Particulate Nature of Matter

Look around and almost everything you can point at is matter. The desk, your water bottle, the water inside it, the chalk on the board, even the air you cannot see. In science we define matter as anything that has mass and occupies space. A steel spoon clearly has both. So does a glass of milk. So does air, which is why a properly pumped football feels firm while a flat one folds under your foot.

Now look closely at a drop of water, or at the edge of a steel ruler. It looks smooth and unbroken, as though it were one continuous lump all the way through. That is exactly what people believed for a very long time. The modern picture is quite different, and it is what this chapter is about. Matter is particulate: it is made of an enormous number of unbelievably small pieces called particles, and those particles are not stacked perfectly tight against one another. There are spaces in between.

How small is one particle? Far smaller than anything a school microscope can show you. A useful way to feel the size is this. A single drop of water holds so many particles that if you tried to count them at one particle per second, you would not finish in a whole lifetime. That is not a figure of speech. The number really is that large.

You can get a hint of it yourself. Take a small piece of chalk and crush it, then grind the powder finer, then finer again. Every speck, however tiny, is still chalk. It still writes on the board and it still feels like chalk between your fingers. Nothing tells you that you have reached the smallest possible piece. What stops you is your hands and your mortar, not the chalk. The particle idea simply pushes that same thought much further than your hands can go.

Through the rest of this chapter we will keep coming back to four short statements. Almost everything else follows from them:

• Matter is made of very small particles.
• There are spaces between those particles.
• The particles are always moving, and they move faster when the substance is hotter.
• The particles pull on one another with a force of attraction, and this pull is strong in some substances and weak in others.

None of these four statements can be checked by simply staring hard at a substance. They earn their place because they explain things you can actually see, smell and feel: sugar vanishing into tea, an agarbatti scenting a whole room, air squashing inside a syringe while water refuses to. That is how science usually works. An idea is accepted because it explains real observations better than the alternatives do, and because it keeps working when you test it on something new.

Saying that matter has gaps inside it sounds odd, because a steel rod feels completely full. So let us look at three simple activities that are hard to explain any other way.

Try this: dissolving. Fill a glass to a marked level with water and stir in two spoons of sugar. The sugar disappears from sight, yet the water level rises by less than the space the sugar filled in the spoon, and the water now tastes sweet right through. So where did the sugar go? It did not stop existing, because the sweetness proves it is still there. The particle model says the sugar broke up into its own particles, and those particles slipped into spaces that were already present between the water particles, rather like fine sand poured into a jar of marbles settling into the gaps. The same thing happens with salt, and your tongue can tell the difference.

Try this: the colour that refuses to vanish. Drop one small crystal of potassium permanganate into a beaker of water and watch a deep purple colour spread out. Now take a little of that coloured water, add it to a fresh beaker of plain water, and repeat that dilution several times over. Even after many rounds, the water still carries a visible tint. One tiny crystal has managed to colour a very large amount of water. That only makes sense if the crystal was built out of a colossal number of separate particles, each able to spread out on its own.

Try this: the syringe test. Take a syringe with no needle, pull the plunger back so it fills with air, seal the nozzle tightly with your thumb and press the plunger. It moves in a long way, because the air squashes. Now do exactly the same with the syringe full of water. Press as hard as you like and the plunger barely shifts. Air can be squeezed into a smaller volume because its particles are far apart with plenty of empty space to close up. Water cannot be squeezed much at all, because its particles are already almost touching.

Put the three together and a single explanation covers all of them. Sugar fits into water because water has room inside it. One crystal colours a huge volume because it contains a huge number of particles. Air compresses and water does not because the amount of empty space differs enormously between the two.

One warning about the picture in your head. The spaces between particles are not holes filled with air, the way a sponge is full of air. In a glass of water, the gaps between the water particles are simply empty space. Water that has been boiled, which drives out the small amount of air dissolved in it, still pours the same way, still dissolves sugar and still refuses to be squeezed in a syringe, so it cannot have been trapped air doing that work. Keep the picture as simple as it really is: particles, and empty space in between.

Light an agarbatti in one corner of a room and before long someone at the far end can smell it, even with the fans switched off and nobody walking about. Nobody carried the smell across. The scented particles travelled there by themselves, weaving through the moving particles of the air. This spreading of one substance through another, all on its own, is called diffusion.

Diffusion happens in liquids too, and there it is easy to watch. Place a drop of ink or a crystal of potassium permanganate at the bottom of a tall glass of still water and do not stir it. At first there is only a small coloured patch. Come back to it later and the colour has crept upwards and outwards, and if you leave the glass undisturbed for long enough the whole glass takes on an even colour. Nothing pushed it. The only thing that could have moved that colour is the motion of the particles themselves.

Diffusion tells you two things at the same time. First, the particles must be moving, otherwise nothing would spread anywhere. Second, there must be gaps for them to move into, otherwise they would be locked in place. So one observation supports two of the four ideas from the start of the chapter.

