Class 8 Science Chapter 7: Particulate Nature of Matter

Start on a Riverbank

Pick up a handful of sand on a riverbank and you are holding the end of a long process. High in the mountains, rock breaks apart. Rivers carry the broken pieces down, grinding them against one another the whole way, until what arrives in the plains is pebbles, then grit, then fine sand and clay.

The Obvious Question

So is a grain of sand the smallest piece a rock can be broken into, or could it be broken further? The river stopped, but did nature? That is the question this whole chapter grows from, and the way to answer it is to try the breaking yourself with something softer than rock.

Breaking Chalk

Take a stick of chalk and snap it in two. Snap the halves. Keep going until your fingers cannot manage it, then grind what is left in a mortar with a pestle until it is a fine powder. Look at the powder through a magnifying glass.

Still Chalk

Every tiny grain you can see is still a speck of chalk. Nothing has turned into anything else. In the language of last year, grinding chalk is a physical change: only the size of each piece has been reduced, and no new substance has appeared.

Keep Going, In Your Head

Now imagine the grinding carrying on far past what any pestle could do. Sooner or later you arrive somewhere that nothing divides again. The tiny units waiting there are the blocks the chalk was built from.

Constituent Particles

Those units have a name: a constituent particle, meaning the unit at which dividing a material has to stop. So one stick of chalk was an enormous crowd of them held together, and a grain of sand is no different. It is not the smallest piece a rock can be, but a crowd of particles in its own right.

Too Small to See

These particles are extremely small. Not merely hard to see, but far beyond an ordinary microscope. That makes them awkward: you cannot point at one. So the evidence for them has to be indirect, which is what the next experiment provides.

Where Does the Sugar Go?

Fill a tumbler with drinking water and put in two teaspoons of sugar without stirring. Taste a small spoonful from the top layer. It is not sweet. Now stir until every grain has gone, and taste the top layer again.

Sweet at the Top

This time it is clearly sweet. That matters, because sweetness at the top means sugar is at the top, and yet there is not a grain to be seen anywhere in the glass. The sugar has not been destroyed and it has not stayed at the bottom.

What Dissolving Is

When sugar dissolves, every grain comes apart into the constituent particles it was built from, and nothing divides those again. Each tiny grain you spooned in held millions upon millions of them. They are much too small to see, and taste is the only sense that reports them.

And Where They Went

The sugar particles separate and settle into the room that is already there between the water particles. Those gaps have a name of their own: interparticle spaces. This is the first hint that matter is not solid all the way through, even where it looks it.

Two Activities, One Conclusion

Put the chalk and the sugar together and they say the same thing. Matter is composed of a very large number of extremely small particles. The chalk shows that breaking has an end; the sugar shows that the pieces at that end are small enough to hide in the spaces of a liquid.

An Old Idea

This is not a new thought. Acharya Kanad, an ancient Indian philosopher, wrote in the Vaisheshika Sutras about tiny, indivisible, eternal particles he called Parmanu, and argued that matter is made of them. People have been asking how far things can be broken down for a very long time.

The Next Question

If a piece of chalk is a crowd of separate particles with spaces between them, something must be holding it together. Nothing stops a pile of loose sand falling apart, and yet a stick of chalk does not. What holds the particles of a solid in place?

Forces That Pull

The answer is that the constituent particles attract one another. These pulls are called interparticle attractions, and their strength depends on the substance and, heavily, on the gap between them. Open that gap a little and the pull falls away sharply. It is the strength of these pulls that decides the physical state of a substance.

What To Carry Forward

Matter is made of constituent particles far too small to see. Grinding and dissolving both break a substance into smaller pieces without changing what it is. There are spaces between those particles, which is where dissolved sugar goes. And what holds them together is attraction between the particles, which is where the next topic begins.

Scenario:

Two pupils set out to settle an argument about dissolving. Ravi says the sugar sinks and stays at the bottom. Meera says it spreads everywhere. They fill a tumbler with drinking water and add two spoons of sugar without stirring. A spoonful from the top layer tastes of nothing at all, and Ravi is pleased. Then they stir until no grains are left and taste the top layer again. This time it is clearly sweet. Ravi says the sugar must have been destroyed, because he can no longer see any of it anywhere in the glass.

