Chapter 7

Particulate Nature of Matter

You might have collected pebbles and stones from the sand while playing on a riverbank or a beach. Where do these come from? In the mountains, rocks gradually break down due to erosion, and rivers carry the eroded pieces along, wearing them down further into pebbles, stones, sand, and finally very fine grains of clay.

Is a grain of sand the smallest possible unit of a bigger rock, or can it be broken down even further?

What Is Matter Composed of?

Think about grinding a stick of chalk into a fine powder with a mortar and pestle. Every tiny speck you're left with is still chalk — as you learnt in Class 7, grinding is a physical change, so only the size of each speck reduces, not what it's made of.

Now imagine that grinding continues, speck after speck, until the particles simply cannot be broken down any further. The tiny units reached at that stage are the basic building blocks the chalk was made of — meaning the whole piece of chalk was really a huge number of these smaller units bundled together. These are called constituent particles: the basic unit that makes up a larger piece of a substance or material. Just like chalk, grains of sand and clay are also made up of a large number of their own constituent particles, not the smallest units themselves.

Now recall dissolving sugar in water. Once dissolved, the sugar particles can no longer be seen — but their presence can still be sensed by taste, since the water tastes sweet throughout. That's because the sugar breaks up into constituent particles so small they can't be observed, even though each tiny grain of sugar is made of millions and millions of them.

These particles don't vanish — they separate out and occupy the available spaces between the water's own particles. These gaps between particles are known as interparticle spaces.

What Decides Different States of Matter?

The constituent particles of matter are held together by attractive forces called interparticle attractions. How strong these forces are depends on the nature of the substance and on the distance between particles — even a small increase in that distance drops the force drastically. It's ultimately the strength of these forces that decides whether a substance is a solid, a liquid, or a gas.

Solid state

Pick up any solid object — an iron nail, a piece of rock salt, a stone, a block of wood — and you'll notice it has a definite shape and volume, and hammering barely dents it. That's because in solids, the particles are packed tightly and the interparticle attractions between them are very strong, holding each particle in a fixed position. A particle can only vibrate to and fro about that position; it cannot move past its neighbours.

Heat a solid, and its particles vibrate more and more vigorously, until the vibrations become strong enough that particles start leaving their fixed positions altogether — the interparticle forces weaken, and the solid turns into a liquid. The temperature at which this happens is called the melting point: the minimum temperature at which a solid melts into a liquid at atmospheric pressure.

Some solids have weak interparticle forces and so melt at low temperatures; others, with strong attractive forces, have high melting points. A few examples:

• Ice — 0 °C
• Urea — 133 °C
• Iron — 1538 °C

Liquid state

Pour water from one container into another of a different shape, and it takes on the new container's shape every time — yet however you pour it, the water level always settles at the same volume you started with. That's because a liquid's particles are free to move, so liquids have no fixed shape but do have a definite volume.

Run your finger through a shallow dish of water and you can move it freely, without permanently breaking the water apart — unlike a solid. That tells us the interparticle attractions in a liquid are slightly weaker than in a solid, though still strong enough to keep the particles close together.

Heat a liquid far enough and it starts to boil: the particle movement becomes so vigorous that particles break free from the liquid entirely and escape as vapour. The temperature at which this happens is the boiling point — where a liquid boils and turns to vapour at atmospheric pressure. At the boiling point, vapour forms rapidly both at the surface and within the liquid, seen as bubbles. Below the boiling point, a slower version of this — vapour forming only at the surface — is called evaporation.

Gaseous state

Trap some smoke inside a gas jar and let it spread into a second jar placed against it, and the smoke fills the entire available space in the new jar. Gases, in other words, have no fixed shape or volume — they simply expand to occupy whatever space they're given, just as liquids take the shape of their container. This happens because gas particles move freely in every direction, and the interparticle attractions between them are negligible.

Because liquids and gases both flow and don't hold a fixed shape of their own, the two are grouped together and called fluids.

How Does the Interparticle Spacing Differ in the Three States of Matter?

Trap some air in a syringe with your thumb over the opening, and pushing the plunger inward visibly shrinks the air's volume — the particles are simply forced closer together, showing there's a good deal of space between gas particles in their natural state, space that external pressure can squeeze down. Let go, and the particles spread back out again. Try the same thing with water instead of air, though, and you'll find it's practically incompressible.

Dissolve sugar in a half-filled glass of water and mark the water level before and after: the level rises when the sugar is added, but drops back a little once it fully dissolves — the volume of the solution ends up less than the sum of the water and sugar's separate volumes. That's the dissolved sugar particles slotting into the interparticle spaces between the water particles, rather than adding on top of them. Try it instead with sand, which doesn't dissolve, and the opposite happens: the sand particles simply settle and take up their own space, so the total volume increases.

Solids show the same interparticle-space story in a different way: their constituent particles are held together strongly and packed closely, so they can't move from place to place — but even so, some space is left between them. That space isn't filled with air; it contains nothing at all.

How Particles Move in Different States of Matter?

Drop a few grains of potassium permanganate into a tumbler of water and, without stirring, you'll first see pink streaks spreading out from the grains — and given enough time, the whole tumbler turns a uniform pink. The water particles are in constant motion: they pull particles of potassium permanganate off the grain and then keep hitting them, spreading them evenly through the liquid. Some substances, like sand, are held together so strongly that water particles can't pull any particles free at all — which is exactly why sand is insoluble in water.

Light an incense stick in one corner of a room, and though the fragrance is felt only nearby at first, it soon reaches every corner. The particles of air are moving constantly, colliding with the fragrance particles and carrying them outward until they fill the room.

Putting all this together: matter is made of small particles held together by attractive forces, and the strength of that attraction depends on the distance between particles, which itself depends on how much thermal (heat) energy they carry. In a solid, particles have low thermal energy, so they stay close together under strong attraction and can only vibrate in place. At the melting point, that thermal energy overcomes enough of the attraction for particles to leave their fixed positions — the interparticle distance grows slightly, the attraction weakens, and the particles can now move within a limited space, as a liquid. In a gas, particles carry enough energy to overcome the attraction between them almost entirely and move freely in every direction.