Activities 7.1 and 7.2 — What Matter Is Composed Of — Particulate Nature of Matter

1. 14 marksCuriosity Grade 8, Chapter 7, page 99, Activity 7.1

   Describe Activity 7.1 with the chalk. Is grinding chalk a physical change or a chemical change, and how do you know?

   Hint. The answer comes from a Grade 7 chapter, and it is the whole point of grinding rather than burning.

   The activity. Take a stick of chalk (Fig. 7.1a) and break it in two (Fig. 7.1b). Keep breaking it until you cannot break it further by hand. Grind the small pieces with a mortar and pestle (Fig. 7.1c), then look at the fine powder through a magnifying glass (Fig. 7.1d).

   What you observe. Each tiny grain you observe is still a speck of chalk. However fine the powder, it is still chalk.

   Physical or chemical? A physical change. Recalling Changes Around Us: Physical and Chemical from Curiosity Grade 7: the chalk does not change into a new substance on grinding. It is a physical change in which only the size of each speck of chalk has reduced further.

   ✦ This is why the chapter grinds chalk instead of burning it. If grinding produced something new, the experiment would tell you nothing about what chalk is made of — you would be looking at a different substance. Because nothing new is formed, every grain you produce is a genuine sample of chalk, and you are entitled to ask how far the process could go.

   The magnifying glass matters too. Even magnified, the smallest grain you can make by hand is still recognisably a speck of chalk — nowhere near the end of the road.

2. 24 marksCuriosity Grade 8, Chapter 7, page 100

   What are constituent particles? Define the term and explain how the chapter argues its way to them from a stick of chalk.

   Hint. The argument goes past what you can actually do, and it says so.

   The argument. These specks of chalk powder can be broken further into smaller particles by further grinding. Let us imagine that this process of grinding continues. Eventually, we would reach a stage where the chalk particles cannot be broken down any further. The tiny units obtained at this stage are the basic building blocks that the chalk was made up of.

   Definition. A constituent particle is the basic unit that makes up a larger piece of a substance or material.

   So one whole piece of chalk was made up of a large number of smaller units, and 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.

   ✦ Notice the words let us imagine. You cannot actually grind chalk down to its constituent particles with a mortar and pestle — the chapter is not pretending you can. It takes a process you have carried out, and asks what would happen if it continued. That is a thought experiment, and the chapter is honest about labelling it one.

   Why the conclusion is still trustworthy. It does not rest on the thought experiment alone. Activity 7.2 with dissolving sugar gives independent evidence for particles far too small to see, and every activity that follows — smoke spreading, a syringe compressing, potassium permanganate colouring a whole tumbler — makes sense only if matter is particulate.

   These particles are so small that they cannot be seen even through an ordinary microscope.

3. 34 marksCuriosity Grade 8, Chapter 7, pages 100-101, Activity 7.2

   Describe Activity 7.2 with sugar and water. Why are you asked to taste the top layer both before and after stirring?

   Hint. The first taste is a control, exactly like touching the cold wire in the last chapter.

   Safety first. Perform the activity under the supervision of a teacher or an adult. Never eat or drink anything unless asked to.

   The activity. Fill a glass tumbler with drinking water and add two teaspoons of sugar. Do not stir. Taste a small spoonful from the top layer — it does not taste sweet, because the sugar is sitting at the bottom. Now stir until the sugar dissolves completely (Fig. 7.2), and taste the top layer again — now it is sweet.

   Why both tastes are needed. The first one is the control. Without it you could not be sure the water was not sweet to begin with, or that the top of the tumbler was not somehow different. Tasting before and after, with only the stirring in between, shows that the sweetness arrived because of the dissolving.

   What it establishes. Since the top layer of water tastes sweet after dissolving sugar, it must be present in the solution. And yet sugar particles can no longer be observed but their presence can be sensed by taste.

   ✦ This is the strongest evidence in the chapter for particles too small to see. The sugar has not vanished — you can taste it at the far end of the tumbler, so it is genuinely there and genuinely spread out. But nothing is visible. Something must have become very small indeed and travelled throughout the water.

