00:00:00 Proper regard for safety glasses, shields, protective gloves, laboratory coats, and

00:00:08 visible fire extinguishers.

00:00:11 The principle of safety first would be explicitly present as part and parcel of a modern tempest

00:00:18 in a test tube.

00:00:48 A series of experiments designed to explain the mysteries of chemistry and the laws that

00:01:17 govern it, produced by KQED San Francisco, in cooperation with the California section

00:01:32 of the American Chemical Society, for the Educational Television and Radio Center.

00:01:44 And now let's go to our laboratory and meet Dr. Harry Sello.

00:02:00 Hello, the number I have just written down, 602 followed by 21 zeros, is the number of

00:02:26 molecules I have in this little ounce bottle of water, which is half full.

00:02:34 Pretty large number.

00:02:37 If we wanted to name it or count it off, we'd have to start off and say, well, here, the

00:02:42 first six are millions, then trillions, quadrillions, now back off, we have too many already, millions,

00:02:53 trillions, quadrillions, quintillions, sextillions.

00:02:58 That's how I remember where I should end up here, sextillions, there's a key word here.

00:03:03 602 followed by 21 zeros.

00:03:06 Well, the chemist has a shorthand way of writing this, he doesn't like to write so many zeros,

00:03:10 being sort of a naturally lazy person, and he will write it in the following way.

00:03:18 602 times 10 with a 21 as a superscript.

00:03:25 This means if you multiply 10 by itself 21 times, that is 10 times 10 times 10 times

00:03:31 10 and so on, 21 times, and then multiply the resulting number by 602, you have in this

00:03:38 shorthand way a way of expressing this large number.

00:03:47 This number has meaning to the chemist and to physicists, and was named after an early

00:03:51 Italian physicist by the name of Avogadro, and it's called Avogadro's number.

00:04:09 Avogadro's number, and is the number of molecules in this amount of water.

00:04:15 Well, this is very hard to conceive of, that is you can't quite picture a number like this.

00:04:20 What are some numbers that we can compare this to that maybe we can think of?

00:04:25 Three billion is the number of people on earth about, and well, the 300 billion would

00:04:47 be about the estimated national income of the United States in the year 1955.

00:04:54 However, these numbers, as large as they are themselves, don't come anywhere near Avogadro's

00:04:59 number, nothing like it.

00:05:04 Because of the size of this number, it is then possible to realize that molecules and

00:05:10 atoms are indeed very small, small is hardly a word to use, infinitesimal, so tiny that

00:05:18 of course they cannot be seen, and are present in extremely large quantities and even the

00:05:23 tiniest drop of any liquid or the tiniest amount of any gas.

00:05:27 Now in this half ounce or so of water, there are Avogadro's number of molecules.

00:05:34 If all this water were to be vaporized, that is changed over into steam, at ordinary conditions

00:05:40 as exist in the room around us, then that number of molecules would occupy a box about

00:05:47 this large, just under one cubic foot or so.

00:05:51 To the chemist it's 22.4 liters at room temperature and pressure.

00:05:56 So here's a comparison.

00:05:59 The same number of molecules in the bottle as there are in the box, of course the box

00:06:03 contains air, the bottle contains water.

00:06:07 And then this brings us to the topic of the talk, which is atoms and molecules.

00:06:13 The word atom means indivisible, and an atom is the smallest part of an element which still

00:06:23 retains the properties of that element.

00:06:27 If we were to take a piece of calcium, which is a metal, an element, and cut it down as

00:06:31 fine as we could and then cut it down even finer and cut it down if we had an instrument

00:06:35 to do it, many, many, many, many times, the last little piece we could get down to that

00:06:40 would still be calcium is an atom of calcium.

00:06:43 Now I've said the word atom means indivisible.

00:06:46 This is an old dictionary meaning which doesn't apply in modern times because we know now

00:06:51 that atoms can be divided, but they then no longer have the properties that they had as

00:06:57 a whole atom.

00:06:58 That is, if an atom is broken up, something new is formed, and we'll talk about that later

00:07:03 in another talk.

