NCERT Solutions: Chapter7 Particulate Nature of Matter (class 8 Science)

Stones and sand are solids. In solids, particles are tightly packed and held together by relatively strong interparticle attractions. This fixed arrangement gives solids a definite shape and allows them to rest on one another, so they can be piled up.

Water is a liquid; its particles have weaker attractions and can move past one another, so liquid flows and cannot keep a free-standing pile of its own shape.

Water is a liquid. Its particles can move around and rearrange themselves to fit the shape of the container (in this case, folded hands). When the hands are opened, gravity and the ability of the particles to move cause the water to flow and change shape again, because liquids have a definite volume but no fixed shape.

Air is a mixture of gases made of tiny particles (molecules) that cannot be seen individually. When a balloon is inflated, these gas particles occupy space inside the balloon and add mass to it. Because mass is present, the balloon becomes heavier — that is, air inside the balloon contributes to its weight.

Yes. Matter, including air, is continuously recycled in nature through processes such as respiration, photosynthesis, and weather cycles. The atoms and molecules in the air today have existed for a very long time and keep circulating through the environment.

Questions that spark scientific curiosity include:

• How small are the tiniest particles of matter, and can we ever see them?
• Why do some solids melt easily while others need very high temperatures?
• If gases have no fixed volume, how do they stay contained in the atmosphere?
• What happens to the particles when a substance changes from solid to liquid?
• Why don't all solids dissolve in water like sugar does?

Yes. Even after breaking or grinding, each speck of chalk powder is chemically the same as the original chalk. Grinding is a physical change that reduces particle size but does not alter the substance's chemical composition.

No. The particles obtained by grinding chalk are smaller pieces but not the smallest possible units. Each of these pieces is still made of many smaller constituent particles, which are the true basic units of the substance.

The constituent particles in solids are held together by interparticle forces of attraction. These forces keep the particles closely packed and fixed in position, giving solids a definite shape and volume.

In a solid, particles can only vibrate about fixed positions because they are very close together and strongly attracted to one another. To move them further apart, we must supply energy (for example, by heating) so that the solid melts into a liquid, where particles can move more freely.

• Liquids: Liquids have a definite volume but no definite shape; they take the shape of the container that holds them.
• Gases: Gases have neither definite shape nor definite volume; they expand to fill the entire container or space available to them.

No. Gases do not have a fixed volume because their particles are far apart and move freely. They spread out to fill the space available, so the volume of a gas depends on the container it is in.

Sugar dissolves in water because water molecules can surround and separate the sugar particles (molecules), breaking the forces that hold sugar crystals together so the sugar disperses at the molecular level.

Sand does not dissolve in water and settles down because its particles are held by very strong internal forces that water molecules cannot pull apart.

We can use visible tracers such as smoke or coloured vapours to show gas motion. For example, when an incense stick (agarbatti) is burnt in one corner of a room, its fragrance spreads throughout the entire room due to the continuous random movement and diffusion of air and smoke particles.

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

Explanation: In solids, particles are closely packed and fixed in position due to strong interparticle attractions. In liquids, particles are still close but can move or slide past each other, allowing the liquid to flow and take the shape of its container.

(i) True: Melting is the process where a solid becomes a liquid on heating; ice turning into water is a direct example.

(ii) True: As a solid melts, particles gain energy, move farther apart, and can move past one another; therefore, interparticle attraction decreases.

(iii) True: Solids keep their own shape and volume because their particles are closely packed and held in fixed positions.

(iv) True: Strong attractions hold particles close together in solids, leaving very small spaces between them.

(v) True: Camphor sublimes on heating: it changes directly from solid to gas and its vapour spreads through the room by the random motion of gas particles, carrying the smell everywhere.

(vi) False: Correction: Heating adds thermal energy to camphor, causing sublimation ($\text{solid} \rightarrow \text{gas}$). The smell is due to camphor molecules entering the air as vapour and diffusing to our nose, not because energy itself is released as a smell.

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

Explanation: A chair is made up of its constituent particles (atoms and molecules). If all these particles were removed, there would be no material left to give shape, mass, or structure — the chair would cease to exist.

Gas particles are far apart and move rapidly in all directions. Because of these large interparticle separations and high kinetic energies, gases spread out and mix quickly with other gases (diffusion). Solids have particles packed very closely together in fixed positions, so they cannot move past each other or mix unless they are melted or ground into fine powders.

Milk is a liquid; its particles can move past each other, so milk flows and spreads on the table to take the shape of the surface.

The glass tumbler is a solid; its particles are held tightly in a rigid, fixed arrangement by strong interparticle forces, giving the tumbler a definite shape that does not change when milk is spilled.

Pictorial Characteristics of the Three States:

• (i) Aluminium Foil (Solid): Particles are depicted as tightly packed spherical atoms arranged in an orderly, rigid geometric grid lattice with almost zero interparticle gaps.
• (ii) Glycerin (Liquid): Particles are shown close together but irregularly arranged, with small spaces allowing them to slide past one another.
• (iii) Methane Gas (Gas): Particles are depicted as widely separated points scattered far apart throughout the container, with arrows indicating rapid, random motion in all directions.

Scientific Process: The burning candle illustrates melting ($\text{solid} \rightarrow \text{liquid}$) and evaporation ($\text{liquid} \rightarrow \text{gas}$), demonstrating transitions between fixed and mobile particle structures.

Ocean water contains salts (predominantly sodium chloride, $\text{NaCl}$) dissolved as individual sodium and chloride ions at the molecular level. These dissolved particles occupy the intermolecular spaces between water molecules and are far too small to be seen with the naked eye, leaving the water clear. However, their chemical presence activates our taste receptors, giving ocean water its characteristic salty taste.

Both grains of rice and rice flour are solids.

Explanation: Each individual grain of rice or particle of flour retains its own fixed shape and definite volume. They appear to conform to the shape of the container only because a bulk collection of countless tiny solid particles can slide over one another and settle under gravity. However, the fundamental constituent units remain rigid solids.

Observation: The balloon inflates and expands.

Scientific Explanation: Heating the bottle warms the air trapped inside it. The thermal energy increases the kinetic energy of the air particles, causing them to move faster and spread farther apart. The increased pressure inside the bottle forces air upward into the balloon, causing it to inflate. This demonstrates that gas particles respond to heat by expanding and moving more vigorously.

Design Guidelines for Three-State Models:

• Solid Model: Pack clay balls or beads tightly in a transparent box in regular rows and columns so that they touch each other closely. This illustrates minimal interparticle spacing and fixed positions.
• Liquid Model: Place beads loosely in a shallow tray so they remain close but have enough space to slide and roll over each other when the tray is tilted. This illustrates slight spacing and fluidity.
• Gas Model: Place only a few beads inside a large, clear container with ample empty space between them so they rattle freely when shaken. This illustrates large interparticle separations and rapid random motion.

Role-Play Choreography:

• Solid Stage (Low Temperature): Students stand in a tight grid, holding elbows together and vibrating in place without moving their feet, representing closely bound particles with fixed positions.
• Liquid Stage (Medium Temperature): Students release elbows, stay close, and slowly weave around each other in smooth gliding movements, simulating liquid flow.
• Gas Stage (High Temperature): Students move quickly and freely in random straight paths across the room, gently bouncing off walls, representing high kinetic energy and rapid diffusion.

Temperature Modulation: As the teacher signals an increase in temperature, students increase their movement speed, transitioning from solid vibration into liquid flow (melting), and then into rapid gas motion (vaporization).