Temperature changes the speed. Set up two identical glasses, one with cold water and one with hot water, and drop the same amount of ink into each at the same moment without stirring either. The colour spreads through the hot water noticeably faster. Heating gives the particles more energy, so they move faster and mix more quickly. This is the everyday reason sugar dissolves faster in hot tea than in cold water, and why the smell of food fills the house sooner when it is bubbling on the stove than when it is standing cold.

The state of matter changes the speed too. Diffusion is quickest in gases, slower in liquids, and so slow in solids that for everyday purposes we treat it as not happening. In a gas the particles are far apart and move freely, so they cover ground quickly. In a liquid the particles are close together and keep bumping into neighbours, so progress is slow and halting. In a solid the particles are held in fixed positions and can only vibrate, so they cannot travel through the material in any time you would notice.

You meet diffusion constantly without giving it a name. Perfume or deodorant spreading through a room. The smell of cooking reaching you two rooms away. The smell of a leak from a cooking gas cylinder alerting the whole kitchen, which is no accident, because a strong smelling substance is deliberately mixed into the gas so that diffusion warns you early. Fish survive because oxygen from the air dissolves at the surface of the water and then spreads through it. Tea leaves colour hot water long before anyone stirs the cup.

Ice, water and steam are the same substance in three different states. They look and behave completely differently, yet nothing has been added and nothing has been taken away. The whole difference lies in how the particles are arranged, how strongly they hold on to one another, and how much freedom they have to move.

In a solid the particles sit very close together, usually in a neat repeating arrangement. The force of attraction between them is strong, strong enough to hold each particle in a fixed position. They are not perfectly still, though. Each particle vibrates about its own place, rather like a student fidgeting while staying in the same seat. They cannot swap places or wander off, and that is why a solid keeps its own shape.

In a liquid the particles are still close together, only a little further apart than in a solid, but the attraction between them is weaker. It is strong enough to keep the particles together in one body, and too weak to pin each one to a fixed spot. So the particles slide and roll past one another, rather like a crowd shuffling out of a school gate. Everybody stays in the crowd, but nobody keeps the same neighbour for long.

In a gas the particles are very far apart compared with their own size, and the attraction between them is so weak that it hardly matters. Each particle moves quickly in a straight line until it collides with another particle or with the wall of the container, bounces off and heads somewhere else. There is nothing holding them together, so a gas simply spreads out to fill whatever space it is given.

Because a gas is mostly empty space, the same amount of a substance takes up far more room as a gas than it does as a liquid or a solid. That is also why solids and liquids are usually far denser than gases: the same volume of them holds a great deal more matter. A bucket of water is heavy; the same bucket full of air is not.

Two more points worth carrying with you. First, whether a substance is a solid, a liquid or a gas at this moment depends on the temperature and the pressure, not on the substance being permanently one thing. Water is a solid in the freezer, a liquid in your bottle and a gas above a boiling pan, and it is the same water throughout. Second, some familiar materials do not fit the three neat boxes. Toothpaste, curd and jelly hold their shape when left alone but flow when you push them. Those in between materials are interesting, and the three states are still the right place to begin.

Once you have the picture of spacing and attraction in your head, the familiar properties of the three states stop being a list to learn by heart and become things you can work out for yourself.

Why a solid has a fixed shape and a fixed volume. Its particles are held in position by strong attraction. They cannot move to new places, so the block cannot rearrange itself. A steel rod stays a rod whether it is lying on a table or shut inside a box. To change the shape of a solid you have to force it, by hammering, cutting or bending, and the whole time you are working against those strong forces.

Why a liquid has a fixed volume but no fixed shape. The attraction is strong enough that the particles stay together, so the amount of space the liquid takes up does not change. It is too weak to fix each particle in one spot, so the particles flow into whatever container they are poured into. Pour half a litre of water from a jug into a bowl and you still have half a litre. It has only taken the shape of the bowl. This is also why liquids can be poured at all, and why water settles to a level surface.

Why a gas has neither a fixed shape nor a fixed volume. With almost no attraction holding them back, the particles keep moving until they strike the walls of the container, so a gas fills the whole container evenly. Release a little scented gas in a room and it does not stay in one corner. It spreads until it occupies everything available.

Why gases can be compressed. The particles of a gas are separated by large empty spaces. Push on them and there is room for them to come closer together, so the volume shrinks a great deal. This turns out to be genuinely useful. Cooking gas is squeezed so hard that it turns into a liquid, and a cylinder of it fits neatly in the corner of a kitchen. Vehicles that run on compressed natural gas carry it in strong cylinders at high pressure. A bicycle pump works on the same idea, and so does the firmness of a football or a tyre.

