Particulate Nature of Matter

1. Where this chapter starts — breaking things down

Pebbles, stones and sand on a riverbank come from rocks that erosion and flowing rivers slowly break into finer and finer grains, right down to clay. This raises a deep question: is a grain of sand the smallest unit of a rock, or can it be broken further? The whole chapter is built to answer this.

Activity 7.1 (chalk): break a chalk stick into pieces, keep breaking until you can't by hand, then grind it in a mortar and pestle and look at the powder under a magnifying glass. Each tiny grain is still chalk — grinding is a physical change (recall Class 7), only the size has reduced, not the substance.

Imagine grinding on and on. Eventually you reach units that cannot be broken down further — these are the basic building blocks the chalk is made of, called constituent particles.

2. What is matter composed of? (7.1)

Activity 7.2 (sugar in water): add sugar to water without stirring; the top tastes plain. Stir until it dissolves; now even the top layer tastes sweet. The sugar is everywhere in the solution, yet you can see no sugar particles. On dissolving, each grain of sugar breaks up into its constituent particles — millions and millions of them — too small to see.

Together, Activities 7.1 and 7.2 support one big idea:

But where did the sugar go? The tiny sugar particles separate and slip into the gaps between the water particles. These gaps are called interparticle spaces.

Our scientific heritage: the ancient Indian philosopher Acharya Kanad proposed that matter is made of tiny, indivisible, eternal particles he called Parmanu (atom), written in his work Vaisheshika Sutras.

3. Interparticle attraction — what holds it together (7.2)

Particles do not fly apart; they are held by forces that are attractive in nature, called interparticle forces of attraction (or interparticle attractions).

• The strength of these forces depends on the nature of the substance and the interparticle distance.
• Even a slight increase in the distance decreases the force drastically.
• This strength decides the physical state — solid, liquid or gas.

So the recipe for every state is: how close the particles are → how strong the attraction is → how freely they can move.

4. The solid state (7.2.1)

Activity 7.3: collect an iron nail, rock salt, a stone, a wooden block, a key and a piece of aluminium; observe and try hammering. All are solids — they have a definite shape and a definite volume.

• Particles are tightly (closely) packed; interparticle spacing is minimum.
• Interparticle attractions are very strong (maximum), holding particles in fixed positions.
• Particles cannot move past each other — they can only vibrate (oscillate to and fro) about their fixed positions.

On heating a solid: particles vibrate more vigorously. At a certain stage the vibrations become so strong that particles start leaving their positions, the attractions weaken, and the solid turns into a liquid. The temperature at which this happens is the melting point.

Weak attractions → low melting point; strong attractions → high melting point. From Table 7.1: Ice melts at 0 °C, Urea at 133 °C, Iron at 1538 °C. (Note: ice is an exception — its particles are actually farther apart than in liquid water.)

5. The liquid state (7.2.2)

Activity 7.4: pour 200 mL of water through three containers of different shapes (A → B → C). The water takes the shape of each container, but the volume stays 200 mL.

• Liquids have no fixed shape (they take the container's shape) but a definite (fixed) volume.
• Particles can move freely, but only within a limited space; spacing is a little more than in solids.
• Attractions are slightly weaker than in solids, yet still strong enough to keep particles close together.

Pushing a finger through water (Fig. 7.6) only temporarily displaces it — the water flows back, unlike a solid which would be cut.

On heating a liquid: particle movement becomes so vigorous that particles move apart, attractions decrease, and particles escape from the liquid into the gaseous state (vapour). The temperature at which a liquid boils and turns to vapour at atmospheric pressure is the boiling point.

• Boiling: very fast vapour formation, both at the surface and inside the liquid (seen as bubbles), only at the boiling point.
• Evaporation: slow vapour formation, only at the surface, happening at all temperatures — which is why spilled water dries up.

6. The gaseous state (7.2.3)

Activity 7.5 (smoke in gas jars): trap incense smoke in Gas Jar A, cover with a glass plate, place empty Gas Jar B above it, then remove the plate. The smoke spreads to completely fill both jars. Smoke (or iodine vapour) is used because its tiny suspended particles, hit by invisible gas particles, let us see the motion of gases.

• Gas particles move freely in all directions; interparticle spacing is maximum.
• Interparticle attractions are negligible (minimum).
• Gases have no fixed shape and no fixed volume — they spread to occupy the entire available space.

Both liquids and gases flow and lack a fixed shape, so they are together classified as fluids.

