Activities 7.6 and 7.7 — Interparticle Spacing — Particulate Nature of Matter

1. 14 marksCuriosity Grade 8, Chapter 7, page 107, Activity 7.6

   Describe Activity 7.6 with the syringe. What happens when you push the plunger in, and what does it show?

   Hint. Then repeat it with water, and compare.

   The activity. Take a syringe without a needle. Pull the plunger fully out (Fig. 7.9a). Place your thumb firmly over the open end so no air can escape (Fig. 7.9b). Now push the plunger slowly and steadily inwards (Fig. 7.9c).

   What you observe. The volume of air inside the syringe decreases. And if you stop pushing, the gas particles spread, and the plunger moves back to its original position.

   What it shows. When you compress the air by pushing the plunger, the particles are forced to come closer. This shows that the gas particles have a lot of space between them in their natural state, and this space can be reduced by applying external pressure.

   Repeat with water. You would observe that water is practically incompressible.

   	Air	Water
   Can it be compressed?	Yes, easily	No — practically incompressible
   What that says about spacing	Large interparticle spaces	Very little space between particles

   ✦ The water trial is the more important half of the activity. Compressing air alone shows only that air can be squeezed. Filling the same syringe with water and finding it will not budge shows that the compressibility belongs to the gas, not to the syringe, and that the difference between the two states is the amount of empty space in them.

   Why the thumb must seal the end completely. Otherwise the air simply escapes and the plunger moves in for a quite different reason — you would be emptying the syringe, not compressing anything.

2. 24 marksCuriosity Grade 8, Chapter 7, pages 108, Activity 7.7

   Describe Activity 7.7 with the three water levels A, B and C. What is observed, and what does it prove?

   Hint. Predict where level C will be before you stir — the activity asks you to.

   The activity. Half-fill a glass vessel with water and mark the level A (Fig. 7.10a). Add two teaspoons of sugar and mark the new level B (Fig. 7.10b). Predict whether the level will rise or fall once the sugar dissolves. Stir with a glass rod (Fig. 7.10c) and mark the final level C (Fig. 7.10d).

   What you observe. Initially, when sugar is added, the level of water increases, but after dissolution, it may decrease to some extent. So C is lower than B.

   What it proves. Since the volume of the solution is less than the sum of the volumes of water and sugar, it indicates that there is some space between the water particles. The particles of the dissolved substance occupy these spaces (Fig. 7.11).

   ✦ This is a measurement, not just a demonstration. Activity 7.2 established that sugar disappears into water and can still be tasted. This activity goes further and shows that the mixture takes up less room than its two ingredients did separately — which can only happen if some of the sugar has gone into gaps that were already there.

   Why you are asked to predict first. Most people expect the level to stay at B, or to rise further. Being wrong is the point: a result that matches what you already believed teaches you nothing, and writing the prediction down beforehand is what makes the surprise honest.

   Compare the previous chapter, where you were also asked to predict before Activity 6.5 and again before blowing over a paper strip. The habit is deliberate.

3. 34 marksCuriosity Grade 8, Chapter 7, pages 108-109

   Activity 7.7 asks you to repeat the experiment with insoluble solids such as sand and stone pieces. What happens, and why is the result different?

   Hint. Ask whether the added solid can get into the gaps.

   What happens with sand. The sand does not dissolve. Sand is a solid that does not dissolve in water. When added to water, the sand particles settle down and occupy some space in the container, causing the total volume to increase.

   	Sugar (soluble)	Sand (insoluble)
   Does it dissolve?	Yes	No — it settles at the bottom
   Where does it go?	Its particles separate and occupy the interparticle spaces between water particles	It stays as grains, taking up space of its own
   Effect on the level	The level falls back somewhat after dissolving	The level increases

   Why the difference. Section 7.4 gives the reason: the water particles first pull out the particles of potassium permanganate from its grain, and later hit these particles so that they get spread throughout the liquid. But in the case of many substances, the constituent particles are held together strongly that the water particles are unable to pull these out. Such substances, like sand, are insoluble in water.

   ✦ The comparison is what makes the sugar result meaningful. On its own, a falling water level could be blamed on almost anything. Add sand instead, under identical conditions, and the level rises — so the fall with sugar must be caused by something the sugar does and the sand does not: breaking up into constituent particles and slipping into the gaps between the water particles.

