NCERT Solutions Curiosity Chapter 7 End-of-chapter question set (continues on page 114) — Keep the curiosity alive

Book page 113 Updated on2026-09-05

Q1.

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.

Answer

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

This is the one statement that gets both halves right. In a solid the particles are tightly packed and held in fixed positions by very strong attractions, so they can only vibrate about those positions. In a liquid the attractions are a little weaker, the spacing a little more, and the particles are free to move past one another — which is exactly why you can pass your finger through water but not through a stone.

Why the others are wrong:

(i) — the first half is right, but liquid particles are certainly not stationary; their motion is what spreads potassium permanganate through a whole tumbler.
(ii) — both halves are the wrong way round. Solid particles are close together, not far apart; and it is solids, not liquids, whose particles have fixed positions.
(iii) — solid particles do not move from place to place at all, and liquid particles do not have fixed positions.

Q2.

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.

Answer

Statements (i), (ii), (iii), (iv) and (v) are true. Only (vi) is false.

Statement	True / False	Reason
(i)	True	Ice is a solid and water is a liquid, so melting ice is precisely a solid turning into a liquid — the change shown in Fig. 7.4
(ii)	True	On heating, the vibrations become vigorous enough for the particles to leave their fixed positions; the interparticle distance grows and the forces of attraction get weakened
(iii)	True	Strong attractions and minimum spacing hold every particle in place, so a solid keeps both its shape and its volume
(iv)	True	These are the two defining features of the solid state; the small spaces contain nothing at all, not air
(v)	True	The same thing you observe with an incense stick in Activity 7.9 — the particles spread through the whole room
(vi)	False	Energy is not what reaches your nose; camphor particles are

Corrected form of (vi): On heating, we are adding energy to the camphor. This energy increases the motion of its particles until they escape from the solid and spread through the air of the room; the smell we notice is caused by these camphor particles reaching our nose, not by energy being released as a smell.

Why the original is wrong: a smell is not a form of energy. It is our response to the actual particles of a substance arriving at our nose. The energy supplied does a different job — it frees the particles from the solid and keeps them moving.

Q3.

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.

Answer

(iii) Nothing of the chair will remain.

Justification. A chair is not an object that contains particles, the way a box contains marbles. The chair simply is a very large number of constituent particles held together by interparticle attractions, arranged in the shape of a chair. Its wood, its weight, its hardness and its shape are all properties of those particles and of the forces between them.

Take every particle away and there is no material left to be a chair — no wood, no legs, no seat, no mass, nothing at all. Even the interparticle spaces would go, because a space between particles cannot exist where there are no particles.

Why the other options fail:

(i) assumes the chair is something extra, over and above its particles, that could survive on its own. The whole chapter shows it is not: break a piece of chalk far enough and you are left with its particles, and nothing else.
(ii) treats the particles as only a part of the chair, so that removing them leaves a lighter chair behind. But they are all of it. Removing all of something does not leave a lighter version of it.

Q4.

Why do gases mix easily, while solids do not?

Answer

Because in a gas the particles are free to travel, and in a solid they are not.

In gases. The interparticle attraction is negligible and the interparticle spacing is maximum. Every particle moves freely in all directions and never stops. Bring two gases together and the particles of each simply wander into the wide spaces between the particles of the other, hit them, get knocked further along, and keep going until both are spread uniformly. No stirring, no shaking and no heating is needed — Activity 7.9 shows a fragrance crossing a whole room by itself.

In solids. The interparticle attractions are very strong and the spacing is minimum. Each particle is held in a fixed position and can only vibrate to and fro; it cannot move past its neighbours and cannot leave the solid. So when two solids are placed together, nothing can cross from one into the other. They only touch at their surfaces.

What about mixing two powders? Grinding turmeric and salt together and shaking them mixes the grains, not the particles. Look closely and each grain is still pure turmeric or pure salt. In a mixture of two gases there is nothing corresponding to a grain — the mixing goes all the way down.

Q5.

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.

Answer

Because the milk is a liquid and the tumbler is a solid, and the two differ in how strongly their particles are held and how freely they can move.

The milk flows and spreads. In a liquid the interparticle attractions are slightly weaker than in a solid, so the particles are free to move past one another — though only within a limited space. Inside the tumbler, the walls provide that space and the milk takes the shape of the tumbler. Spilled on the table there are no walls, so nothing holds the particles in place; they slide over one another and the milk spreads out into a thin layer, taking the shape of the table top. Its volume, however, does not change — the same amount of milk is now merely wide and shallow instead of tall and narrow.

The tumbler keeps its shape. In a solid the particles are closely packed and the interparticle attractions are very strong, holding each particle in a fixed position. They can only vibrate about those positions and cannot move past each other. So the glass has a definite shape and a definite volume that a fall does not alter — it will crack if the blow is hard enough, but it will never flow across the table.

The one sentence to take away: the same event, on the same table, at the same temperature, gives two completely different results — and the only difference between the two materials is the strength of their interparticle attractions.

Q6.

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

Answer

Three arrangements of the same particles, joined by two changes of state:

Magnified schematic pictures. Left to right the interparticle spacing grows and the interparticle attraction weakens — but the particles themselves are identical throughout.

What each stage shows, and why:

Ice. Particles closely packed, held in fixed positions by strong attractions. They can only vibrate. Spacing minimum, so the ice has a definite shape and volume.
Heating to the melting point. The thermal energy supplied is used to overcome the attractions. The vibrations become vigorous enough for the particles to leave their fixed positions; the interparticle distance increases slightly, the attractions weaken, and the ice becomes water.
Water. Particles a little more loosely packed, moving — but still close together and still within a limited space. Fixed volume, no fixed shape.
Heating to the boiling point. The movement becomes so vigorous that the particles move far apart, the attractions fall away, and the particles escape from the liquid.
Water vapour. Attraction negligible, spacing maximum, particles moving freely in all directions. Neither shape nor volume is fixed.

