Stones and sand are solids. Each grain or stone keeps its own fixed shape because its constituent particles are tightly held together by strong interparticle forces, so a grain does not flow or spread out — many such rigid grains can simply be stacked on top of one another into a pile.

Water, on the other hand, is a liquid. Its particles are held together only loosely and are free to move past one another. Because of this, water cannot hold a heap-like shape — it always flows and spreads out to take the shape of whatever surface or container it is in, so it cannot be “piled up” the way solid grains can.

Liquids like water do not have a fixed shape of their own — their particles are free to move and slide over one another, so water simply flows into and takes up the shape of whatever container or boundary holds it. While your hands are folded, they act as a temporary container, so the water takes that cupped shape.

The moment you open your hands, that boundary disappears. Since the water particles are not held in any fixed arrangement, they are free to move again and the water flows away, losing the shape it had only because it was being contained, not because it has any tendency to remain in that form.

Even though air particles are too small to be seen, they are still made of matter and therefore have mass. When we inflate a balloon, we are forcing many extra air particles into it (in addition to the air already present around it). This larger number of air particles inside the balloon adds extra mass compared to a deflated balloon, and since weight is simply the force of gravity acting on mass, the inflated balloon weighs slightly more than the same balloon when empty — invisibility does not mean an absence of mass.

The basic constituent particles of the gases that make up air (such as nitrogen, oxygen, carbon dioxide, etc.) are extremely small and are believed to be largely indestructible under normal conditions — matter is neither created nor destroyed in ordinary physical or even most chemical processes, only rearranged. So, in that sense, many of the same particles that existed thousands of years ago are likely still present today, having been continuously recycled through air, water, soil, and living things.

However, the composition (the relative proportion of different gases) of the air may have changed over thousands of years, due to natural processes and, more recently, human activities such as burning fuels, deforestation, and industrial emissions — for example, the proportion of carbon dioxide in today’s atmosphere is understood to be higher than it was in the distant past.

Every speck of the fine chalk powder is still composed of the same substance — chalk. Breaking and grinding chalk is a physical change: only the size of each piece of chalk is reduced into smaller and smaller specks; no new substance is formed in the process. This can be confirmed by recalling the ‘Changes Around Us: Physical and Chemical’ chapter from Grade 7 — grinding only changes the size/form of the chalk, not its chemical identity.

Yes. If we imagine this process of grinding continuing again and again, we would eventually reach a stage where the resulting tiny units cannot be broken down any further (while still remaining chalk). These tiniest units are called the constituent particles of chalk — they are the basic building blocks that make up the whole piece of chalk.

The sugar has not disappeared — it has broken up into its tiny constituent particles, which are far too small to be seen. These tiny sugar particles spread out and occupy the empty spaces that exist between the water particles, called interparticle spaces. Since the sugar particles are now mixed uniformly throughout the water at this microscopic level, we cannot see them, but their presence can still be detected because the water tastes sweet.

The constituent particles of any matter are held together by attractive forces called interparticle attractions. In a solid piece such as chalk or a sugar crystal, these attractive forces are very strong, pulling the particles close together and locking them into fixed positions. This is what gives solid objects their definite, rigid shape and keeps them from simply falling apart.

Yes — by supplying heat (thermal energy) to the solid. When a solid is heated, its particles absorb this energy and vibrate more and more vigorously about their fixed positions. If enough heat is supplied, the vibrations become strong enough to partly overcome the interparticle forces of attraction, allowing the particles to move apart from their fixed positions. At this point, the solid melts and changes into the liquid state, where particles have more freedom and are spaced slightly farther apart.

Liquids also have a definite (fixed) volume — as shown in Activity 7.4, the same quantity of water always measures 200 mL no matter which container it is poured into, even though its shape keeps changing. This is because liquid particles, although free to move, remain within a limited space due to interparticle attraction.

Gases, however, do not have a fixed volume. As shown in Activity 7.5, smoke (representing gas particles) expands to fill the entire available space of any container it is placed in, however large — because interparticle attraction in gases is negligible, allowing particles to spread apart freely in all directions.

