Activities 7.8 and 7.9 — How Particles Move — Particulate Nature of Matter

1. 14 marksCuriosity Grade 8, Chapter 7, pages 109-110, Activity 7.8

   Describe Activity 7.8 with potassium permanganate. What do you observe, and how does the chapter explain it?

   Hint. The explanation has two separate stages — pulling out, then spreading.

   Safety first. Do not touch potassium permanganate with your hands. Use a spoon or a spatula to handle it.

   The activity. Take a glass tumbler of water and drop in a few grains of potassium permanganate. Do not stir.

   What you observe.

   1. Initially, you will see some streaks of pink colour spreading out from the grain (Fig. 7.13a).
   2. With the passage of time, the entire bulk of water will acquire a uniform pink colour (Fig. 7.13b).

   The explanation, in two stages. This happens because the water particles are in constant motion. First they pull out the particles of potassium permanganate from its grain, and later they hit these particles so that they get spread throughout the liquid.

   Stage	What the water particles do
   Pulling out	They detach potassium permanganate particles from the solid grain
   Spreading	They collide with the freed particles and knock them throughout the liquid

   ✦ The key word is without stirring. Nobody mixes the water, and yet the colour reaches every part of the tumbler. Something must be moving of its own accord — and the only candidates are the particles themselves. That is the whole argument of section 7.4: the water particles are in constant motion.

   And it explains solubility too. 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. Whether something dissolves is a contest between the water particles' pull and the substance's own interparticle attractions.

2. 24 marksCuriosity Grade 8, Chapter 7, page 110, Think like a scientist

   Repeat Activity 7.8 with hot water, water at room temperature and ice-cold water. What do you observe, and what does it establish?

   Hint. Three tumblers, one variable, and the conclusion is the chapter's most important one.

   The test. Take three clean glass tumblers. Pour hot water into one, water at room temperature into the second and ice-cold water into the third. Drop a small grain of potassium permanganate into each. Watch carefully and compare.

   What you observe. The potassium permanganate spreads the fastest in hot water, less quickly in water at room temperature, and the slowest in ice-cold water.

   Why. Water particles move faster in hot water compared to water at room temperature, and even slower in ice-cold water.

   The conclusion. Hence, the movement of particles increases when heat is provided.

   ✦ This is the experiment that ties the whole chapter together. Everything else has described what particles do in each state; this one shows what changes them from one state to another. Heat makes particles move faster — and faster movement is exactly what melts a solid (vibrations become vigorous enough to leave fixed positions) and boils a liquid (movement becomes vigorous enough to escape altogether).

   A fair test. The only difference between the three tumblers is the temperature — same water, same size of grain, same tumblers, no stirring in any of them. So the difference in spreading can be attributed to the temperature and to nothing else.

   The chapter asks you to try to depict it by drawing a diagram — three tumblers side by side, with the pink colour spread furthest in the hot one and barely started in the cold one.

3. 33 marksCuriosity Grade 8, Chapter 7, pages 110-111, Activity 7.9

   Describe Activity 7.9 with the incense stick. What does it show about the movement of gas particles?

   Hint. You cannot see gas particles — so what is the fragrance doing for you?

   The activity. Light an incense stick in one corner of a room (Fig. 7.14). Wait a few minutes and observe. Do you notice the fragrance from a distance?

   What happens. When an incense stick is burnt in one corner of the room, initially, the fragrance is felt only around the incense stick. Shortly, you can smell the fragrance throughout the room.

   The explanation. This happens because the particles of the fragrance spread, filling the entire room. This shows that the particles of air are moving constantly. The air particles hit the particles of the fragrance and help them spread throughout the room.

   ✦ This is Activity 7.8 done with a gas instead of a liquid, and the structure is identical. In both, a substance released at one point spreads by itself through the whole body of the fluid, with nobody stirring or fanning it — and in both, the explanation is that the surrounding particles are in constant motion and knock the new particles about.

   	Activity 7.8	Activity 7.9
   Medium	Water (liquid)	Air (gas)
   What is released	Potassium permanganate	Fragrance from incense
   How you detect it	Colour	Smell
   Conclusion	Water particles are in constant motion	Air particles are in constant motion

   The chapter's own follow-up: Can you share a few other real-life situations where you have experienced the movement of gas particles? — the smell of cooking reaching another room, perfume noticed across a hall, the smell of a leaking gas cylinder, camphor scenting a room.

