Activity 7.4 — The Liquid State and Boiling — Particulate Nature of Matter

1. 14 marksCuriosity Grade 8, Chapter 7, page 104, Activity 7.4

   Describe Activity 7.4 with the three containers. What two conclusions follow, and why must the containers be clean and dry?

   Hint. Two things are watched at every transfer — the shape and the level.

   The activity. Take three clean and dry containers of different shapes, labelled A, B and C (Fig. 7.5). Mark the 200 mL level in each with a marker or a paper strip. Fill container A to its mark, then carefully transfer the water to B, observing the shape and level, then from B to C and observe again.

   Two conclusions, one from each observation:

   Observation	Conclusion
   The water takes the shape of the container into which it is poured	Liquids do not have a fixed shape and take the shape of the container they are kept in — because the particles of liquids are free to move
   In all three containers, the water level remains at 200 mL and no change in volume is observed	Liquids have a definite volume

   So: liquids have no fixed shape but have a fixed volume.

   ✦ Why clean and dry containers. The chapter is unusually honest about this: if a container is not clean, some water may stick to its walls, causing the water level in the next container to be slightly less than 200 mL after pouring. That is a real source of error, and it is named rather than glossed over — a small drop in level would look like evidence that liquids do not have a definite volume, when it is really just water left behind on the walls.

   Why three containers rather than two. Two could be a coincidence. Three of clearly different shapes, all giving 200 mL, makes the point convincingly.

2. 23 marksCuriosity Grade 8, Chapter 7, pages 104-105

   You can move a finger through water but not through a stone. What does this tell you about interparticle attractions in liquids compared with solids?

   Hint. Watch what the water does after you take your finger out.

   The observation. You can move your finger through water without breaking or cutting it permanently, which cannot be done in the case of solids. When you try this, you are temporarily displacing water. As soon as you remove your finger, the position of the water is restored (Fig. 7.6).

   The conclusion. In liquids, the interparticle attractions are slightly weaker than in solids, but still strong enough to keep the particles close together.

   ✦ Both halves of the observation are needed, and the second is the more informative one.

   What happens	What it shows
   The finger goes through	The attractions are weaker than in a solid — particles can move past each other
   The water closes up again	The attractions are still strong enough to hold the particles together — the liquid does not stay parted

   If only the first were true, water would behave like a gas and disperse. If only the second, it would behave like a solid and resist your finger. A liquid is precisely the state in between, and this one simple test shows both sides of it.

   And it explains why liquids are grouped with gases as fluids. Both liquids and gases flow and do not retain a fixed shape. These properties distinguish them from solids and classify them as fluids.

3. 34 marksCuriosity Grade 8, Chapter 7, page 105

   Define boiling point and explain what happens to the particles of a liquid as it boils.

   Hint. Follow the particles from vigorous movement to escape.

   Definition. The temperature at which a liquid boils and turns into vapour at atmospheric pressure is called its boiling point.

   What happens to the particles. The movement of particles becomes so vigorous that they move apart from each other, resulting in a decrease in the interparticle forces of attraction. Eventually, the constituent particles can escape from the liquid state. The liquid is converted into vapour or the gaseous state.

   The chain, step by step:

   1. Heating makes the particles move more vigorously.
   2. They move apart from one another.
   3. The increased distance decreases the interparticle forces — recall that even a slight increase in the distance decreases the interparticle forces drastically.
   4. Particles can now escape the liquid altogether.
   5. The liquid becomes vapour.

   ✦ Compare this with melting, and notice they are the same story twice. Melting: vibration grows until particles leave their fixed positions and the forces weaken. Boiling: movement grows until particles leave the liquid altogether and the forces become negligible. In both cases heat increases motion, motion increases distance, and increased distance destroys the attraction.

   Note the at atmospheric pressure in the definition, exactly as in the definition of melting point. Leaving it out costs a mark and changes the meaning.