Analysis:

The first taste supported Ravi only because the sugar had not yet spread. The second taste settles it. Sweetness at the top means sugar particles are at the top. Nothing was destroyed: the grains broke up into particles too small to see, and those particles moved into the spaces between the water particles.

Activity:

Repeat the test with common salt and write down which taste tells you it has spread.

Title:

Try it at home: grind and look

Safety:

Do not taste the chalk, and wash your hands afterwards.

What You Notice:

The powder gets finer the longer you rub, and it never turns into anything else. Both halves are still chalk. Rubbing changes the size of the pieces and nothing more.

Acharya Kanad argued for indivisible eternal particles he called Parmanu in the Vaisheshika Sutras, long before any instrument could look for them.

A single grain of sugar is made of millions upon millions of constituent particles, which is why it vanishes so completely in a glass of water.

The spaces between particles are what allow a dissolved solid to take up almost no extra room in a liquid.

BREAKING HAS AN END — Grind far enough and a stage comes where nothing breaks further. Those units are the constituent particles.

SMALLER, NOT DIFFERENT — Chalk powder is chalk. A change of size with no new substance is a physical change.

Something Is Holding It

The last topic ended on a question. A stick of chalk is a crowd of separate particles with room between them, so why does it not fall apart the way a heap of dry sand does? Something must be pulling the particles towards one another.

Interparticle Attraction

That pull has a name: interparticle attraction. Two things govern how strong it is. One is what the substance happens to be. The other is the gap between the particles, and the gap matters enormously, because opening it even slightly makes the pull much weaker.

Why That Matters

It is the strength of those pulls that fixes whether a substance is a solid, a liquid or a gas. That single sentence is the spine of this chapter, and the three topics that follow are really one question asked three times: how strong is the hold here?

Pick Up Some Solids

Collect a key, a stone, a piece of aluminium, a block of wood, an iron nail and a lump of rock salt. Look at them. Try tapping each with a hammer. Every one keeps the shape it had and takes up the same room it took before.

Fixed Shape, Fixed Volume

A solid holds its own shape and its own volume, and neither depends on what you keep it in. Put the key in a box or in your hand and it is the same key, the same shape, taking up the same space.

Packed Tight

The reason sits at the particle scale. In a solid the particles sit very close to one another and the pulls between them are as strong as they get. Those pulls keep every particle in one place and will not let it wander off.

But Not Still

That does not mean nothing moves. Each particle shakes to and fro about its own place, which is called vibrating or oscillating. What it cannot do is slide past its neighbours, and that restriction is exactly what gives a solid its shape.

Now Add Heat

Warm a solid and the shaking grows. The hotter it gets, the more violently each particle swings about its place, and a point arrives where the swinging is too much for the hold to contain.

The Hold Gives Way

At that point the particles start abandoning the places they were kept in. The pulls between them weaken, the particles are able to move about, and what was a solid is now a liquid. The temperature at which this happens has a name.

Melting Point

The melting point of a solid is the lowest temperature at which it turns into a liquid under ordinary air pressure. It is a property of the substance, not of the size of the lump, so a nail and a girder of the same iron melt at the very same temperature.

Slightly Further Apart

In a liquid, the particles generally sit a little further from one another than they did in the solid. Water and ice are the famous exception the other way: the particles in ice are further apart than those in liquid water.

Weak Hold, Low Point

Now the useful consequence. Where the pulls between particles are weak, not much heat is needed to overcome them, so the melting point is low. Ice gives way at zero degrees Celsius, which is why an ice cube melts in your hand.

Strong Hold, High Point

Where the pulls are strong, a great deal of heat is needed and the melting point is high. Urea melts above a hundred and thirty degrees. Iron does not give way until well past fifteen hundred, which is why a cooking fire will never melt a nail.

Reading That Backwards

This is the most useful trick in the chapter. A melting point is a measurement you can look up, and it tells you something you cannot see: how strongly the particles of that substance hold one another. A high number means a strong hold.

What Heat Is Spent On

Notice what the heat actually does. It is not used up changing the particles, which stay exactly as they were. It is spent overcoming the attraction between them. That is why melting needs heat even though the substance afterwards is the same substance.

And What Comes Next

A solid, then, is the state where the hold wins. The particles are close, strongly held and unable to travel, so the shape and the volume are both fixed. Weaken that hold a little and you get a liquid, which is where the next topic goes.