When you draw this in your notebook: use the same number of circles of the same size in all three boxes. Only their spacing and arrangement should change. Drawing bigger circles for the gas would say that the particles themselves expand on heating, which is not what happens.

Did you know? Water does not have to reach 100 °C to become vapour. Vapour forms at every temperature, slowly and only at the surface — that slow process is evaporation, and it is why a spilled puddle disappears on an ordinary day.

Q7.

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

Answer

Aluminium foil is a solid, glycerin is a liquid and methane is a gas — so the three pictures are the three standard arrangements, drawn to the same scale.

Each box is the same size and holds particles of the same size. Only the spacing and the freedom of movement differ.

Substance	State	How to draw it	Why
Aluminium foil	Solid	Circles closely packed in a regular arrangement, almost touching, filling the whole box	Interparticle attraction maximum, spacing minimum, particles only vibrate about fixed positions
Glycerin	Liquid	The same circles, a little more loosely packed and irregularly placed, but still close together and filling the box	Attraction slightly weaker than in a solid; particles move, but only within a limited space
Methane gas	Gas	Only a few circles, scattered far apart with large empty gaps between them	Attraction negligible, spacing maximum, particles move freely in all directions

Two mistakes to avoid: do not draw the gas particles larger than the solid ones — the particles are the same in every state. And do not shade the gaps: the interparticle spaces contain nothing at all, not air.

Q8.

Observe Fig. 7.16a which shows the image of a candle that was just extinguished after burning for some time. Identify the different states of wax in the figure and match them with Fig. 7.16b showing the arrangement of particles.

Answer

A candle that has just been blown out shows all three states of the same substance — wax — at the same moment.

Where you see it in Fig. 7.16a	State of the wax	Which arrangement in Fig. 7.16b
The body of the candle, and the hardened white wax that has run down the sides and set	Solid	The box in which the particles are closely packed and fill it completely (the upper left one)
The molten pool of wax in the hollow around the wick, and the drops still trickling down	Liquid	The box in which the particles are a little more loosely packed, with slightly larger gaps but still filling it (the upper right one)
The white trail rising from the just-extinguished wick	Gas (wax vapour)	The box with only a few particles, far apart, most of it empty (the lower one)

How one candle produces all three: the flame heats the solid wax near the wick. Its particles gain thermal energy, the interparticle attractions weaken, and the wax melts into the pool you see. That liquid is drawn up the wick, heated further, and turns into wax vapour — and it is this vapour, not the solid, that actually burns. Blow the flame out and the last of the vapour escapes unburnt, which is the white trail. Cool it and the whole sequence runs backwards: the vapour condenses, the pool sets, and solid wax is left on the sides.

Check it yourself: hold a burning matchstick in the white trail a moment after blowing the candle out. The flame runs down the trail and relights the wick — proof that the trail is wax vapour and not smoke.

Q9.

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

Answer

Because the salt is dissolved — it is present as separate constituent particles, spread through the water and far too small to see.

When salt dissolves it breaks up into its constituent particles. These particles are extremely small, so small that they cannot be seen even through an ordinary microscope, and they move into the interparticle spaces that already exist between the water particles. The constant motion of the water particles then spreads them evenly through the whole body of water. Once that has happened there are no grains of salt left anywhere for the eye to find — but every drop of the sea contains salt particles, and your tongue detects them at once.

This is exactly Activity 7.2 on a very large scale. Two teaspoons of sugar disappear completely into a tumbler of water, yet a spoonful taken from the top layer tastes sweet. Activity 7.7 adds the second piece of evidence: the level falls from B to C, showing that the dissolved particles have slipped into space that was already there.

The general point: invisible does not mean absent. Seeing is only one way of detecting matter; taste, smell and a change of volume are others, and in this chapter they are the ones that work.

Did you know? The salt gets there the same way the pebbles on the beach did. Rivers wear down rocks as they flow (page 99); along with the sand and clay they also carry dissolved salts to the sea. Water leaves the sea by evaporation, but the salt particles stay behind — so the sea keeps getting saltier.

Q10.

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

Answer

Both are solids. What takes the shape of the jar is the heap, not the material.

Apply the particle test rather than the eye test:

Look at one grain. A single grain of rice has its own definite shape and its own definite volume, and it keeps them wherever you put it. Its constituent particles are closely packed, held in fixed positions by strong attractions, and can only vibrate. That is the definition of a solid. Rice flour is the same thing ground finer — every speck is still a small piece of solid rice, exactly as every speck of ground chalk was still chalk in Activity 7.1.
Ask what is actually moving. When you tilt the jar, it is the grains that roll over one another. Inside each grain nothing moves from its place. In a true liquid it is the particles themselves that slide past one another, and there is nothing smaller for them to be arranged into.
Pour it out again. Rice comes back as the same countable grains, unchanged. Water poured out does not come back as separate identical pieces.

Test	Rice grains / rice flour	A true liquid such as water
Can it be piled into a heap?	Yes — a heap of rice or flour keeps a sloping surface	No — it flows until the surface is level
Are there visible gaps in it?	Yes, air-filled gaps between the grains	No, the liquid is continuous
Does each piece keep its own shape?	Yes, every grain does	No, the water has no pieces

Why the question is a good trap: flowing and filling a container look like liquid behaviour, so it is tempting to judge by that alone. But those are properties of a heap of small solid pieces. The state of matter is settled by how the constituent particles are arranged and held — and in rice they are locked in fixed positions.

Link back to page 98: this is the same reasoning as the opening question about heaping sand. Sand pours, yet sand is undoubtedly a solid.

Was this helpful? Report an error