Four questions to open the chapter with. Tap to bring up the next one.
Probe and ponder · 1
Why is it possible to pile up stones or sand — but not water?
Probe and ponder · 2
Why does water take the shape of your folded hands, then lose that shape the moment you release it?
Probe and ponder · 3
We cannot see air. So how does it add weight to an inflated balloon?
Probe and ponder · 4
Is the air we breathe today the same air that existed thousands of years ago?
Chapter 7 · What we will cover
The chapter, in four questions
7.1What is matter composed of?chalk · sugar · constituent particles
7.2What decides the state of matter?solid · liquid · gas · melting & boiling
7.3How does interparticle spacing differ?the syringe · the missing volume
7.4How do particles move?potassium permanganate · incense · soap
One big ideaएक बड़ा विचारEverything you see is built from particles too small to see — and how much energy they carry decides whether you get a stone, a river, or the air.
Chapter 7 · Where it begins
From a mountain to a grain of sand
You have picked up pebbles on a riverbank. Where did they come from?
Step 1 · The mountain
Rock breaks down slowly by erosion.
Step 2 · The river
Flowing water carries the pieces and keeps breaking them smaller — boulder, stone, pebble.
Step 3 · The plain
What finally arrives is sand and clay.
So ask the class
Is a grain of sand the smallest piece a rock can become — or can it go smaller still?
Class 8 · Chapter 7 · Particulate Nature of Matter
What is matter made of?
Break a stick of chalk. Grind it to powder. It is still chalk. How far can this go?
Remember this
Grinding is a physical change. Only the size changes — never the substance.
Too small to see
These particles cannot be seen even with an ordinary microscope.
Ask the class
Are sand and clay the smallest bits of rock?
Constituent particleसंघटक कणthe basic unit a substance is made of — it cannot be broken down any further
Class 8 · Chapter 7 · Particulate Nature of Matter
Where does the sugar go?
Two spoons of sugar into water. Stir. It disappears — yet the water is sweet.
The taste test
Sip from the top layer. It is sweet — so sugar is spread through the whole glass.
Look at the gaps
Water particles do not touch. There are spaces between them.
So remember
Sugar is not destroyed — only spread out.
Interparticle spacesअंतरकण स्थानthe gaps between constituent particles — this is where the sugar went
Class 8 · Chapter 7 · Recap
Two ideas to carry away
Constituent particle
The smallest unit of a substance. It cannot break further.
Interparticle space
The gap between particles. Sugar hides in here.
Q1If you grind chalk forever, where does it stop?
Q2Why does sugar vanish but the water stay sweet?
Q3Is grinding a physical or a chemical change?
7.1 · Our scientific heritage
India asked this 2,500 years ago
People have wondered how far matter can be broken since long before there were microscopes.
Acharya Kanad · आचार्य कणाद
An ancient Indian philosopher who said matter is made of tiny, indivisible, eternal particles.
Written where
In his work the Vaisheshika Sutras · वैशेषिक सूत्र
ParmanuपरमाणुKanad's word for the smallest, uncuttable piece of matter — the same idea we now call the constituent particle
7.2 · What decides the state of matter?
What holds particles together?
Constituent particles pull on each other. That pull is called interparticle attraction.
Distance rules everything
Move particles even slightly apart and the attraction drops sharply.
The nature of the substance, and the distance between its particles.
Interparticle attractionअंतरकण आकर्षणthe force pulling constituent particles towards each other — it weakens fast with distance
7.2.1 · Activity 7.3 · Let us find out
Put six solids on the table
An iron nail, rock salt, a stone, wood, a key, a piece of aluminium. Look. Then hammer.
What is the same about all six
Each one has a definite shape and a definite volume. Put it down anywhere — it stays that shape.
Ask the class
In which of them do you think the particles are held most strongly? What did the hammer tell you?
7.2.1 · The solid state
Solids: locked in place
Iron, rock salt, wood, a key. All keep a definite shape and volume.
Why they hold shape
Particles are tightly packed with very strong attraction, fixed in position.
They can still move
Only to and fro about their own position — they never slip past each other.
Melting points
Ice 0 °C · Urea 133 °C · Iron 1538 °C
Vibrationकंपनthe only motion a solid particle is allowed — to and fro about its own fixed position. Heat it and the vibration grows until it breaks free.
7.2.1 · Table 7.1
Every solid has its own melting point
Heat a solid and the vibration grows. At one exact temperature the particles finally break free. That temperature is the melting point.
Melting pointगलनांकthe lowest temperature at which a solid turns into a liquid at atmospheric pressure
7.2.2 · The liquid state
Liquids: shape of the vessel
Pour 200 mL from A to B to C. The shape changes. The level does not.
What that proves
No fixed shape — but a definite volume. Particles move within a limited space.