What To Carry Forward

Interparticle attraction holds the particles of a solid in fixed places and lets them do nothing but vibrate, which is why both the shape and the room it takes are its own. Heating makes the vibration fiercer until the hold fails, at the melting point. A low melting point means a weak hold; a high one means a strong hold.

Scenario:

A class is given a block of ice, a spoon of urea from the farm shop and an iron nail, and asked which will melt first on a hot plate. Anil says the nail, because metal gets hot fastest. Priya says the ice, because it melts in her hand already. The teacher sets the plate going and the ice is water within a minute. The urea takes much longer. The nail does not change at all, even after twenty minutes, and the class decides the plate is broken. The teacher shows them a table of melting points instead.

Analysis:

Getting hot quickly and melting are two different things. The order is set by how strongly the particles attract one another: ice gives way at zero degrees, urea at well over a hundred, and iron not until more than a thousand. The hot plate never reaches the last of those, so the nail was never going to melt on it.

Activity:

Find the melting point of two more solids and place them in order beside ice, urea and iron.

Title:

Try it at home: watch ice give way

Safety:

Keep the saucers somewhere nobody will slip on spilled water.

What You Notice:

The warm one goes first. Warmth makes the particles vibrate harder until they break out of their fixed positions. Neither cube turned into a new substance; both became water.

A melting point lets you compare forces you can never see: the higher the figure, the more tightly that substance holds itself together.

The heat that melts ice does not change a single water particle. All of it goes on loosening the grip between them.

Ice breaks the usual rule. Its particles sit further apart than those in the water it melts into, which is why ice floats.

SHAKE, BUT DO NOT TRAVEL — A particle in a solid vibrates about its own place and can never slide past its neighbours.

HIGH NUMBER, STRONG HOLD — A melting point read backwards tells you how tightly the particles of that substance hold one another.

Loosen the Hold

A solid is the state where the hold wins: particles close together, kept in their places, able to shake but not to travel. Loosen that hold a little and you get this topic. Not much has to change at the particle scale for a great deal to change in what you can see.

The Pouring Test

Take three clean, dry containers of different shapes and call them A, B and C. Mark two hundred millilitres on each one. Fill A to its mark, pour that water carefully into B, and then from B into C, watching both the shape and the level each time.

Two Different Answers

The shape is new in every container: the water simply fills whatever it is poured into. The level, though, arrives at the mark every time. One experiment, two separate answers, and the whole character of a liquid is in the difference between them.

No Shape of Its Own

So a liquid has no shape of its own. Whatever holds it lends it one. That is the first half, and it follows from the particles being free to move about instead of being pinned in place the way a solid’s are.

But a Volume of Its Own

The second half is that the amount of room a liquid takes is its own. Pour it anywhere and it is still two hundred millilitres. The particles can move, but only within a limited space, and that limit is what fixes the volume.

When the Level Comes Out Low

Sometimes the mark is not quite reached after pouring, and the honest explanation is not about the water. If a container was not perfectly dry, some water clings to its walls and is left behind. Dry the containers and the level holds.

Put a Finger In

Here is the second test, and it takes five seconds. Pour a shallow layer of water into a wide dish and push a finger through it. The water parts, and the moment you lift the finger out, the water closes over the gap and the surface is as it was.

What That Shows

You could not do that to a stone. The parting means the particles are free to move past one another, which a solid’s cannot. The closing means something is still pulling them together. Both halves of the liquid state are in that one movement.

Slightly Weaker, Still Holding

So the attractions in a liquid are a shade weaker than in a solid, and they are not weak. They keep every particle near its neighbours. That is the whole difference between the two states: not the presence of a hold, but the strength of it.

Now Heat It

Heat a liquid and the movement of the particles becomes more and more vigorous. A point arrives where they are moving violently enough to get right away from each other. The attractions between them fall off, and particles start escaping the liquid altogether.

Boiling Point

That escape is boiling, and the temperature at which a liquid turns into vapour under ordinary air pressure is its boiling point. At that temperature the vapour forms very fast, and not only at the top: it forms inside the body of the liquid too.

Why There Are Bubbles

Those bubbles rising through a pan of boiling water are vapour being made down inside the liquid. That is the visible difference between boiling and the slower process, and it is worth watching for, because it settles which of the two you are looking at.