4. 43 marksCuriosity Grade 8, Chapter 7, page 111, Ever heard of ...

   How does soap remove an oil stain from cloth? Explain in terms of particles.

   Hint. One soap particle does two jobs at once, at its two ends.

   The chapter's account. When we wash clothes stained with oil using soap, numerous soap particles surround the oil particles on the fabric. One end of the soap particle attaches to the oil, and the other mixes with water, thus helping lift the oil off and wash it away (Fig. 7.15).

   Why this is needed at all. Oil does not mix with water — water particles alone cannot pull oil particles off the fabric, in the way they pull potassium permanganate off its grain in Activity 7.8. Something has to bridge the two.

   How a soap particle bridges them:

   End of the soap particle	What it attaches to
   One end	The oil on the fabric
   The other end	The water

   With soap particles gripping the oil at one end and the water at the other, the oil can be lifted off the cloth and carried away when the water is poured out.

   ✦ The box is placed here to show that the chapter's ideas are not only about laboratory activities. The particulate nature of matter plays a crucial role in many everyday processes. Washing clothes is a particle-level process, and it works because one kind of particle happens to have two different ends.

   How far to take this. The chapter says 'one end' and 'the other end' and stops there — it does not name the parts or explain why they behave differently. That comes in later grades, and a Class 8 answer should stop where the book stops.

5. 54 marksCuriosity Grade 8, Chapter 7, page 112

   The chapter concludes that it is the thermal energy of the particles that determines the physical state of matter. Set out that argument.

   Hint. The chain has three links: thermal energy, distance, attraction.

   The chapter's summing up. Matter is made up of small particles which are held together by the force of attraction. The strength of attractive forces between particles depends on the distance between them, which in turn depends on their thermal (heat) energy. Thus, it is the thermal energy of the particles that determines the physical state of matter.

   The chain:

   thermal energy → interparticle distance → strength of attraction → physical state

   Applied to the three states:

   State	Thermal energy	Consequence
   Solid	Low	Particles remain close to each other and experience strong interparticle attractive forces. This restricts their motion to only small vibrations
   Melting point	Rising	The thermal energy is used to overcome the attractive forces between particles, allowing the solid to change into a liquid
   Liquid	Higher	The interparticle distance increases slightly, reducing the strength of the attractive forces to a level that allows the particles to move around, though still within a limited space
   Gas	Highest	The particles have enough energy to overcome the forces of attraction between them and move freely in all directions

   ✦ This is why the chapter ends here rather than earlier. Sections 7.2, 7.3 and 7.4 described attraction, spacing and movement as three separate topics. This paragraph reveals them as one: thermal energy sets the distance, distance sets the attraction, and attraction sets the state. There is really only one variable in the whole chapter.

   You will learn more about these particles that constitute matter in higher grades.

6. 64 marksCuriosity Grade 8, Chapter 7, page 115, A step further

   What are the tiny particles that make up all matter called? Explain the difference the chapter draws, with its examples.

   Hint. Some elements' atoms can exist alone and some cannot.

   The chapter's account. The tiny particles that make up all matter are atoms and molecules. For example, iron is made up of atoms of iron, and gold is made up of atoms of gold. Atoms of many elements like hydrogen, oxygen, and sulfur are not able to exist independently. In such cases, a certain number of atoms of the same element combine to form a molecule. For example, two atoms of hydrogen combine and form a stable particle called a molecule of hydrogen. A water molecule is made up of two hydrogen atoms and one oxygen atom.

   Example	Made of
   Iron	Atoms of iron
   Gold	Atoms of gold
   Hydrogen	Molecules, each of two hydrogen atoms — its atoms cannot exist independently
   Water	Molecules, each of two hydrogen atoms and one oxygen atom

   ✦ This box finally names what the chapter has been calling 'constituent particles' all along — and notice that it is left to the very last page, after a whole chapter of arguing that such particles must exist. The chapter earns the names rather than starting with them, because a name handed over before the evidence would be nothing but a word to memorise.

   And it closes with an honest limit: You will learn about atoms and molecules in higher grades. Everything about how atoms are built, how they combine, or how to write formulae belongs to a later year. In Class 8 you should use constituent particles as your working term, and know that they are atoms and molecules.

   Which also settles the SPM warning from page 109: a dust particle is made of a very large number of constituent particles, i.e., atoms and molecules.

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.