Weaker, not weak
Push a finger through water and it closes behind you. Try that with a stone.
Boiling vs evaporation
Boiling makes bubbles throughout. Evaporation is slow, and only at the surface.
Definite volume, no fixed shapeनिश्चित आयतन, अनिश्चित आकार200 mL stays 200 mL in every container — but the shape changes every time you pour
7.2.2 · Figure 7.6
Put your finger through the water
Try the same with a piece of wood. You cannot. So how strong is the pull in a liquid?
What happened
You pushed the water aside for a moment. Lift the finger and the water closes back — nothing was cut, nothing broke.
So the answer is
In a liquid the attraction is a little weaker than in a solid — but still strong enough to hold the particles close together.
7.2.2 · Heating a liquid
Boiling and evaporation are not the same
Both turn a liquid into vapour. But look at where and when each one happens.
Boiling
Only at one temperature — the boiling point. Vapour forms inside the liquid too, which is why you see bubbles. Fast.
Evaporation
At any temperature, even a cold day. Only from the surface. Slow — which is why spilled water disappears overnight.
Boiling pointक्वथनांकthe temperature at which a liquid turns to vapour at atmospheric pressure — for water, 100 °C
7.2.3 · The gaseous state
Gases: fill everything
Trap incense smoke in jar A. Open it to jar B — the smoke fills both.
No shape, no volume
A gas takes the whole space available to it, whatever the container.
Why
Interparticle attraction is negligible. Particles move freely in every direction.
The smoke is only a marker
Invisible gas particles are constantly hitting the smoke, making their motion visible.
No fixed shape, no fixed volumeन आकार, न आयतनthey expand until they occupy every bit of the space they are given
7.2.3 · Figure 7.8 · Another way to show it
Iodine vapour does the same thing
No incense stick? Leave solid iodine in a closed gas jar.
What you see
Purple vapour rises on its own and fills the whole jar — corner to corner, evenly.
Safety first · सावधानी
Iodine vapour irritates the eyes and nose. Keep the jar closed. Do not let students handle it.
Fluidsतरल · द्रव और गैस दोनोंliquids and gases together — both flow, and neither keeps a fixed shape. This is what separates them from solids.
7.3 · Interparticle spacing
Which one can you squeeze?
Seal a syringe of air with your thumb. Push the plunger. It moves in.
What you just proved
Gas particles have large gaps between them, and pressure closes those gaps.
Now fill it with water
It will barely budge. Water is practically incompressible.
Let go
The plunger springs back — the particles push apart again.
Gases compressगैसें संपीड़ित होती हैंbecause the space between particles is huge in a gas and tiny in a liquid
7.3 · Activity 7.6 · Repeat it with water
Now try the same with water
Thumb on the nozzle. Push the plunger as hard as you like. It will not move.
Water is practically incompressible
Its particles are already close together. There is almost no spare space to squeeze out.
Air was different
Gas particles sit far apart, so pressure pushed them closer. Release the plunger and they spread out again.
Incompressibleअसंपीड्यa substance whose volume will not reduce under pressure, because its particles have almost no spare space between them
7.3 · Interparticle spacing
Where did the volume go?
Water at A. Add sugar → level rises to B. Now stir it. The level drops to C.
The solution is smaller
Its volume is less than water and sugar measured separately.
Because
Dissolved particles slip into the gaps that were already between the water particles.
Careful — a common mistake
The gaps in a solid are not filled with air. They contain nothing at all.
Interparticle spacingअंतरकण दूरीlargest in gases → smaller in liquids → smallest in solids
7.3 · Repeat Activity 7.7 with sand
Sugar dissolves. Sand does not. Why?
Both are solids. Both go into the same glass of water. Yet the water level behaves in opposite ways.
Sugar · चीनी
Water particles are strong enough to pull sugar particles apart and slot them into the gaps. Level barely rises.
Sand · रेत
Sand particles are held together too strongly for water to pull out. They settle at the bottom and take up space — so the level rises.
Insolubleअघुलनशीलa substance whose particles water cannot pull apart, so it never spreads through the liquid — sand, stone, chalk
7.3 · The part everyone gets wrong
A solid has gaps — and they hold nothing
Particles in a solid are packed tight and cannot travel. But packed is not the same as touching.
The common mistake
"The gap must be filled with air." Almost every class says this.
The truth
There is nothing at all in it. Not air, not anything. Air is itself made of particles — it cannot fit inside a solid.
Teacher's line
"Empty means empty."
7.3 · A step further
One word, two very different meanings
You will hear the word in the news and in this chapter — and they are not the same thing.
SPM · Suspended Particulate Matter
In air pollution reports this means tiny dust floating in air. You can see it in a sunbeam.
Constituent particle
In this chapter it means the building block of a substance — unimaginably smaller. One speck of dust is itself made of a huge number of them.