The Slower Way

Spilled water on a floor dries up without anybody heating it, at whatever temperature the room happens to be. Vapour forms at every temperature, well below the boiling point included. It is simply slow, and it happens only at the surface.

Evaporation

That slower process is evaporation, which you met in earlier grades. The pair is worth holding together: boiling is fast, needs the boiling point, and happens throughout the liquid; evaporation is slow, needs no particular temperature, and happens at the surface alone.

Hard to Squeeze

One more property, because it will matter next topic. Draw water into a syringe, seal the end and push: the plunger barely moves. Water is practically impossible to compress, and the reason is that its particles are already close together.

Liquids and Gases Together

Both liquids and gases flow, and neither keeps a shape of its own. That shared pair of properties earns them a shared name: fluids. It is a useful word, because it groups them by behaviour rather than by how far apart their particles happen to be.

What To Carry Forward

A liquid has no shape of its own and a volume that is its own, because its particles can move but only within a limited space. The hold is a little weaker than in a solid and still real, which a finger pushed through water shows both ways. Boiling is fast, at the boiling point and throughout; evaporation is slow, at any temperature and at the surface.

Scenario:

A group is given the pouring test and told to report whether water has a fixed volume. They mark two hundred millilitres on three containers and start with the widest. After the first pour the level is a whisker low, and after the second it is lower again. The group writes that water loses volume each time it is poured and that the fixed-volume rule is wrong. Their teacher asks one question: were the second and third containers dry when you began? They were not. One had just been rinsed and the other had been used for juice.

Analysis:

Water clinging to the walls of a container that was not dry is left behind, so less arrives in the next one. Nothing has happened to the volume of the water itself. The rule holds; the method was at fault, and the way to show it is to dry every container and pour again.

Activity:

Repeat the pouring test with dry containers and report whether the level holds.

Title:

Try it at home: three shapes, one volume

Safety:

Work over a sink or a tray so a spill does not matter.

What You Notice:

The water takes a new shape in every container, and the amount in the measuring cup at the end is the same as at the start. If it is a little less, look for water left on the walls.

The finger test settles two questions at once: the water parting proves the particles move, and the water closing proves they still pull.

Water is so hard to compress that a sealed syringe full of it will barely let the plunger travel at all.

Vapour is coming off every open glass of water in the room right now, at whatever temperature that room happens to be.

SHAPE BORROWED, VOLUME OWNED — The container lends a liquid its shape. The amount of room it takes travels with the liquid.

BUBBLES MEAN BOILING — Vapour forming down inside the liquid is what a bubble is, and only boiling does that.

Loosen It Further

Two states down, one to go. A solid has strong pulls and particles that cannot travel; a liquid has slightly weaker pulls and particles that travel inside a limit. Take the hold away almost entirely and you have the third state.

Smoke in Two Jars

Here is how to watch it. Burn an incense stick, hold a gas jar upside down over the smoke until the jar is full of it, then turn it the right way up and lay a glass plate across its mouth. Stand a second, empty jar on top of that plate.

Take the Plate Away

Now slide the plate out, keeping the two jars mouth to mouth so nothing escapes at the join. Within moments the smoke has spread evenly through both. Nobody blew it, nobody shook it, and nothing was heated.

What That Settles

The smoke took every bit of room that was open to it. So a gas has no volume of its own: it spreads into whatever space it can reach. And like a liquid, it ends up the shape of the vessel it is in, which means a gas has neither property fixed.

Why It Happens

At the particle scale the pulls between gas particles are so weak that they count for almost nothing. With nothing to hold them, the particles travel in every direction at once, and they keep going until a wall stops them.

Iodine Instead

The same demonstration works with iodine vapour in place of incense smoke: leave a little solid iodine in a closed jar and the vapour fills it. Smoke and vapour are both standing in for a gas here, because a gas itself is invisible.

A Shared Name

Liquids and gases have something in common worth naming. Both flow, and neither keeps a shape of its own. Those two properties set them apart from solids, and they are called fluids because of it.

Now the Spaces

The second half of this topic is about the room between particles. Interparticle spacing is not the same question as the strength of the hold, although the two are tied together, and it has its own experiments.

Push a Syringe

Take a syringe with no needle and pull the plunger right out, so the barrel is full of air. Seal the open end with your thumb so no air can get away, then push the plunger slowly inwards. It travels a long way.