7. Interparticle spacing in the three states (7.3)

Activity 7.6 (syringe): seal air in a needle-less syringe with your thumb and push the plunger. The air compresses — gas particles have large gaps that external pressure can squeeze. Repeat with water: it is practically incompressible (liquids have very small gaps).

Activity 7.7 (sugar + water levels): mark the water level (A), add and dissolve two teaspoons of sugar, mark the new level (C). After dissolving, the level does not rise by the full sugar volume — the dissolved sugar particles slip into the interparticle spaces of the water. (Insoluble sand instead settles and the volume simply increases.)

So even solids have some space between their particles (Fig. 7.12a) — though it is the minimum. Important: this space contains nothing at all (not air).

A step further: the word "particle" changes with context — Suspended Particulate Matter (SPM) means dust particles in air, which are themselves made of huge numbers of constituent particles (atoms and molecules).

8. How particles move — diffusion (7.4)

Activity 7.8 (potassium permanganate): drop a grain of potassium permanganate into water without stirring. First pink streaks appear; with time the whole water turns uniformly pink. The constantly-moving water particles pull the coloured particles out of the grain and spread them everywhere. This spreading of particles of one substance into another is diffusion. (In sand, the attractions are too strong for water to pull particles out — so sand is insoluble.)

Think like a scientist: drop the same grain into hot, room-temperature and ice-cold water. Colour spreads fastest in hot water, slowest in ice-cold. So particle movement increases when heat (thermal energy) is supplied.

Activity 7.9 (incense stick): light it in one corner of a room; soon the fragrance reaches everywhere. Moving air particles hit the fragrance particles and carry them through the room — this is why perfume and food smells reach us (diffusion in gases, which is the fastest).

Ever heard of… the particulate nature explains cleaning with soap: one end of a soap particle grabs the oil, the other end mixes with water, lifting the oil off the fabric.

9. Thermal energy decides the state — the big picture

Pulling it all together: the strength of attraction depends on the distance between particles, which in turn depends on their thermal (heat) energy. So it is thermal energy that decides the physical state.

• Solid: low thermal energy → particles stay close → strong attraction → only small vibrations.
• At the melting point: thermal energy overcomes some attraction → particles move apart a little → liquid.
• Gas: enough thermal energy to overcome attraction → particles move freely in all directions.

A step further (atoms & molecules): the tiny particles that make up matter are atoms and molecules. Iron is made of iron atoms; gold of gold atoms. Some atoms (hydrogen, oxygen, etc.) cannot exist alone, so they combine into molecules — two hydrogen atoms form a hydrogen molecule, and a water molecule has two hydrogen atoms and one oxygen atom (learnt in detail in higher grades).

10. Master comparison — solid vs liquid vs gas

This NCERT "States of Matter" summary chart is the single most exam-useful table in the chapter.

• Interparticle spacing: Solid = minimum · Liquid = a little more than solids · Gas = maximum.
• Packing: Solid = closely packed · Liquid = a little more loosely packed than solids · Gas = particles are free.
• Interparticle attraction: Solid = maximum · Liquid = slightly weaker than solids · Gas = minimum (negligible).
• Movement of particles: Solid = negligible (only vibrations) · Liquid = restricted to a limited space · Gas = in all the available space.
• Shape & volume: Solid = definite shape + definite volume · Liquid = no fixed shape + definite volume · Gas = no fixed shape + no fixed volume.

11. Common mistakes to avoid

• Saying interparticle spaces are "filled with air" — they contain nothing at all.
• Thinking grinding chalk or dissolving sugar makes a new substance — both are physical changes; the substance stays the same.
• Mixing up boiling (whole liquid, only at boiling point, bubbles inside) with evaporation (surface only, at all temperatures).
• Calling liquids "shapeless and volumeless" — liquids have no fixed shape but a definite volume.
• Forgetting solids do have tiny interparticle spaces — just the minimum of the three states.
• Writing that attraction is strongest in gases — it is strongest in solids, negligible in gases.

12. Quick revision checklist

• Matter = a huge number of extremely tiny particles, held by interparticle attraction, with interparticle spaces.
• Strength of attraction (set by interparticle distance / thermal energy) decides the state.
• Attraction & packing: solid > liquid > gas; spacing & movement: gas > liquid > solid.
• Solid: fixed shape + volume; Liquid: fixed volume only; Gas: neither.
• Melting point = solid→liquid; Boiling point = liquid→gas; heating increases particle motion.
• Diffusion (KMnO₄, incense, perfume) proves particles move constantly — faster when hot.
• Liquids + gases = fluids; tiny particles of matter are atoms & molecules.