Probe and Ponder — The Opening Questions — Particulate Nature of Matter

1. 13 marksCuriosity Grade 8, Chapter 7, page 98

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

   Hint. The whole chapter is about what holds particles in place.

   Because the particles of a solid are held in fixed positions, while those of a liquid are free to move.

   	Stones and sand	Water
   Interparticle attraction	Strong	Slightly weaker
   Can particles move past each other?	No — they only vibrate about fixed positions	Yes, within a limited space
   Shape	Definite	Takes the shape of its container
   Can it be piled?	Yes	No — it spreads out and flows

   ✦ A grain of sand is itself a solid, and that is the point. A heap of sand can be piled because every grain keeps its own shape and sits where it is put, resting on the grains beneath it. Water cannot be piled because its particles slide past one another the moment they are stacked, so any heap collapses and spreads.

   A caution about the heap. The heap of sand does take the shape of whatever jar you pour it into — but the grains do not change shape at all. Confusing the two is exactly the trap in exercise question 10 about rice and rice flour.

2. 23 marksCuriosity Grade 8, Chapter 7, page 98

   Why does water take the shape of folded hands but lose that shape when released?

   Hint. Two properties of a liquid are on show here, not one.

   Because a liquid has no fixed shape of its own — it only borrows the shape of whatever holds it.

   The particles of liquids are free to move, so they arrange themselves to fit the container. Cupped hands are a container, and the water fills them. Open your hands and the container is gone, so the particles flow apart and the shape is lost at once.

   ✦ What does not change is the amount of water. Liquids have no fixed shape but have a fixed volume. The same water that filled your hands spreads on the ground — it has changed shape, not quantity. Activity 7.4 makes exactly this point by pouring 200 mL of water through three differently shaped containers: the shape changes every time, the level stays at 200 mL.

   Why the particles stay together at all rather than flying apart like a gas: in liquids, the interparticle attractions are slightly weaker than in solids, but still strong enough to keep the particles close together. Weak enough to flow, strong enough not to disperse — that is what being a liquid means.

3. 33 marksCuriosity Grade 8, Chapter 7, page 98

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

   Hint. Being invisible is not the same as being nothing.

   Because air is matter — it is made of constituent particles, and those particles have weight, even though they are far too small to see.

   The chapter's central claim is that matter is composed of a large number of extremely small particles which cannot be seen even through an ordinary microscope. Not being visible tells you the particles are small; it tells you nothing about whether they exist.

   Blowing up a balloon puts more air particles inside it. More particles means more matter, and more matter weighs more.

   ✦ The evidence that air is really there is all around this chapter. In Activity 7.5 smoke spreads to fill a second gas jar, pushed about by invisible particles of gases. In Activity 7.6 a syringe full of air resists being compressed, and springs back when released. In Activity 7.9 the fragrance from an incense stick crosses a room because the air particles hit the particles of the fragrance and help them spread. Something invisible is doing all of that.

   And you met air's weight in the previous chapter. Pressure, Winds, Storms, and Cyclones found that the air column above a 15 cm square patch presses with about 2250 N. Air is not nothing.

4. 44 marksCuriosity Grade 8, Chapter 7, page 98

   Is the air we breathe today the same that existed thousands of years ago?

   Hint. The chapter does not answer this. Say what it does let you conclude, and where you would have to stop.

   What the chapter lets you say. Matter is made of constituent particles, and a constituent particle is the basic unit that makes up a larger piece of a substance or material. Grinding, dissolving and changing state all move these particles about and rearrange them — Activity 7.1 shows that grinding chalk is a physical change in which only the size of each speck of chalk has reduced. The particles themselves are not destroyed by such changes.

   So the particles making up the air in your lungs right now have been in existence for a very long time, and have been through countless other places on the way.

   ✦ Where honesty requires you to stop. The chapter says nothing about how long particles last, whether the composition of the atmosphere has changed over thousands of years, or how air circulates around the planet. All three are real scientific questions with real answers, and none of those answers is in this book.

   The right response is to separate two different questions:

   Question	What you can say from this chapter
   Are these the same particles?	Physical changes rearrange particles rather than destroying them, so many are indeed very old
   Is the air the same mixture?	The chapter gives you nothing on this — it is a question for a later grade, and for sources you would have to check

   A Probe-and-ponder question is meant to be carried around, not closed off. Saying this is what I can establish, and this is where my evidence runs out is a better answer than a confident one that goes beyond the book.

5. 53 marksCuriosity Grade 8, Chapter 7, page 99

   The chapter opens with pebbles, stones and sand on a riverbank. Trace where they come from, and say what question this leads to.

   Hint. The opening is not scene-setting — it is the chapter's first question in disguise.

   Where they come from. In the mountains, rocks gradually break down due to erosion. Rivers flowing through these regions carry along the eroded rock pieces. As the rivers flow, they continue to break down the rocks further into pebbles, stones, sand; and transport large quantities of them to the plains. The bigger rocks are eventually broken down into finer grains of sand and clay.

   The question it leads to. Is this grain the smallest unit of a bigger rock or can these grains of sand and clay be broken down further?

   ✦ The opening is doing real work. A river spends thousands of years doing by accident what Activity 7.1 asks you to do by hand in five minutes — breaking something into ever smaller pieces. The natural question is whether that process has an end, and the whole of section 7.1 is the answer: yes, it ends at the constituent particles.

   And the chapter returns to the answer explicitly: Just like chalk, the grains of sand and clay are not the smallest units of bigger rocks. These are also made up of a large number of their constituent particles. The riverbank is not decoration, because it is the same question as the chalk, asked at the scale of a mountain — one the river has already been answering, slowly, for thousands of years.

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.