Class 8 Science Curiosity Chapter 7 Solutions (NCERT 2026–27) – Particulate Nature of Matter

These Class 8 Science Curiosity Chapter 7 solutions cover Particulate Nature of Matter from the new NCF-2023 textbook (2026–27), with every in-text activity and the complete “Keep the curiosity alive” exercise solved step by step.

Class: 8 Subject: Science Book: Curiosity Chapter: 7 Exercise: Keep the curiosity alive (10 Qs) Session: 2026–27
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Class 8 Science Curiosity Chapter 7 Solutions – Overview

Chapter 7 of Curiosity, Particulate Nature of Matter, explains that all matter is made up of an extremely large number of very small constituent particles that cannot be seen even under an ordinary microscope. These particles are held together by interparticle attractions, and there are tiny gaps between them called interparticle spaces. The chapter shows how the strength of these attractions and the size of these spaces decide the three states of matter — solid, liquid and gas — and their properties of shape, volume and compressibility. It introduces melting point and boiling point, explains how heat (thermal energy) drives changes of state, and uses diffusion of sugar, potassium permanganate and fragrance to demonstrate the constant motion of particles.

Key Concepts & Definitions

Constituent particle: the basic unit that makes up a larger piece of a substance or material. A grain of chalk, sand or sugar is itself made up of millions and millions of these particles.

Interparticle attractions: attractive forces that hold the constituent particles together. They are strongest in solids, slightly weaker in liquids and negligible in gases; they weaken sharply as interparticle distance increases.

Interparticle spaces: the empty gaps between constituent particles. Smallest in solids, a little more in liquids and largest in gases. (Sugar dissolving in water fills these spaces, so the solution’s volume is less than water + sugar.)

Melting point: the minimum temperature at which a solid melts to become a liquid at atmospheric pressure (e.g. ice 0 °C, urea 133 °C, iron 1538 °C).

Boiling point: the temperature at which a liquid boils and turns into vapour at atmospheric pressure; vapour forms throughout the liquid (bubbles), unlike slow surface evaporation.

Fluids: liquids and gases — they flow and do not have a fixed shape. Thermal energy of the particles ultimately decides the physical state of matter.

In-text & Activity Questions Answered

Probe and ponder: Why is it possible to pile up stones or sand, but not a liquid like water?

ANSWERIn stones and sand (solids), the constituent particles are held by strong interparticle attractions and stay in fixed positions, so the pieces keep a definite shape and can be heaped up. In water (a liquid) the particles can move freely past one another, so water has no fixed shape and simply flows and spreads out instead of forming a pile.

Probe and ponder: Why does water take the shape of folded hands but lose that shape when released?

ANSWERA liquid has no fixed shape and takes the shape of its container. When held, the cupped hands act as a container; once released, there is no container, so the water flows away and loses that shape, though its volume stays the same.

Probe and ponder: We cannot see air, so how does it add weight to an inflated balloon?

ANSWERAir is matter made of constituent particles that have mass, even though they are too small to see. When a balloon is inflated, a large number of these gas particles are pushed inside, so their combined mass makes the inflated balloon weigh slightly more than the empty one.

Probe and ponder: Is the air we breathe today the same that existed thousands of years ago?

ANSWERThe particles of matter, including those of air, are not created or destroyed in everyday processes — they are only rearranged and recycled. So the same air particles keep moving through the atmosphere, living things and water over time, and air similar to today’s has existed for thousands of years.

Activity 7.1 (Let us explore): On grinding a stick of chalk into fine powder, what do you observe — is every speck still composed of the same substance, or has it changed?

ANSWEREach tiny grain of powder is still a speck of chalk — the substance has not changed. Grinding is only a physical change in which the size of each speck is reduced; no new substance is formed.

Activity 7.2 (Let us perform): After dissolving sugar in water, does the top layer taste sweet, and can you see any sugar particles?

