Activity 7.3 — The Solid State and Melting — Particulate Nature of Matter

1. 14 marksCuriosity Grade 8, Chapter 7, page 102, Activity 7.3

   Describe Activity 7.3. What do all six objects have in common, and what does that tell you about their particles?

   Hint. The objects are deliberately as different from one another as possible.

   The activity. Collect a few solid objects — a piece of iron or an iron nail, a piece of rock salt, a stone, a piece of wood, a key, and a piece of aluminium (Fig. 7.3). Observe their shapes and sizes, and try hammering them. Then ask: in which of the above six objects do you think particles are strongly held together?

   What they have in common. You must have noticed that all these objects are solids. They have a definite shape and volume.

   What that says about their particles. In solids, the particles are tightly packed and the interparticle attractions are very strong. These strong forces of attraction hold the particles in fixed positions, preventing them from moving freely (Fig. 7.4a). The particles can only move to and fro about their positions (vibrate or oscillate) but cannot move past each other.

   ✦ Why six very different objects rather than one. Iron, rock salt, stone, wood, a key and aluminium are made of quite different substances, and hammering affects them very differently. Yet every one keeps a definite shape and volume. That is what makes 'solid' a genuine category rather than a description of one material.

   What the hammering shows. It takes real force to change a solid's shape at all — and even then the pieces keep their own definite shapes. Compare this with a liquid, where you can push a finger straight through and the position of the water is restored the moment you take it out.

2. 23 marksCuriosity Grade 8, Chapter 7, page 102

   Why do solids have a definite shape and a definite volume?

   Hint. Two features of the particles, and each explains one of the two properties.

   Because the particles are tightly packed and held in fixed positions by very strong interparticle attractions.

   Property	Reason
   Definite shape	These strong forces of attraction hold the particles in fixed positions, so they cannot move past each other — the arrangement itself cannot be rearranged
   Definite volume	The particles are tightly packed, with minimum interparticle spacing, so the solid cannot be squeezed into a smaller space

   What the particles can still do. They are not motionless: the particles can only move to and fro about their positions (vibrate or oscillate). A solid particle jiggles in place; it does not travel.

   ✦ Definite shape is the property that separates solids from both other states, and it comes entirely from the particles being unable to change places. A liquid has a definite volume too — but its particles can slide past one another, so the shape is whatever the container decides. Take away the ability to move past each other and you take away the ability to flow.

   The Snapshot version: Solids have a fixed shape and size due to strong interparticle attraction, minimum interparticle space, and no free movement of the constituent particles.

3. 34 marksCuriosity Grade 8, Chapter 7, pages 102-103

   Explain what happens to the particles of a solid when it is heated, and how it melts. Define melting point.

   Hint. Follow Fig. 7.4 from (a) to (c) — three pictures, three stages.

   The three stages of Fig. 7.4.

   Stage	What the particles are doing
   (a) Solid	Tightly packed, held in fixed positions, vibrating about those positions
   (b) Heated solid	When solids are heated, their particles vibrate more vigorously
   (c) Liquid	A stage is reached when these vibrations become so vigorous that the particles start leaving their respective positions. The interparticle forces of attraction get weakened and the solid gets converted into the liquid state

   Definition. The minimum temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point.

   ✦ Note the chain of cause and effect, because it is easy to state it backwards. Heat makes the particles vibrate harder; harder vibration lets them leave their positions; leaving their positions increases the interparticle distance; and even a slight increase in the distance decreases the interparticle forces drastically. So the attraction weakens because the particles moved apart — the heat does not weaken the forces directly.

   And note the two conditions in the definition: minimum temperature, and at the atmospheric pressure. Both matter, and both are easy to leave out.

   What the liquid is like. Generally, in a liquid state, particles are somewhat farther away from each other as compared to those in the solid state — with one exception the chapter names: ice is an exception — its particles are farther apart than those in water.

4. 43 marksCuriosity Grade 8, Chapter 7, page 103

   Why is ice described as an exception? What follows from it?

   Hint. Compare where the particles are in ice and in liquid water.

   The chapter's statement. Generally, in a liquid state, particles are somewhat farther away from each other as compared to those in the solid state (ice is an exception — its particles are farther apart than those in water).

   Why this is exceptional. For almost every substance, melting spreads the particles out — the solid is the more closely packed of the two. Water does the reverse: its particles are further apart in the solid (ice) than in the liquid.

   ✦ A consequence you can observe, though the chapter does not spell it out. Particles spread further apart means the same amount of matter occupies more space — which is why ice floats on water rather than sinking, and why a bottle of water left in a freezer can crack. Both are everyday facts that would be impossible if water behaved like other substances.