7.3 · Figure 7.12 · The picture to remember
Three states. Three spacings.
Spacing
Minimum. Packed — but not touching.
Spacing
A little more than in a solid.
Spacing
Maximum. Mostly empty space.
7.4 · How particles move
Nobody stirred it
Drop a grain of potassium permanganate into still water. Pink streaks spread on their own.
What this shows
Water particles are in constant motion, even when the water looks perfectly still.
How it spreads
They pull particles off the grain, then knock them right through the liquid.
Try three glasses
Hot, room, ice-cold. Pink spreads fastest in hot — heat means faster particles.
Particles never stop movingकण कभी रुकते नहींand the hotter they are, the faster they go
7.4 · Activity 7.8 · The explanation
Nobody pushed it. So what moved it?
The water was still. Yet the pink reached every corner of the glass.
Step 1 · Pull
Water particles are in constant motion. They pull permanganate particles off the grain.
Step 2 · Hit
Then they keep hitting them, knocking them further and further until the colour is even.
And when they cannot pull
In sand the particles are held too strongly. Water fails to pull them out — so sand is insoluble.
7.4 · Think like a scientist · Try it yourself
Three tumblers. One grain each. Watch the clock.
Hot water, room-temperature water, ice-cold water. Drop one grain of potassium permanganate into each.
The ruleऊष्मा बढ़ी → कणों की गति बढ़ीGive particles more heat and they move faster. That is the whole of this chapter in one sentence.
7.4 · How particles move
Why you can smell it from there
Light an incense stick in one corner. Minutes later the whole room smells of it.
What carried it
Moving air particles hit the fragrance particles and push them across the room.
And why sand never dissolves
Its particles are held too strongly for water particles to pull them apart.
Soap works the same way
One end of a soap particle grips oil, the other holds water — lifting the stain away.
Diffusionविसरणparticles of one substance spreading right through another, with no stirring at all
7.4 · Ever heard of… · Figure 7.15
How soap lifts an oil stain
Water alone will not lift oil off cloth. Add soap and it goes. Particles explain it.
A soap particle has two ends
One end grips oil. The other end mixes with water.
What happens on the cloth
Many soap particles surround the oil drop, oil-end inward. The water-ends face out.
Then the water pulls
The whole ball lifts off the fabric and rinses away.
Chapter 7 · Putting it together
It is thermal energy that decides the state
How strong the attraction feels depends on the distance between particles — and that distance depends on their thermal (heat) energy.
Solid · low thermal energy
Particles stay close, attraction is strong, motion is cut down to small vibrations.
At the melting point
The added heat is spent overcoming the attraction. Particles leave their fixed positions and can move around — but only within a limited space.
Gas · high thermal energy
Enough energy to beat the attraction completely — free in every direction.
Chapter 7 · The whole idea
Heat decides the state
Solid
Packed. Strong pull. Only vibrates.
Liquid
Loose. Weaker pull. Flows, fixed volume.
Gas
Far apart. Almost no pull. Fills everything.
Q1Why do solids keep their shape but liquids do not?
Q2Why can you squeeze air but not water?
Q3Why does the smell of food reach the next room?
Chapter 7 · Let us wrap up
The whole chapter on one screen
SOLIDठोस
Interparticle spacing minimum
Particles closely packed
Attraction is maximum
Movement negligible — only vibration
LIQUIDद्रव
Spacing a little more than a solid
Particles loosely packed
Attraction slightly weaker than a solid
Movement within a limited space
GASगैस
Spacing maximum
Particles are free
Attraction is minimum — negligible
Moves through all available space
Chapter 7 · Snapshots
Six lines worth copying into the notebook
1Matter is composed of extremely small particles.
2They are held together by interparticle forces of attraction.
3That attraction is strongest in solids, weaker in liquids, weakest in gases.
4Solids have a fixed shape and size — strong attraction, minimum space, no free movement.
5Liquids have a definite volume but no fixed shape — the particles move within a limited space.
6Gases have neither fixed shape nor fixed volume — attraction is negligible and the particles are completely free.
Chapter 7 · A step further
These particles have names
You will study this properly in higher classes. For now, just the names.
Atom · परमाणु
Iron is made of iron atoms. Gold is made of gold atoms.
Molecule · अणु
Some atoms cannot stay alone. Two hydrogen atoms join to make one hydrogen molecule.
Water
Two hydrogen atoms + one oxygen atom = one water molecule.
Chapter 7 · Keep the curiosity alive
Ten questions from the book
Every one of them is straight out of Keep the curiosity alive, page 113–114.
Tap once to read it. Tap again for the answer.
How to use theseइनका उपयोग कैसे करेंRead the question aloud, let the class answer, then tap. The answer stays on screen while they copy it.