   When sugar dissolves in water, it breaks up into its constituent particles which cannot be broken down further. Each tiny grain of sugar is made up of millions and millions of such constituent particles.

4. 43 marksCuriosity Grade 8, Chapter 7, page 101

   Where does the sugar go when it dissolves? What are interparticle spaces?

   Hint. The answer requires that the water is not solid all the way through.

   Where it goes. The tiny sugar particles separate and occupy the available spaces between the water particles.

   Definition. These spaces between the particles are known as interparticle spaces.

   ✦ This answer only works if water has gaps in it, which is a genuinely surprising claim. Water looks perfectly continuous — there is no visible room in it for anything to fit into. Yet two teaspoons of sugar disappear into a tumbler that was already full, and the level barely changes. The chapter's explanation is that the water was never continuous in the first place: it is particles with spaces between them, so the sugar particles slip into room that was already there.

   Activity 7.7 turns this from an explanation into a measurement. There you mark the water level before adding sugar, after adding it, and after it dissolves — and find that the volume of the solution is less than the sum of the volumes of water and sugar, which indicates that there is some space between the water particles.

   Interparticle spaces exist in all three states, and how big they are is what distinguishes the states — minimum in solids, a little more in liquids, maximum in gases (Fig. 7.12). Section 7.3 is entirely about this.

5. 54 marksCuriosity Grade 8, Chapter 7, page 101, section 7.2

   What holds the constituent particles of a substance together? What decides how strong that hold is?

   Hint. Two things decide the strength, and one of them is very sensitive.

   What holds them together. The constituent particles of matter are held together through forces which are attractive in nature. These forces are called interparticle attractions.

   What decides their strength. The strength of these attractions depends on the nature of the substance and the interparticle distance.

   Factor	Effect
   The nature of the substance	Different substances attract with different strengths — which is why iron melts at 1538 °C and ice at 0 °C
   The interparticle distance	Even a slight increase in the distance decreases the interparticle forces drastically

   And this is what the whole chapter turns on: The strength of these forces ultimately decides the physical state of the substances.

   ✦ The word drastically is doing a great deal of work. Because the attraction falls away so sharply with distance, a small change in spacing produces a large change in behaviour. That is why melting is not a gradual softening: once the particles are pushed a little further apart, the forces holding them collapse and the solid becomes a liquid.

   It also explains the chapter's structure. Sections 7.2, 7.3 and 7.4 look at the same three states through three different windows — the strength of the attractions, the spacing between particles, and how freely the particles move. All three are the same story, because spacing and attraction are linked.

6. 63 marksCuriosity Grade 8, Chapter 7, page 101, Our scientific heritage

   What did Acharya Kanad say about the composition of matter, and in which work?

   Hint. Report exactly what the chapter says — and notice what it does not say.

   The chapter's account. Acharya Kanad, an ancient Indian philosopher, first spoke about the idea of a Parmanu (atom). He believed that matter is made up of tiny, indivisible eternal particles called Parmanu. This idea was written in his work called Vaisheshika Sutras.

   This appears under the heading Our scientific heritage, introduced with: Do you know that since ancient times, people have been thinking about how far things could be broken down and what is matter made up of?

   ✦ Notice what the chapter does not give: a date. It says ancient and nothing more precise. When Kanad lived is genuinely uncertain and scholars disagree, so adding a century to your answer would be supplying a precision the book deliberately avoids. If you want to write about the dating, take it from a cited source and say that estimates differ.

   Three features of the idea are worth naming, because they are the same three the chapter has just argued for:

   Kanad's Parmanu	The chapter's argument
   Tiny	Particles cannot be seen even through an ordinary microscope
   Indivisible	The grinding thought experiment reaches units that cannot be broken down any further
   Eternal	Physical changes rearrange particles rather than destroying them

   The box is placed exactly where it belongs — immediately after the chapter reaches the same conclusion by experiment, so you can see that a very old idea and a modern argument arrive at the same place.

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.