ANSWERYes, the top layer tastes sweet, which shows the sugar is present throughout the solution. However, no sugar particles can be seen — on dissolving, sugar breaks up into its extremely small constituent particles that spread into the interparticle spaces between the water particles.

But where did the sugar go (after it dissolved)?

ANSWERThe tiny sugar particles separate from one another and occupy the available interparticle spaces between the water particles, so they are still in the solution but are too small to be seen.

Activity 7.3 (Let us find out): In which of the solid objects (iron nail, rock salt, stone, wood, key, aluminium) are the particles strongly held together, and why do solids have a definite shape and volume?

ANSWERIn all these objects the particles are strongly held together, because they are all solids. In solids the particles are tightly packed with very strong interparticle attractions that hold them in fixed positions (they can only vibrate). This is why every solid has a definite shape and definite volume.

Activity 7.4 (Let us try and find out): When the same water is poured into containers A, B and C of different shapes, what happens to its shape and its level (volume)?

ANSWERThe water takes the shape of each container it is poured into, so liquids have no fixed shape (their particles move freely). But the level stays at 200 mL in every container, showing that a liquid has a definite (fixed) volume.

Are you able to move your finger through the water (Fig. 7.6)?

ANSWERYes. You can move your finger through water because the interparticle attractions in a liquid are weaker than in a solid, so the particles part to let the finger through and then flow back. The water is only temporarily displaced and its position is restored at once — something you cannot do with a solid.

Activity 7.5 (Let us investigate): When Gas Jar B is placed over Gas Jar A filled with smoke and the plate is removed, what do you observe about the smoke?

ANSWERThe smoke spreads upward and fills the entire space of Gas Jar B as well. This shows that a gas has no fixed volume and occupies all the available space, and (like a liquid) takes the shape of its vessel — because gas particles move freely in all directions and their interparticle attractions are negligible.

Activity 7.6 (Let us experiment): When you push the syringe plunger inward with the open end closed, what happens to the air, and what about water?

ANSWERThe volume of the trapped air decreases — the gas is compressed because its particles have a lot of space between them, which can be reduced by pressure. On releasing, the plunger springs back. When the activity is repeated with water, it is found to be practically incompressible, because liquids have very little interparticle space.

Activity 7.7 (Let us observe): When sugar is added to water and dissolved, what difference do you observe in the water levels A, B and C, and what does it tell you?

ANSWERWhen sugar is added, the level first rises (to mark B). After the sugar dissolves, the level falls a little (mark C is below B). Since the volume of the solution is less than water + sugar, it shows there are interparticle spaces between the water particles, which the dissolved sugar particles occupy.

Sugar and sand are both solids. Why does sugar dissolve in water but sand does not?

ANSWERIn sugar, the constituent particles are held together loosely enough that the moving water particles can pull them apart and spread them into the interparticle spaces — so sugar dissolves. In sand, the particles are held together by very strong forces that water particles cannot pull apart, so sand stays as separate grains, settles down and remains insoluble.

Activity 7.8 (Let us experiment): On putting a few grains of potassium permanganate in water without stirring, what do you observe and why?

ANSWERFirst, streaks of pink colour spread from the grains; in time the whole water turns a uniform pink. This happens because the water particles are in constant motion — they pull the potassium permanganate particles out of the grains and, on colliding, spread them throughout the liquid. This spreading is diffusion.

Think like a scientist: A grain of potassium permanganate is dropped into hot water, room-temperature water and ice-cold water. What do you observe?

ANSWERThe colour spreads fastest in hot water, more slowly in water at room temperature and slowest in ice-cold water. This is because water particles move faster when they have more heat (thermal energy), so the movement and mixing of particles increases on heating.

Activity 7.9 (Let us find out): On lighting an incense stick in one corner of the room, do you notice the fragrance from a distance, and why?