Keep the curiosity alive · Question 1
01
The primary difference between solids and liquids is that the constituent particles are:
(i)closely packed in solids, while they are stationary in liquids.
(ii)far apart in solids and have fixed position in liquids.
(iii)always moving in solids and have fixed position in liquids.
(iv)closely packed in solids and move past each other in liquids.
Why (iv)
In a solid, particles are packed and locked — they only vibrate. In a liquid the attraction is weaker, so they slide past one another. That sliding is what makes a liquid flow.
Question 2 · True or false? Correct the false ones.
02
(i)Melting ice into water is an example of the transformation of a solid into a liquid.
(ii)The melting process involves a decrease in interparticle attractions during the transformation.
(iii)Solids have a fixed shape and a fixed volume.
All three are true
(i) Ice → water is solid → liquid. (ii) Heat pushes the particles apart, so attraction weakens. (iii) Strong attraction + no free movement = fixed shape and volume.
Question 2 · continued
02
(iv)The interparticle interactions in solids are very strong, and the interparticle spaces are very small.
(v)When we heat camphor in one corner of a room, the fragrance reaches all corners of the room.
(vi)On heating, we are adding energy to the camphor, and the energy is released as a smell.
(iv) true · (v) true · (vi) FALSE
Energy is not "released as smell". The heat makes camphor particles move fast enough to leave the solid and spread through the air. What reaches your nose is the camphor particles themselves — not energy.
Question 3 · Choose, then justify
03
If we could remove all the constituent particles from a chair, what would happen?
(i)Nothing will change.
(ii)The chair will weigh less due to the lost particles.
(iii)Nothing of the chair will remain.
Justification
The chair is its constituent particles. Take them all away and there is no wood, no shape, no weight — nothing at all is left behind.
Question 4
04
Why do gases mix easily, while solids do not?
In a gas
Attraction is negligible and the spaces are huge. Particles fly in all directions and slip straight into each other's empty space.
In a solid
Strong attraction holds every particle in a fixed position. It can vibrate but never travel — so it cannot go and sit inside another solid.
Question 5 · Justify the statement
05
Spilled milk flows and spreads out on the table — but the glass tumbler stays exactly the same shape.
The milk is a liquid
Its particles attract each other weakly enough to slide past. It has a definite volume but no fixed shape, so it takes the shape of whatever it lands on.
The tumbler is a solid
Very strong attraction, minimum spacing, particles fixed in position — so it keeps both its shape and its volume no matter where you put it.
Question 6 · Draw it
06
Show the particles as ice melts, then boils away
What the drawing must show
Same number of particles throughout — only the spacing and freedom change. Packed and vibrating → slightly apart and sliding → far apart and flying everywhere.
Question 7 · Draw the particles present in each
07
Aluminium foil · Glycerin · Methane gas
(i) Aluminium foil
Solid — packed, tiny gaps, fixed positions.
(ii) Glycerin
Liquid — a little apart, sliding past.
(iii) Methane gas
Gas — far apart, scattered, free.
Question 8 · Figure 7.16 · The extinguished candle
08
A candle has just been blown out. Find all three states of wax in the picture.
Wax vapour · gas
The white smoke curling up from the wick.
Molten wax · liquid
The clear pool sitting around the wick.
Solid wax
The body of the candle you are holding.
Matching to the particle diagrams
Solid wax → closely packed. Molten wax → slightly apart, able to flow. Wax vapour → far apart and free. One substance, three arrangements, all visible at once.
Question 9
09
Why does the water in the ocean taste salty, even though the salt is nowhere to be seen?
The salt did not vanish
It dissolved. Water particles pulled the salt apart into its constituent particles.
Where it went
Those particles slipped into the interparticle spaces between the water particles, spread evenly through the whole ocean.
Why you cannot see it
Constituent particles are far too small to see — even an ordinary microscope will not show them. But your tongue still finds them.
Question 10 · The tricky one
10
Rice grains and rice flour take the shape of the jar you pour them into. So are they solids or liquids?
✓They are solids.
Look at one grain, not the heap
A single grain of rice keeps its own fixed shape and volume wherever you put it. That is the test for a solid.
What is really moving
The heap changes shape because separate grains slide over each other — not because the particles inside a grain moved. In a liquid, the particles themselves move within the liquid.
Chapter 7 · Discover, design, and debate
Four things to do in the classroom
1Fix a balloon on a bottle's neck and stand the bottle in hot water. Predict first — then watch.
2Build the three states with clay balls or beads, showing the interparticle spacing in each.
3Become the particles. Act out solid, liquid and gas at different temperatures as a role-play.
4Debate: "Gases spread and fill all available space." Is that property useful or dangerous?
Chapter 7 · Complete
That is the whole chapter
Matter is made of particles. How much heat energy those particles carry decides whether you are holding a stone, pouring a river, or breathing the air.
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