This is correct, and it is the phenomenon of evaporation. Unlike boiling (which happens only at a liquid’s fixed boiling point), vapour formation actually occurs at all temperatures, not just at the boiling point. At any given moment, some particles at the surface of the liquid have enough energy to escape into the air as vapour, even though the bulk of the liquid is far below its boiling point. This surface-level, slow escape of particles — which can happen at any temperature — is what causes spilled water to gradually dry up and “disappear” over time.

No. Activity 7.5 (the smoke/gas-jar activity) shows that when smoke from Gas Jar A is allowed to enter empty Gas Jar B, it does not stay confined to a fixed volume — instead it spreads out and fills the entire available space of Gas Jar B. This happens because interparticle attraction in gases is negligible, so the particles move freely in all directions and keep expanding until they occupy whatever space is available to them. Hence, gases have no fixed volume (and no fixed shape either).

This depends on how strongly each solid’s constituent particles are held together, and on how those particles interact with water particles.

In sugar, the interparticle forces of attraction are weak enough that the moving water particles are able to pull the sugar’s constituent particles away from each other. These freed sugar particles then spread out and occupy the interparticle spaces of water — this is what we observe as “dissolving.”

In sand, the constituent particles are held together so strongly that the water particles are unable to pull them apart. As a result, sand particles cannot separate into water’s interparticle spaces — they simply settle down at the bottom of the container as an insoluble solid, without dissolving.

We can demonstrate it indirectly using our sense of smell. If an incense stick is lit in one corner of a room, the fragrance is felt only near it at first, but within a few minutes it can be smelled throughout the entire room. This spreading happens because the gas (air) particles in the room are in constant, random motion; they continuously collide with the tiny fragrance particles released by the incense stick and push them outward in all directions, until the fragrance has diffused evenly through the whole room. The fact that the smell reaches distant corners is indirect evidence that invisible gas particles are indeed in constant motion.

• Smelling food cooking in the kitchen from another room.
• The smell of fresh rain (petrichor) spreading across an open area after it starts raining.
• Smelling petrol/diesel fumes near a vehicle even while standing a short distance away.
• The fragrance of flowers in a garden being noticeable from quite far away, especially on a breezy day.
• Smelling smoke from a fire or from someone’s cooking, well before you can see its source.

In every case, gas particles (carrying the smell) move and diffuse through the air, constantly colliding with air particles, until they reach our nose from a distance.

• (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.

In solids, particles are tightly/closely packed and locked in fixed positions by strong interparticle forces — they can only vibrate, not travel. In liquids, the interparticle attraction is slightly weaker, so particles are free to slide and move past one another (though still confined within the liquid’s volume) — this is exactly why liquids can flow and take the shape of their container. Option (iv) correctly captures both halves of this difference.

(i) Melting ice into water is an example of the transformation of a solid into a liquid. TRUE

(ii) Melting process involves a decrease in interparticle attractions during the transformation. TRUE
Heat energy makes particles vibrate more vigorously until the strong interparticle forces of the solid weaken — this drop in attraction lets particles move out of their fixed positions, turning the solid into a liquid.

(iii) Solids have a fixed shape and a fixed volume. TRUE

(iv) The interparticle interactions in solids are very strong, and the interparticle spaces are very small. TRUE

(v) When we heat camphor in one corner of a room, the fragrance reaches all corners of the room. TRUE
Heating speeds up the release of camphor vapour, and these gas particles diffuse — moving freely and colliding with air particles — until they spread throughout the room.

(vi) On heating, we are adding energy to the camphor, and the energy is released as a smell. FALSE
Correction: On heating, energy is added to the camphor’s particles, increasing their motion and helping them overcome interparticle attraction so that they escape as vapour (a physical change called sublimation). It is not the “energy” that is released as smell — the smell is caused by the actual camphor particles (now in gaseous form) diffusing through the air and reaching our nose.