ANSWERYes — soon the fragrance is smelt all over the room. The particles of the fragrant vapour spread out, and the constantly moving air particles strike them and carry them throughout the room. This shows that gas particles are in continuous, rapid motion (diffusion in gases).

Class 8 Science Curiosity Chapter 7 Solutions — Keep the Curiosity Alive

1. 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. In solids the particles are tightly packed and only vibrate in fixed positions; in liquids the attractions are weaker, so the particles can slide and move past one another. The other options are wrong because liquid particles are not stationary (i, ii) and solid particles are not always moving (iii).

2. 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.

ANSWER (i) True. Solid ice changes to liquid water on melting. (ii) True. On melting, the particles gain energy and move apart, so the interparticle attractions become weaker. (iii) True. A solid keeps both its shape and its volume fixed. (iv) True. In solids the interparticle forces are very strong and the interparticle spaces are very small. (All four statements are correct, so none needs correcting.)

2 (continued). State whether true or false; correct the false statements. (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 (v) True. On heating, camphor turns to vapour whose particles diffuse and are carried by the moving air particles to every corner of the room. (vi) False. The heat energy is not “released as a smell.” Correct statement: On heating, energy is supplied to the camphor so its particles move faster and escape as vapour; we sense a smell because these vapour particles reach our nose, not because energy turns into smell.

3. 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: The chair is made up entirely of its constituent particles. They are the basic building blocks of the matter the chair is made of, so if every particle is removed there is no matter left — the whole chair disappears.

4. Why do gases mix easily, while solids do not?

ANSWER In gases, the interparticle attractions are negligible and there are very large interparticle spaces, so the particles move freely and fast in all directions and quickly diffuse into one another — gases mix easily. In solids, the particles are tightly packed with very strong attractions and can only vibrate in fixed positions, so they cannot move from place to place. Hence solids do not mix on their own.

5. 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 Milk is a liquid: its particles have weaker interparticle attractions and can move past one another freely, so when spilled it has no fixed shape and flows and spreads over the table. The glass tumbler is a solid: its particles are held in fixed positions by very strong interparticle attractions, so it keeps its definite shape and does not flow or spread.

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

ANSWER Show three boxes side by side (described in words, since no copyrighted image is used): Ice (solid): particles drawn very close together in a neat, regular, tightly packed pattern — smallest spacing. Water (liquid): particles a little farther apart, still close but irregularly arranged so they can slide past one another — slightly more spacing. Water vapour (gas): only a few particles, far apart and scattered all over the box, moving freely in all directions — largest spacing. Arrows between the boxes labelled “heating →” show ice → water (melting) → water vapour (boiling/evaporation).

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

ANSWER (i) Aluminium foil (solid): draw circles packed very close together in a regular, tightly packed arrangement with the smallest gaps. (ii) Glycerin (liquid): draw circles fairly close but loosely and irregularly arranged, with slightly larger gaps so they can move past one another. (iii) Methane gas (gas): draw only a few circles spread far apart and randomly across the space, indicating free movement and the largest gaps.

8. 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 burning candle shows wax in different states at the same time: Solid wax — the cool, hard body of the candle away from the flame. Matches the diagram with particles tightly and regularly packed (smallest spacing). Liquid wax — the molten wax (the pool/drips) near the wick that has melted in the heat. Matches the diagram with particles loosely and irregularly packed with a little more spacing. Gaseous wax (wax vapour) — the wax that has vaporised, seen as the rising smoke/vapour just after the flame is put out. Matches the diagram with a few widely separated particles (largest spacing).

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

ANSWER Salt dissolved in the ocean breaks up into its extremely small constituent particles, which spread into the interparticle spaces between the water particles. These salt particles are far too small to be seen, so no salt is visible. However, the salt particles are present throughout the water and reach our tongue, so the water still tastes salty — just as sugar made the water taste sweet in Activity 7.2 without being seen.