Why liquids and solids resist compression. Their particles are already almost touching, so there is very little empty space left to remove. Press a syringe full of water as hard as you can and almost nothing happens. This is exactly why the braking system of a vehicle uses a liquid rather than a gas. Push at one end and the liquid passes the push straight on, instead of soaking it up by squashing.

Why gases have a low density. Density compares how much matter is packed into a given amount of space. Because a gas is mostly empty space, a large volume of gas still contains very little matter. That is why a balloon filled with air weighs only a little more than the same balloon empty.

Leave an ice cube on a plate and it becomes water. Boil that water long enough and it disappears into the air. Nothing was added and nothing was destroyed. Only the arrangement and the energy of the particles changed. A change of state is a physical change, and it can usually be reversed by reversing the heating or cooling.

Melting. Heat a solid and its particles vibrate harder and harder. At a certain temperature the vibration becomes violent enough to overcome the attraction that was holding the particles in place. The fixed arrangement collapses and the particles begin to slide over one another, so the solid becomes a liquid. The temperature at which this happens is the melting point. For pure ice at normal atmospheric pressure it is 0 degrees Celsius.

Here is something worth watching for. Put a thermometer into a beaker of crushed ice and heat it gently. The temperature climbs to 0 degrees Celsius and then stops rising, even though you are still supplying heat. It stays there until the last piece of ice has melted, and only after that does the water start getting warmer. The heat you supply during that pause is not wasted. It is being used to break the particles out of their fixed arrangement, rather than to speed them up any further.

Freezing. Cool a liquid and the reverse happens. The particles slow down until the attraction between them is able to pin them into fixed positions once more. Water freezes at the same temperature at which ice melts. Water is unusual in one respect: it expands slightly as it freezes, which is why a completely full bottle of water can crack open in the freezer, and why ice floats on water instead of sinking.

Boiling. Keep heating a liquid and the particles move faster still. At the boiling point they have enough energy to break away from one another altogether and escape as a gas, and this happens throughout the liquid rather than only at the surface, which is why bubbles form deep in the pan and rise up. Pure water boils at 100 degrees Celsius at normal atmospheric pressure. High in the mountains, where the air presses down less, water boils at a lower temperature, and food therefore takes longer to cook there.

Condensation and sublimation. Cool a gas and its particles slow down until attraction can pull them together into a liquid, which is condensation. A few substances go straight from solid to gas without becoming a liquid on the way, a change called sublimation. Camphor left out in the open slowly disappears, and so do naphthalene balls kept in a cupboard, and neither leaves a wet patch behind.

Wet clothes on a line dry even on a cool day. A wet floor dries by itself. Neither one ever comes close to boiling temperature, so something other than boiling must be going on. That something is evaporation: a liquid changing into a gas at temperatures below its boiling point, taking place only at the surface.

Here is why it happens. The particles in a liquid are not all moving at the same speed. Some are slower and some are considerably faster. A fast moving particle that happens to be right at the surface, and happens to be heading outwards, can break free of the pull of its neighbours and escape into the air as vapour. Given time, particle after particle leaves this way, and eventually the puddle is gone. Boiling is different. It needs a definite temperature and it happens throughout the liquid, while evaporation goes on at any temperature and only from the surface.

What makes evaporation faster. Four things, and each one makes sense from the particle picture:

• Higher temperature. More particles are moving fast enough to escape, so drying is quicker on a hot day.
• Larger surface area. Only surface particles can leave, so spreading clothes out dries them faster than leaving them in a bundle, and tea poured into a saucer cools faster than tea left in the cup.
• Faster moving air. Wind or a fan carries the escaped particles away before they can drift back into the liquid, so clothes dry faster on a breezy day.
• Lower humidity. If the air already holds a lot of water vapour, there is less room for more, which is why washing takes so long to dry during the rainy season.

Evaporation causes cooling. The particles that escape are the fastest ones. The particles left behind are, on average, slower, and slower particles mean a lower temperature. So a liquid that is evaporating cools itself and whatever it is touching. Pour a little sanitiser on your palm and your palm feels cold as it dries. Sweating cools you for exactly the same reason, and that is also why cotton clothes feel comfortable in summer, because cotton soaks up sweat and spreads it over a large surface where it can evaporate. Water kept in an earthen pot stays cool because a little water seeps through the tiny pores in the clay and evaporates from the outer surface, drawing heat away from the water inside. Splashing water on a terrace before a hot evening cools it by the same effect.

Condensation is the return trip. Water vapour in the air is invisible, but cool it down and its particles slow enough for attraction to pull them into liquid drops. Take a cold bottle out of the fridge and droplets appear on the outside. That water came out of the air around the bottle; it did not seep through the glass. Dew on grass early in the morning and mist on a cold window are the same process. On a much larger scale, this pair of changes drives the water cycle. The sun evaporates water from oceans, lakes and fields, the vapour rises and cools, it condenses into the tiny droplets that make up clouds, and in time the water falls back to the ground as rain.