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

A chair, like all matter, is made up entirely of constituent particles — there is no “extra” material besides these particles. If every single constituent particle were removed, there would be nothing left to make up the chair’s structure, shape, or substance at all. So it is not simply a case of the chair becoming lighter (option ii) — the chair itself, in its entirety, would cease to exist, since particles are the very matter the chair is made of.

Gases have negligible interparticle attraction, so their particles move freely, rapidly, and randomly in all directions, occupying all the available space. When two gases are brought together, their fast-moving particles quickly intermingle throughout the shared space — this is why gases mix easily and completely (as seen with the spreading smoke/fragrance in Activities 7.5 and 7.9).

Solids, on the other hand, have very strong interparticle attraction that locks their particles into fixed positions, allowing them only to vibrate — not to travel from place to place. Since the particles of one solid cannot break free and move into another solid, two solids placed together cannot mix at the particle level; at most, their surfaces touch.

Milk is a liquid. Its constituent particles are held by relatively weak interparticle forces and are free to move past one another. Because of this, when spilled, milk has no fixed shape of its own and simply flows outward, spreading across the table until it is stopped or contained by some boundary.

The glass tumbler is a solid. Its constituent particles are held together by very strong interparticle attraction, which keeps them locked tightly in fixed positions, allowing them only to vibrate in place. This is why the tumbler retains its definite shape and volume, with or without the milk inside it — solids do not flow or change shape the way liquids do.

Ice (Solid): particles closely packed in a fixed, regular arrangement

Water (Liquid): particles slightly farther apart, free to move within a limited space

Water Vapour (Gas): particles far apart, moving freely in all directions

As ice (solid) is heated, particles vibrate more and more, eventually overcoming the strong interparticle attraction and moving apart slightly to become liquid water — interparticle spacing increases a little and particles can now move within a limited space. On further heating to the boiling point, particles gain enough energy to completely overcome the remaining attraction between them; they move far apart and travel freely in all directions, becoming water vapour (gas) — exactly as summarised in the chapter’s ‘Let us wrap up!’ table.

(i) Aluminium foil — Solid: particles closely and regularly packed, strong interparticle attraction

(ii) Glycerin — Liquid: particles loosely packed, free to move within a limited space

(iii) Methane gas — Gas: particles widely spaced, moving freely in all directions

Aluminium foil is a solid metal at room temperature — its particles should be drawn closely packed in an orderly, tightly-bound arrangement (like Fig. 7.12a).

Glycerin is a liquid at room temperature — its particles should be drawn a little more loosely packed and irregularly arranged, close together but able to move past one another (like Fig. 7.12b).

Methane is a gas at room temperature — its particles should be drawn few and far apart, scattered randomly with large empty spaces between them, representing free movement in all directions (like Fig. 7.12c).

Fig. 7.16(a): The extinguished candle

Solid wax (the candle body)

Liquid wax (the melted wax dripping down the sides)

Wax vapour (the smoke rising from the wick)

The figure shows wax in three different states at the same time:

• The main, hard, unburnt body of the candle is solid wax — its particles are closely and regularly packed (matches the “solid” particle diagram).
• The soft, runny wax dripping down the sides of the candle (melted by the heat of the flame) is liquid wax — its particles are a little more loosely and irregularly packed, but still close together (matches the “liquid” particle diagram).
• The visible smoke/vapour rising from the freshly extinguished wick is wax vapour (gaseous wax) — its particles are far apart and spread out freely (matches the “gas” particle diagram).

This single image is a good real-life example of how the same substance (wax) can exist in all three states of matter simultaneously, depending on how much heat energy its particles in different regions have absorbed.

Ocean water contains large amounts of dissolved salts (mainly sodium chloride). Just like sugar dissolving in water in Activity 7.2, when salt dissolves in ocean water, it breaks up into its tiny constituent particles, which are far too small to be seen with the naked eye. These salt particles spread out and occupy the interparticle spaces between the water particles, mixing uniformly throughout the ocean.