   How far to take this in an answer. The chapter states the exception in a bracket and does not explain why water behaves this way — that needs ideas about how water particles arrange themselves, which come in later grades. State the fact, note that it is exceptional, and do not invent a mechanism.

   The general rule still stands for everything else. Interparticle spacing is minimum in solids and little more than in solids in liquids, as the Let us wrap up! table puts it. Water is the named exception, not a counter-example to the whole scheme.

5. 53 marksCuriosity Grade 8, Chapter 7, page 103, Table 7.1

   Reproduce Table 7.1 and explain what the differences in melting point tell you about the substances.

   Hint. Three substances, and the range between them is enormous.

   Table 7.1: Melting points of some solids

   S.No.	Material	Melting point
   1	Ice	0 °C
   2	Urea	133 °C
   3	Iron	1538 °C

   What the differences mean. Some solids have weak interparticle forces of attraction, so their melting points are low. While others have strong attractive forces and have high melting points.

   Substance	Melting point	Interparticle attraction
   Ice	0 °C	Weakest of the three
   Urea	133 °C	Stronger
   Iron	1538 °C	Strongest

   ✦ A melting point is a measurement of a force you cannot see. You cannot put a number on how strongly iron's particles attract one another — but you can measure the temperature at which they finally let go, and that temperature ranks the substances for you. Iron needs more than fifteen hundred degrees; ice needs none at all above freezing.

   This is the nature of the substance factor from section 7.2 made concrete. The strength of interparticle attraction depends on the substance and on the distance; Table 7.1 shows the substance half of that in numbers.

   And it explains something familiar: why an iron nail stays solid in a fire that would melt a candle in seconds.

6. 64 marksCuriosity Grade 8, Chapter 7, pages 102-103

   A student says: 'The particles in a solid do not move at all.' Correct this, and say why the distinction matters.

   Hint. There is a difference between moving and going somewhere.

   The student is wrong: the particles do move — they just cannot go anywhere.

   The particles can only move to and fro about their positions (vibrate or oscillate) but cannot move past each other. So there are two different things being confused:

   	In a solid
   Motion of any kind?	Yes — every particle vibrates about its fixed position
   Travelling from place to place?	No — the strong attractions hold them in fixed positions

   Why the distinction matters — it is what makes melting possible. If solid particles were completely still, heating them would have nothing to act on. In fact heating makes the existing vibration more vigorous, until these vibrations become so vigorous that the particles start leaving their respective positions. Melting is the vibration winning; without vibration in the first place there would be no route from solid to liquid.

   ✦ The Let us wrap up! table words this very carefully: movement of particles in a solid is negligible (only vibrations). Negligible — not zero, and the bracket tells you exactly which kind of movement remains.

   A way to picture it. People standing shoulder to shoulder in a packed crowd can shuffle and sway on the spot but cannot walk across the room. Heat the crowd enough and the shuffling becomes pushing, and then people do start changing places — which is the liquid.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity, Textbook of Science for Grade 8, Chapter 7 'Particulate Nature of Matter', book pages 98-115 (hecu107.pdf, 18 pages, Reprint 2026-27), downloaded from ncert.nic.in and read page by page. Every activity number, figure number, quantity and quoted sentence below was checked against that PDF. Fig. 7.16 (exercise 8) was rendered and read: 7.16a shows a just-extinguished candle with solid wax in the body and set drips, liquid wax pooled round the wick, and vapour rising from the wick; 7.16b gives three particle boxes - regular close-packed (solid), close but irregular (liquid), and a few widely scattered particles (gas). Table 7.1's melting points were transcribed exactly: ice 0 C, urea 133 C, iron 1538 C. Three deliberate restraints. (1) The chapter covers ONLY melting point and boiling point, with evaporation mentioned in two sentences as prior learning. Sublimation, deposition, condensation, freezing, the factors affecting evaporation, evaporative cooling and the effect of pressure on state changes appear NOWHERE in this book and appear nowhere here. (2) The Acharya Kanad box gives no date - the chapter says only 'ancient Indian philosopher' and names the Vaisheshika Sutras - so no century is supplied, and the solution notes explicitly that dating estimates differ and would need a cited source. (3) Atoms and molecules are introduced only as the chapter's final 'A step further' box introduces them, with its own closing limit quoted: 'You will learn about atoms and molecules in higher grades.' Nothing about atomic structure or chemical formulae is added.. Questions are referenced from the NCERT textbook for identification.