What Gave Way

The air inside takes up less room than it did, and what was squeezed out was never the particles. It was the room between them. So gas particles sit far apart to begin with, and pressure from outside can close that distance up.

And It Springs Back

Stop pushing and the plunger returns to where it started, because the particles spread out again as soon as they are allowed to. Now repeat the whole thing with water in the barrel, and the plunger barely moves at all. Water is very nearly impossible to compress.

The Sugar Measurement

For liquids there is a neater test. Half fill a vessel with water and mark the level A. Add two spoons of sugar and mark the new level B. Stir until the sugar has gone and mark the level once more as C.

Reading the Three Marks

C comes out below B. The solution takes up less room than the water and the sugar did separately, and the only place the missing room can have gone is into gaps that were already there between the water particles. The sugar particles have moved into them.

The Control

Repeat it with sand, which does not dissolve. The grains stay whole, sit in the vessel and take up space of their own, so the level stays where the sand put it. That contrast is what makes the sugar result mean something.

Spaces in a Solid Too

Solids are closely packed, and even so there is room left between the particles. Close packing is not perfect packing. The gaps are smallest here, larger in a liquid and largest of all in a gas, which is the whole ordering of this chapter.

What Is In the Gaps

It is tempting to say air. It is not air. Those spaces hold nothing at all, which makes sense when you remember that air is itself made of particles with their own spaces between them.

Two Meanings of One Word

A warning about language. In talk about air pollution you will meet suspended particulate matter, which means specks of dust and soot floating in the air. Those are not constituent particles. A speck of dust is made of an enormous number of them, and the word simply means something different in that context.

What To Carry Forward

A gas has neither a shape nor a volume of its own, because the pulls between its particles are negligible and they travel in all directions into any space available. A syringe shows that gas particles sit far apart, and the sugar marks show that liquid particles have gaps too. Even solids have gaps, and those gaps hold nothing.

Scenario:

A group measures carefully. They half fill a glass vessel, mark the water level A, add two spoons of sugar and mark the new level B. Then they stir until the sugar has gone and mark the level again as C. To their surprise C is below B. One pupil says some water must have leaked, and checks the vessel for cracks. Another says the sugar has shrunk. A third repeats the whole thing with two spoons of sand instead, and finds that the level stays exactly where the sand put it.

Analysis:

There was no leak and nothing shrank. Sugar breaks into particles that settle into gaps already present between the water particles. So the solution takes up less room than the water and sugar did separately. Sand does not dissolve. Its grains simply sit in the vessel and take up space of their own, which is why that level held.

Activity:

Repeat the test with common salt and with small stones, and record which level falls.

Title:

Try it at home: push and feel

Safety:

Never point the open end at anybody, and never use a syringe with a needle.

What You Notice:

The air lets the plunger travel a long way and pushes it back when you let go. The water hardly lets it move at all. The difference is how much empty space there was to begin with.

Iodine vapour will do the two-jar activity just as well as incense smoke, because both are visible stand-ins for something that is not.

A glass of syrup takes up less room than the water and the sugar did before they were mixed, and the missing room was gaps in the water.

The spaces between the particles of a solid are not filled with air. They hold nothing whatever.

A GAS KEEPS NOTHING OF ITS OWN — Neither shape nor volume. It spreads into whatever space it can reach and takes the shape of the vessel.

ROOM, NOT PARTICLES — What a plunger squeezes out of air is the space between the particles, never the particles themselves.

One Thing Left to Watch

Four topics have described where particles sit and how hard they are held. This one is about whether they are moving, and the answer turns out to tie the whole chapter together.

A Grain of Colour

Take a tumbler of water, drop two or three grains of potassium permanganate in, and then do nothing at all. Do not stir it, do not shake the table, do not warm it. Just watch.

Streaks, Then Pink

Streaks of pink creep out from the grain almost at once. Given time, the whole body of water is an even pink. Something moved the colour through a liquid that nobody touched.

What Did the Moving

The water particles did, because they never stop moving. They first drag the coloured particles off the grain, and then, by knocking into them again and again, they carry them through every part of the liquid. The colour is a tracer for motion we cannot see.

Why Sand Will Not Do That

Try the same thing with sand and nothing happens. In some substances the constituent particles grip one another far too tightly for water particles to prise any of them loose, and such substances will not dissolve. Sand is one of them.

