book a free demo class for personalised coaching.
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