4. 44 marksCuriosity Grade 8, Chapter 7, page 105

   How is boiling different from evaporation?

   Hint. Two differences: where it happens, and at what temperature.

   The chapter's account. At the boiling point, the formation of vapour is very fast and occurs not only at the surface but also within the liquid. This process is observed as bubble formation in the liquid. However, vapour formation occurs at all temperatures, even below the boiling point, though slowly and only at the surface. This slower process is known as evaporation.

   	Boiling	Evaporation
   Temperature	Only at the boiling point	At all temperatures
   Where	At the surface and within the liquid	Only at the surface
   Speed	Very fast	Slow
   What you see	Bubbles forming in the liquid	Nothing — the liquid simply disappears over time

   ✦ Bubbles are the visible signature of the difference. A bubble is vapour forming inside the liquid, and that only happens at the boiling point. Evaporation makes no bubbles because it happens only where the liquid meets the air.

   This answers the character's remark in the chapter: I have seen that spilled water disappears after some time, and it happens at any temperature! Spilled water evaporates — slowly, from the surface, with no boiling and no bubbles.

   The chapter says evaporation — about which you have learnt in earlier grades, and leaves it there. It does not discuss what makes evaporation faster or slower, or the cooling it produces. Those belong to another year, and importing them into a Class 8 answer goes beyond this book.

5. 53 marksCuriosity Grade 8, Chapter 7, pages 103-105

   Compare the melting point and the boiling point as definitions. What do they have in common, and what does each one mark?

   Hint. Both definitions carry the same two qualifying phrases.

   The two definitions side by side:

   	Melting point	Boiling point
   Definition	The minimum temperature at which a solid melts to become a liquid at the atmospheric pressure	The temperature at which a liquid boils and turns into vapour at atmospheric pressure
   Change marked	Solid → liquid	Liquid → vapour
   What the particles do	Leave their fixed positions but stay close together	Escape the liquid altogether
   Attractions afterwards	Weakened, but still hold particles close	Negligible

   What they have in common. Both are stated at atmospheric pressure, and both mark the temperature at which a particular set of interparticle attractions finally gives way under the particles' increasing motion.

   ✦ Each marks a boundary, not a process. A melting point is not melting — it is the temperature at which melting happens. The same substance, named by these two numbers, tells you the whole of its behaviour: below its melting point it is a solid, between the two it is a liquid, above its boiling point it is a gas.

   Why atmospheric pressure has to be specified. Both definitions are about a substance under the conditions we normally live in. The chapter does not explore what happens at other pressures, and neither should a Class 8 answer — but it is careful to say which condition its numbers apply to.

6. 63 marksCuriosity Grade 8, Chapter 7, pages 104-105

   Milk spilled from a glass tumbler flows and spreads out, but the tumbler keeps its shape. Explain both, in terms of particles.

   Hint. Two substances, two states, one explanation each.

   The milk — a liquid. Its particles are held by interparticle attractions that are slightly weaker than in solids, but still strong enough to keep the particles close together. Because they are free to move within a limited space, they slide past one another and rearrange to fit whatever holds them. On a table there is nothing holding them, so the milk spreads out. Liquids have no fixed shape but have a fixed volume — the puddle is a different shape, not a different amount.

   The tumbler — a solid. Its particles are tightly packed, held in fixed positions by very strong interparticle attractions, and cannot move past each other. They can only vibrate where they are. So the tumbler keeps its definite shape and definite volume, whatever happens around it.

   	Milk	Glass tumbler
   State	Liquid	Solid
   Interparticle attraction	Slightly weaker	Very strong
   Can particles move past each other?	Yes	No
   Shape	Takes the shape available	Definite
   Volume	Definite	Definite

   ✦ The single property that separates them is whether particles can change places. Both have a definite volume — the milk that spilled is still the same quantity of milk. What the tumbler has and the milk does not is a shape of its own, and that comes entirely from its particles being unable to move past one another.

   This is exercise question 5.

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.