Three Tumblers

Now add a variable. Fill one tumbler with hot water, one with water from the tap and one with ice-cold water, and drop one grain of potassium permanganate into each. Start a clock and compare.

What the Clock Shows

The hot tumbler goes evenly pink first, by a wide margin. The tap-water one takes noticeably longer. The cold one is still a pale cloud round the grain when the others have finished.

Reading That Result

Warmer water has faster-moving particles, so they prise the coloured particles loose sooner and knock them about more briskly. Colder water has slower ones. Supplying heat increases how much the particles move, and that single sentence does a great deal of work in this chapter.

Watch Out for a Trap

It is tempting to write down that heat dissolves the grain faster. That sentence names the result again instead of explaining it. The cause is the speed of the water particles; the faster spreading is the effect.

Gases, Without Seeing Them

Air particles cannot be traced with a dye, so the demonstration uses a smell instead. Set an incense stick burning at one end of a room, give it a few minutes, and go and stand at the other end.

The Fragrance Arrives

At first the scent is only near the stick. Shortly it is everywhere in the room. Particles carrying the smell have been knocked along by air particles, which tells you that the air in a closed room is never still even when there is no breeze.

Soap, Oil and Cloth

Here is the same physics doing something useful. Washing an oily stain with soap works because soap particles crowd round the oil particles on the cloth. One end of a soap particle takes hold of the oil, and the other end mixes happily with water, so the oil is lifted away and carried off.

Now Put It All Together

Matter is made of small particles with attractions between them. The strength of those attractions depends on the distance, and the distance depends on how much thermal energy the particles have. So thermal energy is the quantity that fixes the state of matter.

The Solid Case

In a solid the thermal energy is low. The particles stay close to one another, feel strong attractions, and can do nothing but vibrate a little about their own places. The hold wins because the energy is small.

Crossing the Melting Point

At the melting point, thermal energy goes into overcoming those attractions. The particles come away from their fixed positions and the distance between them opens slightly, which weakens the hold enough for them to move about inside a limited space. That is the liquid state.

The Gas Case

Give them more energy still and they can defeat the attractions altogether, travelling freely in every direction. That is a gas, and nothing holds it to a shape or a volume.

The Table in One Sentence

Spacing smallest, attraction strongest and movement only vibration in a solid. Spacing a little wider, attraction slightly weaker and movement inside a limit in a liquid. Spacing largest, attraction negligible and movement through all the space available in a gas.

What To Carry Forward

Particles are in constant motion, and a grain of colour in still water proves it. Heating makes them move faster, which makes the colour spread faster. Air particles knock a fragrance across a room. And the thermal energy of the particles is what decides whether a substance is a solid, a liquid or a gas.

Scenario:

A pair of pupils are told to show that warmth changes how fast particles move. They pour hot water into one glass, water from the tap into a second and water with melting ice in a third. Into each they drop one small grain of potassium permanganate and start a clock. The hot glass is evenly pink in under a minute. The tap-water glass takes several. The cold one still has a pale cloud around the grain when the bell rings. One pupil writes that heat dissolves the grain faster. The other writes that heat makes the water particles move faster.

Analysis:

Both sentences fit what they saw, but only the second explains it. Warmer water particles move faster, so they pull particles off the grain sooner and knock them through the liquid more quickly. Saying heat dissolves it faster names the result again instead of giving a cause, which is the commonest way an observation gets written up as an explanation.

Activity:

Try the same three glasses with a drop of ink and report which order you get.

Title:

Try it at home: time a smell

Safety:

An adult must handle anything that burns, and never leave it unattended.

What You Notice:

The smell reaches the far corner with nothing blowing it there, and it gets there sooner in the warmer room. Moving air particles knock the scent particles along, and warmth makes them move faster.

The colour in that tumbler is a tracer: it makes visible a motion that is going on in every glass of water, with or without anything dissolved in it.

Saying that heat dissolves a grain faster is not an explanation at all, because it names the result again instead of its cause.

A soap particle does two jobs at once, holding oil with one end while the other end mixes with the water that carries it away.

NOBODY STIRRED IT — A grain of colour spreading through still water is the proof that particles move on their own.

ENERGY SETS THE STATE — Low energy, a solid. More, a liquid. Most, a gas. Spacing and attraction follow from it.