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

ANSWER Rice grains and rice flour are solids, not liquids. Each tiny grain or flour particle is itself a solid with a definite shape and strong interparticle attractions. When poured into a jar, it is the heap of many separate solid grains that takes the jar’s shape — the individual grains do not change their shape or flow like a true liquid. So the “flowing” is only the loose grains shifting, which is why such solids are sometimes called free-flowing solids.

Common Misconceptions to Avoid

Watch out for these

  • Thinking the interparticle spaces are “filled with air” — in fact they contain nothing at all.
  • Believing dissolved sugar or salt has disappeared — the particles are only too small to see and have moved into the spaces between water particles.
  • Confusing evaporation with boiling — evaporation is slow, occurs only at the surface and at all temperatures; boiling is fast, occurs throughout the liquid and only at the boiling point.
  • Assuming melting increases interparticle attractions — melting actually weakens them as particles move apart.
  • Calling rice grains or flour a liquid because they flow — they are free-flowing solids; each grain keeps its shape.
  • Forgetting that ice is an exception — its particles are farther apart than those in liquid water.

Extra Practice Questions

Very Short Answer Type Questions

Q1. What name is given to the basic unit that makes up a larger piece of a substance?

ANSWERA constituent particle.

Q2. Name the ancient Indian philosopher who first spoke of the “Parmanu” (atom).

ANSWERAcharya Kanad (in his work the Vaisheshika Sutras).

Q3. Which state of matter is the most easily compressed?

ANSWERThe gaseous state (because of its very large interparticle spaces).

Short Answer Type Questions

Q1. Why are liquids and gases together called fluids?

ANSWERBecause both liquids and gases can flow and do not retain a fixed shape — their particles are free to move. This property distinguishes them from solids and lets us classify them together as fluids.

Q2. Arrange solids, liquids and gases in increasing order of interparticle spacing and interparticle attraction.

ANSWERInterparticle spacing (increasing): solid < liquid < gas. Interparticle attraction (increasing): gas < liquid < solid. So solids have the smallest spaces and strongest attractions, while gases have the largest spaces and weakest attractions.

Long Answer Type Question

Q1. Explain how the thermal energy of particles decides whether a substance is a solid, a liquid or a gas.

ANSWER In the solid state the thermal (heat) energy of the particles is low, so they stay close together with strong interparticle attractions and can only vibrate about fixed positions — the solid has a fixed shape and volume. On heating to the melting point, the added thermal energy overcomes some of the attractions; the particles move a little apart and can move around within a limited space — the substance becomes a liquid with a fixed volume but no fixed shape. On further heating to the boiling point, the particles gain so much energy that they overcome the attractions almost completely and move freely in all directions — the substance becomes a gas with no fixed shape or volume. Thus it is the thermal energy of the particles that determines the physical state of matter.

MCQs & Assertion–Reason

1. Matter is composed of:

(a) a single solid block    (b) a large number of extremely small particles    (c) only air    (d) only water

2. The empty gaps between the constituent particles of matter are called:

(a) interparticle attractions    (b) interparticle spaces    (c) melting points    (d) molecules

3. In which state are the interparticle attractions the strongest?

(a) solid    (b) liquid    (c) gas    (d) vapour

4. The minimum temperature at which a solid melts into a liquid at atmospheric pressure is its:

(a) boiling point    (b) melting point    (c) freezing    (d) diffusion point

5. The melting point of iron is:

(a) 0 °C    (b) 100 °C    (c) 133 °C    (d) 1538 °C

6. Spreading of potassium permanganate colour throughout water is an example of:

(a) melting    (b) diffusion    (c) condensation    (d) freezing

7. Which of the following has neither a fixed shape nor a fixed volume?

(a) iron nail    (b) water    (c) air    (d) rock salt

8. A gas can be compressed easily because:

(a) its particles are tightly packed    (b) it has very large interparticle spaces    (c) it has strong attractions    (d) it has no particles

9. Liquids and gases are together classified as:

(a) solids    (b) fluids    (c) crystals    (d) metals

10. Diffusion of a coloured substance in water is fastest in:

(a) ice-cold water    (b) water at room temperature    (c) hot water    (d) it is the same in all

Answer key: 1-(b), 2-(b), 3-(a), 4-(b), 5-(d), 6-(b), 7-(c), 8-(b), 9-(b), 10-(c).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: Solids have a definite shape and a definite volume.

