Particulate Nature of Matter — Class 8 Science (Curiosity)
"Is this grain the smallest unit of a bigger rock, or can these grains of sand and clay be broken down further?" — Curiosity, Grade 8, page 99
1. About the Chapter
This is Chapter 7 of Curiosity (pages 98–115, Reprint 2026-27). It opens on a riverbank. Mountains erode, rivers carry the broken rock down and grind it finer along the way, and what reaches the plains is sand and clay. The chapter's first question is whether that grinding has an end.
| Section | Question |
|---|---|
| 7.1 | What is matter composed of? |
| 7.2 | What decides the different states of matter? |
| 7.3 | How does the interparticle spacing differ in the three states? |
| 7.4 | How do particles move in different states of matter? |
The single thread. Matter is built from constituent particles. Everything after page 100 comes from two facts about them — the spaces between them and the attractions between them — and the chapter ends by naming the third: how much thermal energy they have.
What this chapter is not. It defines exactly two terms — melting point and boiling point. Sublimation, deposition, condensation, the factors affecting evaporation, evaporative cooling and the effect of pressure on state changes belong to Class 9, not here. Evaporation gets two sentences, as prior learning from earlier grades.
2. What Is Matter Composed of?
Activity 7.1 — grinding chalk
Break a stick of chalk in two. Break it again, and again, until you cannot break it further by hand. Grind the pieces in a mortar and pestle. Look at the powder under a magnifying glass.
Each tiny grain you observe is still a speck of chalk.
The chapter pauses here to make you check something from Grade 7: is grinding a physical change or a chemical one?
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.
That matters, because the whole argument depends on it. If grinding changed the chalk into something else, following the process further would tell you nothing about what chalk is made of.
The thought experiment
You cannot grind chalk down to its constituent particles with a pestle, and the chapter does not claim you can:
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.
These units are called constituent particles of chalk. A constituent particle is the basic unit that makes up a larger piece of a substance or material.
And the riverbank question is answered: 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.
Activity 7.2 — the sugar that disappears but is still there
Fill a tumbler with drinking water. Add two teaspoons of sugar. Do not stir. Taste a spoonful from the top layer. Then stir until the sugar dissolves and taste the top layer again.
The top layer is sweet — and no sugar can be seen anywhere.
Sugar particles can no longer be observed but their presence can be sensed by taste. 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.
Taken together, the two activities give the chapter's foundation:
Activities 7.1 and 7.2 support the idea that matter is composed of a large number of extremely small particles. These particles are so small that they cannot be seen even through an ordinary microscope.
Where did the sugar go?
The tiny sugar particles separate and occupy the available spaces between the water particles. These spaces between the particles are known as interparticle spaces.
This is a genuinely surprising claim — water looks perfectly continuous. Activity 7.7 will turn it into a measurement.
Our scientific heritage. 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. The chapter gives no date, and when Kanad lived is genuinely uncertain — do not invent one.
3. What Decides the State of Matter?
The constituent particles of matter are held together through forces which are attractive in nature. These forces are called interparticle attractions. The strength of these attractions depends on the nature of the substance and the interparticle distance. Even a slight increase in the distance decreases the interparticle forces drastically. The strength of these forces ultimately decides the physical state of the substances.
That sentence about distance is worth reading twice. Attraction and spacing are not two independent facts — pull the particles apart a little and the attraction collapses, which is why the three states differ as sharply as they do.
3.1 The solid state — Activity 7.3
Collect six solids: an iron nail, rock salt, a stone, a piece of wood, a key, a piece of aluminium. Observe their shapes. Try hammering them.
They have a definite shape and volume. This is due to the fact that in solids, the particles are tightly packed and the interparticle attractions are very strong. These strong forces of attraction hold the particles in fixed positions, preventing them from moving freely. The particles can only move to and fro about their positions (vibrate or oscillate) but cannot move past each other.
Fixed position is not the same as no motion. Solid particles vibrate. Writing "the particles in a solid do not move" is the standard way to lose a mark on this chapter.
Melting — Fig. 7.4 in three stages
| Stage | What the particles are doing |
|---|---|
| (a) Solid | Packed, held in fixed positions, vibrating gently |
| (b) Heated solid | Vibrating far more vigorously, still in place |
| (c) Liquid | Vibrations so vigorous that particles leave their positions; attractions weaken; the solid becomes a liquid |
The minimum temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point.
