NCERT Solutions Curiosity Chapter 7 Society–Science interdisciplinary projects — Discover, design, and debate

Book page 115 Updated on2026-09-05

Q1.

Fix a balloon over the neck of a bottle and put the bottle in hot water. Explore what will happen?

Answer

The balloon inflates by itself, standing up over the mouth of the bottle — even though you have not blown into it at all.

How to do it. Take an empty, dry bottle and stretch the mouth of a balloon tightly over its neck so that no air can leak out. Stand the bottle in a bowl of hot water so that most of it is under water. Watch the balloon for a minute or two. Then lift the bottle out and stand it in cold water instead, and watch again.

What you will observe.

Bottle placed in	Balloon
Hot water	Slowly swells up and stands erect
Cold water afterwards	Shrinks back down and goes limp again

Why it happens: the bottle is not empty — it is full of air, and air is matter made of particles. Hot water passes heat to that air, so the thermal energy of its particles increases and they move faster. In a gas the interparticle attractions are negligible, so faster particles simply spread further apart and take up more space. A gas has no fixed volume, so the air expands, and the only place it can expand into is the balloon. Cool the bottle and the particles slow down, the air occupies less space, and the balloon collapses.

Keep it a fair test: the balloon must be airtight on the neck, or the air will escape instead of doing the work. And note that no new air has been made — the same particles are simply spread over a larger volume.

Safety first: use hot water from a tap or a kettle that has been allowed to stand, not boiling water, and handle the bowl with an adult's help.

Q2.

Design and create simple models to represent particles of solids, liquids, and gases showing interparticle spacing using clay balls, beads, etc.

Answer

Method. Make all the balls the same size — that is the single most important rule, because the particles do not change from state to state; only their spacing and motion do. Take three identical shallow trays or box lids so that the three models can be compared fairly.

Model	How to build it	What it must show
Solid	Fill the tray with clay balls of one size, packed in neat rows so that they nearly touch, and stick them down so none can move	Minimum spacing, regular packing, particles fixed in position
Liquid	Use the same number of balls in the same tray, but loose and irregularly arranged with slightly bigger gaps; leave them free to be nudged about	A little more spacing than the solid, no fixed arrangement, movement within a limited space
Gas	Put only four or five balls in the tray, well separated, and shake the tray gently so that they roll all over it	Maximum spacing, free movement, particles filling the whole available space

Sample presentation. Label the three trays Solid, Liquid and Gas, and write one sentence under each: "Attraction strongest, spacing minimum, only vibration"; "Attraction slightly weaker, spacing a little more, movement within a limited space"; "Attraction negligible, spacing maximum, free movement in all directions".

Judge your own model honestly. Ask three questions of it: are the balls the same size in all three trays? Are the gaps empty, with nothing drawn or stuffed into them? Does the solid tray really allow no movement at all except a small wobble? A model that fails any of these teaches the wrong idea.

A limit worth stating aloud: real constituent particles are unimaginably smaller than clay balls, are not coloured, and never come to rest. A model shows arrangement and spacing well; it cannot show scale or ceaseless motion.

Q3.

Pretend to be particles of solids, liquids, and gases, at different temperatures—create and perform a role-play/dance showing particles in motion.

Answer

Method. The class becomes the particles and the floor becomes the container. Mark out the container with chalk or with a rope, and keep it the same size for all three scenes so that the spacing is what changes.

Scene	Where you stand	How you move
Solid, cold	Shoulder to shoulder in neat rows, filling the marked area	Feet fixed; only sway gently on the spot — never swap places with a neighbour
Solid being heated	Same rows, same places	Sway harder and harder as a drum beat speeds up, until a few students break away
Liquid	Still close together, but no longer in rows	Walk slowly, brushing past one another, staying inside the marked area
Gas	Spread out, far from each other	Move quickly in straight lines, in all directions, right up to the walls of the room — the chalk boundary no longer applies

Sample script for the narrator: "We are the particles of ice. We are held tightly by our neighbours and can only shiver in place. Heat is coming … we shiver harder … the hold breaks, and we are water — we can move now, but we stay together. More heat … we are moving so fast that we escape altogether. We are vapour. We fill the whole room."

Show the temperature, not just the state: let a drum or a clap set the speed. Faster beat = more thermal energy = faster particles. That single device turns the dance into a correct statement about the chapter — the movement of particles increases when heat is provided.

Q4.

Debate in the class — ‘Gases can spread and fill all the available space’. Is this property of gases beneficial or harmful?

Answer

Both — the property itself is neither good nor bad. What matters is which gas is spreading, and where. That is the position worth arguing, and it should be argued from the same physics on both sides: in a gas the interparticle attraction is negligible, the spacing is maximum, and the particles move freely in all directions, so a gas released anywhere spreads until it occupies all the space available.

Beneficial	Harmful
A leaking LPG cylinder is smelled from another room in time to act — the warning smell spreads faster than the danger builds up	The same leak spreads the cooking gas itself through the whole kitchen, so a single spark can set fire to the entire room
Oxygen from a cylinder spreads evenly to reach a patient; anaesthetic gases fill an operating room mask reliably	Smoke and harmful gases from vehicles and factories do not stay near the chimney — they spread over whole cities
The fragrance of incense, camphor or attar reaches every corner of a room or a temple (Activity 7.9)	A gas leak in a factory can spread over the neighbourhoods around it before anyone can contain it
Air itself keeps mixing, so oxygen reaches every room of a house without any pump	Smoke from burning stubble or garbage spreads far beyond the field where it was lit

How to run the debate. Divide the class in two. Each side must (a) name a real situation, (b) explain it using interparticle spacing, attraction and particle motion, and (c) answer one point made by the other side. Do not allow "gases are dangerous" or "gases are useful" as arguments on their own — every claim must be tied back to the particle model.

A conclusion worth reaching together: because a gas cannot be persuaded to stay put, it must be contained. This is why LPG is sold in sealed cylinders, why laboratories keep iodine in closed jars (Fig. 7.8), why factories fit chimneys and scrubbers, and why we are told never to look for a gas leak with a lighted matchstick. We do not change the property; we design around it.

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