   The chapter's own question: Sugar and sand are both solids. Why does sugar dissolve in water but sand does not? The answer is the strength of the interparticle attractions holding each one together.

4. 44 marksCuriosity Grade 8, Chapter 7, page 109

   Is there any space between the particles of a solid? If so, what is in that space?

   Hint. The chapter anticipates the wrong answer and rules it out explicitly.

   Yes, there is space — but it is minimum. You learnt earlier that the constituent particles in solids are held together by strong forces of attraction. So, these particles do not move from one place to another and are closely packed. However, despite close packing, some space is left between the particles (Fig. 7.12a).

   What is in that space. You might assume that the space between particles is filled with air, but this is not the case. They contain nothing at all.

   ✦ The chapter names the wrong answer before you can give it, which tells you how common it is. 'Air fills the gaps' feels obviously right, and it is obviously wrong once you think it through: air is itself made of particles, and those particles would then need spaces of their own, filled with something else again. The chain has to stop, and it stops with empty space.

   The comparison across the three states (Fig. 7.12):

   State	Interparticle spacing
   Solid	Minimum — but not zero
   Liquid	Little more than in solids
   Gas	Maximum

   Why 'minimum but not zero' matters. If solid particles touched with no gaps at all, a solid could not be compressed even slightly, could not expand on heating, and could not have its particles pushed a little further apart as it melts. The small space is what makes all of that possible.

5. 53 marksCuriosity Grade 8, Chapter 7, page 109, A step further

   The chapter warns that the word 'particle' means different things in different contexts. Explain the warning.

   Hint. Suspended Particulate Matter and a constituent particle differ by an enormous factor.

   The chapter's warning. Often, we use the term 'particle' in different contexts. The meaning of this term changes with the context. For example, while talking about air pollution, the term Suspended Particulate Matter (SPM) is used. This term refers to the tiny dust particles suspended in air and not the constituent particles of matter which are extremely small as compared to the dust particles.

   And the point that settles it: In fact, even these tiny dust particles are also made up of a very large number of constituent particles, i.e., atoms and molecules.

   Term	What it means	Can you see it?
   Dust particle / SPM	A tiny speck of solid floating in air	Sometimes, in a beam of light
   Constituent particle	The basic unit a substance is made of	No — not even through an ordinary microscope

   ✦ The scale difference is the whole warning. A single dust particle contains a very large number of constituent particles. Calling both 'particles' makes them sound comparable, and they are not remotely comparable.

   Where this matters in this very chapter. Activity 7.5 uses smoke particles to reveal the motion of gas particles. It would be easy to come away thinking you had watched gas particles moving. You had not — you watched visible smoke specks being knocked about by invisible gas particles, and the chapter says so carefully: the tiny particles of smoke suspended in the air are constantly hit by invisible particles of gases.

6. 64 marksCuriosity Grade 8, Chapter 7, pages 107-109

   Air can be compressed in a syringe but water cannot. Explain, and predict what would happen if you tried the same thing with a solid.

   Hint. Rank the three states by how much empty space they contain.

   Air compresses easily because the gas particles have a lot of space between them in their natural state, and this space can be reduced by applying external pressure. Pushing the plunger forces the particles closer together into the space that was already there.

   Water is practically incompressible because its particles are already close together, with only a little space between them. There is almost nothing to squeeze out, so the plunger will not move.

   A solid would be even harder to compress. Its particles are tightly packed with the minimum interparticle spacing of the three states — and they are held in fixed positions by very strong attractions, so they cannot even be pushed closer.

   State	Interparticle spacing	Compressible?
   Gas	Maximum	Easily
   Liquid	A little more than in solids	Practically not
   Solid	Minimum	No

   ✦ Compressibility is a direct measurement of how much empty space a state contains, which is why the syringe is such a good experiment. You cannot see interparticle spaces, but you can feel how far the plunger travels — and it travels a long way for air, barely at all for water, and not at all for a solid.

   And note what happens when you let go: the gas particles spread, and the plunger moves back to its original position. Nothing was destroyed by the compression; the particles were simply crowded, and they spread out again the moment they could.

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.