Some solids have weak interparticle forces of attraction, so their melting points are low. While others have strong attractive forces and have high melting points.
| Material | Melting point |
|---|---|
| Ice | 0 °C |
| Urea | 133 °C |
| Iron | 1538 °C |
Table 7.1 is a table of attraction strengths in disguise. Iron needs 1538 °C because its interparticle attractions are enormously stronger than urea's.
The exception the chapter flags: Generally, in a liquid state, particles are somewhat farther away from each other as compared to those in the solid state (ice is an exception — its particles are farther apart than those in water).
3.2 The liquid state — Activity 7.4
Three clean, dry containers of different shapes, A, B and C, each marked at 200 mL. Fill A to the mark, pour into B, then into C, observing shape and level each time.
Two results in one activity:
| Observation | Conclusion |
|---|---|
| The water takes the shape of each container | Liquids have no fixed shape — the particles of liquids are free to move |
| The level stays at 200 mL every time | Liquids have a definite volume — the particles move only within a limited space |
The chapter even tells you why the containers must be clean: if a container is not clean, some water may stick to its walls, causing the water level in the next container to be slightly less than 200 mL after pouring.
The finger test — Fig. 7.6
Move a finger through water in a shallow vessel.
You can move your finger through water without breaking or cutting it permanently, which cannot be done in the case of solids ... As soon as you remove your finger, the position of the water is restored. We can say that in liquids, the interparticle attractions are slightly weaker than in solids, but still strong enough to keep the particles close together.
Both halves carry a mark. The finger goes through because the attractions are weaker than in a solid; the water closes up again because they are still strong.
Boiling
The temperature at which a liquid boils and turns into vapour at atmospheric pressure is called its boiling point. The movement of particles becomes so vigorous that they move apart from each other, resulting in a decrease in the interparticle forces of attraction. Eventually, the constituent particles can escape from the liquid state.
And the one distinction the chapter does draw:
| Boiling | Evaporation |
|---|---|
| Only at the boiling point | At all temperatures |
| Very fast | Slow |
| Throughout the liquid — seen as bubbles | Only at the surface |
Spilled water disappearing on a cool day is evaporation, which is why it happens without any boiling.
3.3 The gaseous state — Activity 7.5
Trap incense smoke in Gas Jar A held upside down; turn it over and cover it with a glass plate. Invert Gas Jar B on top of the plate, then slide the plate out with no gap for smoke to escape.
The smoke fills Jar B completely.
This illustrates that 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.
Why smoke? Gas particles are invisible, so the activity uses something you can see that they push around: 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. Iodine vapour from solid iodine in a closed jar (Fig. 7.8) does the same job.
Both liquids and gases flow and do not retain a fixed shape. These properties distinguish them from solids and classify them as fluids.
4. Interparticle Spacing
Activity 7.6 — the syringe
Pull the plunger of a needle-less syringe fully out. Seal the open end with your thumb. Push the plunger in.
The air compresses.
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.
Let go and the plunger springs back — the gas particles spread.
Now do the identical thing with water: You would observe that water is practically incompressible. Same syringe, same seal, same push — so the difference cannot be the apparatus. It is the spacing.
Activity 7.7 — the falling water level
Half-fill a glass vessel with water and mark the level A. Add two teaspoons of sugar and mark the new level B. Stir to dissolve. Predict, then mark the final level C.
Initially, when sugar is added, the level of water increases, but after dissolution, it may decrease to some extent. 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.
The control matters. The chapter tells you to repeat this with insoluble solids — sand, stone pieces. The sand does not dissolve; it settles and occupies some space in the container, causing the total volume to increase. Because the only thing that differs between the two runs is whether the solid dissolves, dissolving must be what causes the level to fall.
Sugar dissolves, sand does not — and both are solids
The chapter asks this itself. The answer is in section 7.4:
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.
Dissolving is a contest: the pull of the water particles against the substance's own interparticle attractions. Being a solid is not the deciding factor.
Do solids have spaces?
Despite close packing, some space is left between the particles ... You might assume that the space between particles is filled with air, but this is not the case. They contain nothing at all.
Fig. 7.12 summarises all three:
| Solid | Liquid | Gas | |
|---|---|---|---|
| Interparticle spacing | minimum | a little more than in solids | maximum |
| Packing | closely packed | a little more loosely packed | free |
| Interparticle attraction | maximum | slightly weaker than in solids | minimum (negligible) |
| Movement | negligible — only vibrations | restricted to a limited space | in all the available space |
A step further — the word "particle". In air-pollution reports, Suspended Particulate Matter (SPM) means tiny dust particles suspended in air, not the constituent particles of matter which are extremely small as compared to the dust particles. In fact, even these tiny dust particles are also made up of a very large number of constituent particles, i.e., atoms and molecules. If you can see it, it is not a constituent particle.
