Activity 7.5 — The Gaseous State — Particulate Nature of Matter

1. 14 marksCuriosity Grade 8, Chapter 7, pages 105-106, Activity 7.5

   Describe Activity 7.5 with the two gas jars and the smoke. What is observed and what does it show?

   Hint. Watch what the smoke does once the glass plate is removed.

   Safety first. Be careful while burning an incense stick.

   The activity. Take two transparent gas jars, marked A and B. Burn an incense stick to make smoke, and hold Gas Jar A upside down over the smoke so the smoke is trapped inside (Fig. 7.7a). Turn it over and cover it with a glass plate (Fig. 7.7b). Now hold Gas Jar B upside down and place it over the glass plate covering Jar A. Remove the plate slowly, keeping the jars close together so no smoke escapes (Fig. 7.7c), and watch.

   What happens. The smoke fills the entire space in the Gas Jar B, indicating that gases do not have a fixed volume and tend to occupy the entire available space (Fig. 7.7d). Like liquids, they also acquire the shape of the vessel they are in.

   What it shows. The particles in gases move freely in all directions and the interparticle attractions are negligible. As a result, gases do not have a fixed shape or volume.

   ✦ Notice what the smoke was not made to do. Nobody blew it, shook it or heated it. The jars were simply opened to each other and the smoke spread by itself, upward into Jar B against gravity. Nothing but freely moving particles explains that.

   Why the jars must be kept close with no gap. If smoke could escape into the room you would learn nothing — the point is that a fixed quantity of gas spreads to fill whatever space it is given.

2. 23 marksCuriosity Grade 8, Chapter 7, page 106

   Why is smoke used in Activity 7.5 rather than an ordinary gas? What alternative does the chapter suggest?

   Hint. You are not watching gas particles — you are watching something they are pushing about.

   Why smoke. In this activity, smoke is used to represent the gaseous state. The tiny particles of smoke suspended in the air are constantly hit by invisible particles of gases, and their movement helps us observe the motion of gas particles.

   So the smoke is a marker. Gas particles are far too small to see; smoke particles are visible, and because they are being knocked about by the invisible gas particles, watching the smoke lets you watch the gas indirectly.

   The alternative. This activity can also be demonstrated by using iodine vapour instead of smoke from incense sticks. Iodine vapour can be obtained by placing some solid iodine in a closed gas jar for some time (Fig. 7.8). The purple vapour spreads freely to fill the jar in exactly the same way.

   Safety first. Be careful while using solid iodine. Vapours of iodine can cause irritation.

   ✦ This is the same technique the chapter uses again and again: make the invisible visible by giving it something to act on. Sugar's particles are detected by taste; gas particles by the smoke they push about, or by a coloured vapour; the movement of water particles by the pink streaks of potassium permanganate; the movement of air particles by a fragrance crossing a room. In none of these are you seeing particles — you are seeing what they do.

   Which is why the chapter is careful to add a warning about the word 'particle' in its A step further box: smoke and dust particles are enormous compared with constituent particles, and the two must not be confused.

3. 33 marksCuriosity Grade 8, Chapter 7, page 106

   Why do gases have neither a fixed shape nor a fixed volume?

   Hint. One property of the particles explains both.

   Because the interparticle attractions in gases are negligible, so the particles move freely in all directions.

   Property	Reason
   No fixed shape	The particles are not held in any arrangement, so they take the shape of whatever vessel they are in
   No fixed volume	Nothing holds them together at all, so they tend to occupy the entire available space — however large it is

   The Snapshot version: The interparticle attractions in gases are negligible, making their particles completely free to move from one place to another and resulting in maximum interparticle space. Therefore, gases have no fixed shape and volume.

   ✦ 'No fixed volume' is a stronger statement than it sounds, and it is what makes gases different from liquids. Pour a litre of water into a bathtub and you still have a litre of water sitting in the bottom. Release a litre of gas into the same bathtub and it spreads through the whole of it — and would spread through a room, or a hall, if it could. There is no natural volume for a gas to settle at.

   And that is exactly what Activity 7.5 demonstrates, with the smoke from one jar filling two jars the moment it is allowed to.

   Both liquids and gases are fluids, because both liquids and gases flow and do not retain a fixed shape — but only the gas has no volume of its own.

4. 43 marksCuriosity Grade 8, Chapter 7, page 106

   What are fluids? Why are liquids and gases grouped together under this name, and what still separates them?

   Hint. One shared property puts them together; one difference keeps them apart.

   Definition. Both liquids and gases flow and do not retain a fixed shape. These properties distinguish them from solids and classify them as fluids.

   What they share:

   • They flow.
   • They have no fixed shape — both take the shape of their container.
   • In both, particles can move past each other, which solids cannot do.

   What still separates them:

   	Liquid	Gas
   Fixed volume?	Yes	No — fills all available space
   Interparticle attraction	Slightly weaker than in solids	Negligible
   Interparticle spacing	A little more than in solids	Maximum
   Movement of particles	Restricted to limited space	In all the available space

   ✦ 'Fluid' is a classification by behaviour, not by state. It cuts the three states into two groups along a different line from the usual one: solids on one side, liquids and gases on the other. Whether something flows depends on one question only — can its particles move past each other? — and both liquids and gases answer yes.

   You have already used this idea without the name. The previous chapter showed that liquids and gases exert pressure on the walls of a container, and that both exert a force of friction on objects moving through them. Those shared behaviours are why the grouping is useful.

5. 54 marksCuriosity Grade 8, Chapter 7, pages 102-106

   Draw up a full comparison of solids, liquids and gases on shape, volume, interparticle attraction, spacing and movement of particles.

   Hint. The chapter's own *Let us wrap up!* table has four of the five rows.

   The complete comparison:

   	Solid	Liquid	Gas
   Shape	Definite	Takes the container's shape	Takes the container's shape
   Volume	Definite	Definite	No fixed volume — fills all space
   Interparticle attraction	Maximum	Slightly weaker than in solids	Minimum (negligible)
   Interparticle spacing	Minimum	Little more than in solids	Maximum
   Packing of particles	Closely packed	A little loosely packed than in solids	Particles are free
   Movement of particles	Negligible (only vibrations)	Restricted to limited space	In all the available space

   ✦ Read the table down the columns and you find a single story told three times. As you go solid → liquid → gas, the spacing increases, the attraction decreases, and the freedom of movement increases. These are not three independent facts: even a slight increase in the distance decreases the interparticle forces drastically, and weaker forces allow more movement. Spacing is the cause; attraction and movement follow.

   Which row is most useful in an exam? Shape and volume, because they are what you can actually observe. Everything else in the table is the explanation for those two rows.

   One row worth memorising exactly: solids and liquids both have a definite volume; only the solid has a definite shape; the gas has neither.

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.