Reason: In solids the particles are tightly packed and held in fixed positions by strong interparticle attractions.

A-R 2. Assertion: Gases do not have a fixed volume.

Reason: The interparticle attractions in gases are negligible, so the particles move freely and occupy all the available space.

A-R 3. Assertion: Dissolved sugar disappears completely from water.

Reason: Sugar breaks up into extremely small particles that occupy the interparticle spaces of water.

A-R 4. Assertion: A liquid can be compressed as easily as a gas.

Reason: Liquids have very large interparticle spaces.

A-R 5. Assertion: Potassium permanganate spreads faster in hot water than in cold water.

Reason: Particles move faster when their thermal energy increases.

Answer key: 1-(A), 2-(A), 3-(D), 4-(D), 5-(A).

Quick Revision Summary

  • Matter is composed of extremely small particles that cannot be seen even through an ordinary microscope.
  • These particles are held together by interparticle forces of attraction, with empty interparticle spaces between them.
  • Interparticle attractions are strongest in solids, weaker in liquids and weakest (negligible) in gases.
  • Solids: fixed shape and volume, particles closely packed, minimum spacing, only vibrate.
  • Liquids: definite volume but no fixed shape, slightly weaker attractions, particles move within a limited space (fluids).
  • Gases: no fixed shape or volume, negligible attractions, maximum spacing, particles move freely (fluids; easily compressed).
  • Heat (thermal energy) drives changes of state: solid →(melting point) liquid →(boiling point) gas.

Real-life Applications

The particulate nature of matter is at work all around us. The fragrance of incense, perfume or food spreading across a room shows diffusion of gas particles. Soap cleans oily clothes because soap particles surround the oil particles — one end clinging to the oil and the other to water — lifting the dirt away. Sugar and salt dissolving unseen explain sweet tea and salty sea water. Compressed gas in syringes, balloons and tyres relies on the large interparticle spaces in gases, while the incompressibility of liquids is used in hydraulic brakes. Melting and boiling points help us choose metals for tools and explain why ice melts and water boils.

How to score full marks in this chapter

Memorise the three states in a comparison table (spacing, attraction, shape, volume, movement) and quote the melting points of ice (0 °C), urea (133 °C) and iron (1538 °C). Always explain observations in terms of interparticle attractions and spaces. For diagram questions, neatly show particles closely packed (solid), loosely packed (liquid) and far apart (gas), and define melting point and boiling point in the exact textbook words.

Frequently Asked Questions

What is Class 8 Science Curiosity Chapter 7 about?

Chapter 7, Particulate Nature of Matter, explains that all matter is made of extremely small constituent particles held together by interparticle attractions, with interparticle spaces between them. It shows how these decide the solid, liquid and gas states, and covers melting point, boiling point, diffusion and changes of state.

What is the difference between interparticle attraction and interparticle space?

Interparticle attraction is the force that holds particles together (strongest in solids, weakest in gases), while interparticle space is the empty gap between particles (smallest in solids, largest in gases). Together they decide the state and properties of matter.

How many questions are in the “Keep the curiosity alive” exercise of Chapter 7?

There are 10 main questions (with several sub-parts), all solved on this page along with every “Probe and ponder”, activity and “Think like a scientist” question.

Are these Class 8 Science Curiosity Chapter 7 solutions free?

Yes. All ClearStudy NCERT Solutions for Class 8 Science Curiosity are free and follow the official NCERT textbook for 2026–27.

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