5. How Particles Move
Activity 7.8 — potassium permanganate
Drop a few grains of potassium permanganate into a tumbler of water. Do not stir. Pink streaks spread from the grain, and in time the whole tumbler is uniformly pink.
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.
Nobody stirred it. Something had to be moving on its own.
Think like a scientist. Run the same test in hot water, room-temperature water and ice-cold water. The potassium permanganate spreads the fastest in hot water, less quickly in water at room temperature, and the slowest in ice-cold water. Hence, the movement of particles increases when heat is provided.
Activity 7.9 — the incense stick
Light an incense stick in one corner of a room. Wait.
Initially, the fragrance is felt only around the incense stick. Shortly, you can smell the fragrance throughout the 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.
Same argument as the tumbler, one state up. Nothing fans the air; the particles do it themselves.
Ever heard of… Soap and oil (Fig. 7.15). 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.
6. The Conclusion the Chapter Reaches
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.
| State | Thermal energy | What follows |
|---|---|---|
| Solid | Low | Particles stay close, attractions are strong, motion is restricted to small vibrations |
| Liquid | At the melting point, enough to overcome the attractions | Particles leave fixed positions; spacing increases slightly; they move within a limited space |
| Gas | Enough to overcome the attractions entirely | Particles move freely in all directions |
The particles never change. Ice, water and steam are the same substance with the same constituent particles — only the energy differs.
A step further — atoms and molecules. The tiny particles that make up all matter are atoms and molecules. Iron is made of iron atoms; gold of gold atoms. Atoms of many elements like hydrogen, oxygen, and sulfur are not able to exist independently — they combine into molecules, two hydrogen atoms to a hydrogen molecule, two hydrogen and one oxygen to a water molecule. You will learn about atoms and molecules in higher grades. The chapter names them only on its last page, after spending fifteen pages earning the idea.
7. Reading Fig. 7.16 — the just-extinguished candle
The final exercise asks you to find three states of matter in one photograph.
| In the photograph | State | Particle box in Fig. 7.16b |
|---|---|---|
| The body of the candle and the drips set hard on its side | Solid | Particles regularly arranged and closely packed |
| The pool of molten wax around the base of the wick | Liquid | Particles close together but irregularly arranged |
| The wisp rising from the wick after the flame goes out | Gas (wax vapour) | A few particles, widely scattered |
One substance, one moment, three states — because the wax nearest the flame received enough thermal energy to overcome its interparticle attractions and the wax further away did not.
8. The Traps
Rice flour flows — is it a liquid? No. Each speck keeps its own definite shape and volume when poured; what rearranges is the heap, as whole specks slide past one another. In a liquid there is nothing smaller that holds a shape — the constituent particles themselves move. Same for sand, sugar and grain.
"The particles in a solid do not move." False. They vibrate about fixed positions. What they cannot do is change places.
"Melting involves an increase in interparticle attractions." False — the attractions decrease, which is exactly why the particles can leave their positions.
"The sugar disappeared." No. Taste the top layer without stirring.
"The gaps between particles are filled with air." No. They contain nothing at all. And air is itself made of particles, so this explanation would need gaps between the gaps.
"Camphor's energy is released as smell." False. Heating makes the particles move faster and spread; the smell is camphor particles reaching your nose, not energy turning into odour.
Importing Class 9. Sublimation, condensation, evaporative cooling, the effect of pressure — not this chapter. An answer built on them is answering a different question.
9. What to Carry Forward
- Matter is composed of constituent particles, too small to see even through an ordinary microscope.
- Between them are interparticle spaces; holding them together are interparticle attractions, which weaken drastically as the distance grows.
- Solid — definite shape, definite volume. Liquid — definite volume, no definite shape. Gas — neither.
- Melting point: the temperature at which a solid becomes a liquid. Boiling point: the temperature at which a liquid becomes a vapour.
- Fluids = liquids and gases, because both flow.
- Compressibility follows spacing: gas ≫ liquid ≈ solid (both practically incompressible).
- Particles are in constant motion, and that motion increases with temperature.
- Thermal energy versus interparticle attraction decides which state a substance is in.