Even though we cannot see these dissolved salt particles, their presence can still be detected by taste — exactly as the sugar solution tasted sweet in Activity 7.2 without any visible sugar grains. This is why ocean water tastes salty even though no solid salt can be seen floating in it.

Rice grains and rice flour are still solids, not liquids, even though as a bulk collection they appear to “take the shape” of their container.

The key difference is at the level of individual particles/grains. Each separate grain of rice (or each tiny particle of rice flour) keeps its own definite shape and volume — it does not flow, deform, or spread out the way a true liquid’s particles do. What we observe is simply many separate, rigid solid grains rearranging themselves and settling under gravity to roughly fill the shape of the container — this is very different from a liquid, where the same continuous substance’s particles slide past each other and flow as a connected whole. Since true liquid behaviour requires the constituent particles themselves to flow and lack a fixed shape (not just a loose collection of solid grains shifting position), rice and rice flour remain classified as solids.

Observation: As the bottle sits in hot water, the balloon fixed on its neck gradually inflates and stands upright, even though no one is blowing into it.

Why this happens: The hot water transfers heat energy to the air particles trapped inside the bottle. This extra thermal energy makes the air particles move faster and collide with each other and the walls of the bottle more vigorously, pushing them farther apart. Since gas particles already have negligible interparticle attraction, this increased motion makes the air expand to occupy more space — and since it cannot expand within the rigid bottle, the expanding air pushes up into the balloon, inflating it. This activity nicely demonstrates that heating a gas increases the motion and spacing of its particles, causing it to expand.

Solid model: Arrange clay balls or beads tightly together in a neat, regular grid pattern (rows and columns touching each other) on a tray or board, possibly gluing them in fixed positions — representing minimum interparticle spacing and particles that can only vibrate, not move.

Liquid model: Place a similar number of balls/beads in a transparent container, but loosely and irregularly arranged, close together yet able to be gently shaken or tilted so the “particles” shift and slide over each other — representing slightly greater spacing and particles confined to move within a limited space.

Gas model: Place only a few balls/beads loosely inside a large balloon or big box, with lots of empty space between them, so they can be seen moving and bouncing freely in all directions when shaken — representing maximum interparticle spacing and free, random motion.

Solid (cold): Students stand very close together in fixed rows, feet planted, only gently swaying/wobbling in place (representing vibration about a fixed position).

Solid (heated, near melting point): Same close formation, but with more vigorous swaying/shaking in place, hinting that the structure is about to break.

Liquid: Students stand a little more spread out, free to slowly walk around and brush past each other, but staying roughly within a marked-out area on the floor (representing limited free movement within a fixed volume).

Gas: Students spread out widely across the entire room/stage and move quickly and randomly in all directions, occasionally “colliding” and bouncing off one another or the walls — representing free movement filling all available space.

You can change music tempo for each stage (slow for solid, medium for liquid, fast for gas) to represent increasing thermal energy and particle motion.

Beneficial:

• This property allows oxygen and other essential gases to spread evenly and reach every part of a room or the atmosphere, making it possible for living beings everywhere to breathe.
• It allows the smell of food, perfumes, and flowers to be carried and enjoyed across a distance.
• It is essential for many useful technologies, like gas distribution in pipelines, inflation of tyres and airbags, and aerosol sprays, where gases need to fill containers or spaces uniformly.

Harmful:

• The same property allows harmful gases — such as smoke, toxic industrial fumes, vehicle exhaust, or a gas leak — to spread rapidly and fill an entire room, building, or even a wide area, endangering health.
• Greenhouse gases released in one part of the world spread and mix throughout Earth’s atmosphere, contributing to global warming and climate change everywhere.
• In an accidental chemical or LPG leak, this very property is what makes the situation dangerous very quickly, since the gas does not stay confined to one spot.

Conclusion for the debate: The property itself is neutral — it is the nature of the gas and how it is managed/contained that determines whether the outcome is beneficial or harmful. A balanced debate should acknowledge that this single particle property of gases is responsible for both life-sustaining processes and serious safety hazards, depending on context.