NCERT Solutions Class 8 Science Chapter 07 Particulate Nature of Matter

Question 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 simple words: In a solid, particles are tightly packed and stay in fixed spots, but they can shake back and forth a little. In a liquid, particles are still close together but can slide and move around each other.

Exam Tip: Remember that solids have particles locked in place while liquids have particles that can flow freely - this is the key distinction between the two states.

Question 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. (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:
(i) True. This is the standard meaning of melting.
(ii) True. When heat is supplied during melting, the particles get enough energy to weaken the forces that keep them fixed, letting them move about more easily.
(iii) True. These are the two key properties that describe a solid.
(iv) True. This is why solids stay firm and are often heavy. The strong pulls between particles and close packing keep them all in place.
(v) True. Heat causes the solid camphor to become a gas, and these gas particles then travel throughout the room.
(vi) False. The correction is: When we heat the camphor, we add energy that makes its particles turn into a gas. These gas particles spread out and mix with the air, and when they reach our nose, we smell them. The energy creates the change - it does not become a smell itself.
In simple words: Heat makes solid camphor become gas, which travels through the air. When the gas reaches your nose, you smell it. Heat does not turn into smell - it just helps the change happen.

Exam Tip: Pay close attention to statement (vi) - examiners often test whether students understand that energy causes physical change, not that energy becomes a sensory property.

Question 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
In simple words: A chair is made only of its tiny particles arranged in a certain way. If you took away all the particles, there would be no chair left - not even a tiny piece.

Exam Tip: This question tests whether you understand that matter IS its particles, not that it contains particles in a container.

Question 4. Why do gases mix easily, while solids do not?
Answer: It all comes down to how free the particles are to move. Gas particles are very far apart from each other, have very weak forces pulling them together, and zoom around randomly at high speeds. This freedom lets them move about easily and blend with particles from other gases. Solid particles, on the other hand, are held tightly in a fixed structure by strong forces. They can only shake in place and cannot travel from one spot to another, so they cannot mix with other solids by themselves.
In simple words: Gas particles are far apart and move fast, so they can mix easily. Solid particles stay in fixed spots and cannot move, so they cannot mix.

Exam Tip: Link particle movement and interparticle forces to mixing ability - examiners want you to explain the 'why', not just state the observation.

Question 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: This observation shows the key difference between liquids and solids. Milk is a liquid, so its particles are held together by forces that are strong enough to keep them close to each other but weak enough to let them slide past one another. Because particles can slide, milk flows and takes on the shape of whatever surface it is on. The glass tumbler is a solid. Its particles are bound by very strong forces into a fixed, rigid arrangement. Particles cannot move away from their fixed spots, which is why the tumbler keeps its definite shape and does not flow.
In simple words: Milk particles can slide past each other, so milk flows. Glass particles are locked in place, so glass keeps its shape.

Exam Tip: Use particle arrangement and interparticle forces to explain observable differences in behavior between states of matter.

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

Exam Tip: Show clear differences in spacing and arrangement between the three states - examiners look for particle density and order to be visibly different in your diagram.

Question 7. Draw a picture representing particles present in the following: (i) Aluminium foil
Answer:

(ii) Glycerin
Answer:

(iii) Methane gas
Answer:

Exam Tip: Make sure the spacing between particles is clearly different in each diagram - solid particles touching or very close, liquid particles nearby with gaps, gas particles spread far apart.

Question 8. Observe 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 the arrangement of particles.
Answer: The candle shows wax in three different states - Solid wax: The candle's main body is solid wax. Its particles would match a diagram of tightly packed, ordered particles all locked in fixed positions. Liquid wax: At the base of the wick, a small pool of melted wax forms. Its particles would match a diagram of closely packed but randomly arranged particles that can slide past each other. Gaseous wax: The trail of smoke or vapour rising from the hot wick is gaseous wax. Its particles would match a diagram of particles far apart from each other, moving randomly through space.
In simple words: The solid candle body has particles packed tight in order. Melted wax at the wick has particles packed close but mixed up. Smoke rising up has particles spread far apart and moving fast.

Exam Tip: Clearly identify each state in the diagram and link it to a specific particle arrangement - examiners check that you can apply the particulate model to real-world examples.

Question 9. Why does the water in the ocean taste salty, even though the salt is not visible? Explain.
Answer: Salt has dissolved into the ocean water, breaking down into individual salt particles (ions) that are far too tiny to see without special tools. These invisible salt particles spread throughout the water and fill the spaces between water particles. Because the salt particles are spread evenly everywhere in the water, every single drop carries salt taste - even though your eyes cannot detect any salt crystals anywhere.
In simple words: Salt breaks into tiny bits too small to see and mixes evenly with water. That is why ocean water tastes salty everywhere, even though you cannot see salt.

Exam Tip: Emphasize that dissolution is about particles spreading and mixing, not about salt disappearing - it is still there, just invisible because particles are so small.

Question 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 definitely solids. Although you can pour large quantities of them and they will fit the shape of the jar, this is only a property of many tiny solids grouped together - it is not a property of a true liquid. The key difference is that each single rice grain or flour speck is a solid with its own fixed shape and volume. A real liquid flows because its basic particles (molecules) slide past each other. With rice, it is the large grains sliding past each other as a pile, not the inner particles of the rice itself that are sliding.
In simple words: Each rice grain is a solid because it keeps its own shape. Many solids piled together can act like they flow, but that does not make them liquid.

Exam Tip: Distinguish between the bulk properties of a collection of solids and the properties of individual particles - this is a common point of confusion that examiners test.