00:07:05 Now if we combine two or more atoms, we then get a molecule.

00:07:10 The chemist knows that atoms and molecules are indeed small, but he knows their properties

00:07:16 and he describes them or studies them by use of models, which is a very convenient way.

00:07:21 Here are three models of molecules, which are combinations of atoms.

00:07:27 The first shows three parts.

00:07:30 Two parts are the same.

00:07:31 The third is a little different, a little larger, a little different.

00:07:35 This is a model of water, H2O, and it's arranged exactly in proportion to scale.

00:07:43 That is, it shows here that the oxygen atom is about half again as large as the hydrogen

00:07:49 atom.

00:07:50 So this whole molecule is in scale.

00:07:53 The same here, only this time we have a central atom, carbon, and the surrounded and around

00:08:00 it are four hydrogen atoms.

00:08:02 This would be a molecule we know as methane, CH4.

00:08:07 Finally here is another carbon model, a carbon compound, or better still, the smallest part

00:08:13 of a compound, namely a molecule.

00:08:16 This is carbon surrounded by four chlorine atoms, carbon tetrachloride, meaning four

00:08:21 chlorides.

00:08:22 You see, the scale shows up in that the carbon is practically surrounded, blanketed by the

00:08:28 chlorine atoms so that you can hardly see the carbon.

00:08:30 These are in proportion to scale.

00:08:32 We can have an idea of how large Avogadro's number is by the following little analogy.

00:08:38 If I were to take a glass full of water and had a way of coloring all the molecules in

00:08:43 that water, say red, a convenient color that we could recognize, and poured that glass

00:08:49 full of water down the sink and the sink emptied into the storm drain and the storm drained

00:08:53 into the nearest river and the river into the nearest lake and so on out into the oceans.

00:08:57 And I allowed that glass of water to get out into the oceans of the earth and mix itself

00:09:01 by some way, gigantic stir, perhaps, if you could imagine something like that.

00:09:06 If I allowed that glass full of water to mix itself over all the water of the earth, in

00:09:11 all the oceans, and then I took my original glass, which was now empty, and dipped out

00:09:16 a glass of water anywhere on earth, in any one of the oceans, I could find 1,000, about

00:09:22 1,000 of those original red molecules.

00:09:25 Well this is an idea of how tremendous the number is, that even though there is an awful

00:09:29 lot of water on earth, there are still so many molecules in that glass that there is

00:09:34 more than enough to go around all over the earth.

00:09:38 Well as I say, we can do our best study of atoms and molecules simply by the use of all

00:09:43 sorts of models.

00:09:45 For example, it's easy to see that when I pour some of this sand into a beaker, that

00:09:51 the sand flows very much like water, there's a stream of sand running out, each little

00:09:56 grain of sand is tumbling past the other and all its neighbors as it pours out, making

00:10:01 a general overall stream.

00:10:03 And these particles are, you can see the individual grains, these tiny little particles.

00:10:08 Well in the case of water, this is exactly the same thing that happens.

00:10:13 We pour water into a beaker, only you can't see the individual particles.

00:10:18 Still the little molecules of water are rushing past one another, slipping and rubbing, and

00:10:23 tumbling out of one beaker into the other.

00:10:25 In the case of sand, you can see the individual grains.

00:10:27 Of course, each grain of sand is many, many million times larger than the tiniest molecules

00:10:34 of water.

00:10:39 Since molecules are so small, they can be moved around very easily and are in motion

00:10:43 all the time.

00:10:45 The molecules of air around us are in motion all the time.

00:10:49 If the air is even still and we let a little fog loose in the room, why, this fog would

00:10:54 soon find its way through all the corners of the room.

00:10:56 It might take a little time, but eventually it would find its way.

00:11:00 If there are no winds and no currents, the reason that it finds its way out to all corners

00:11:03 of the room is due to molecular motion.

00:11:06 Here's an experiment which will illustrate some of the aspects of molecular motion.

00:11:13 I've got my burner here.

00:11:30 In this tube, next to the burner, there is a little layer of mercury on the bottom, silvery

00:11:38 metal.

00:11:39 I'll tip this up, take it out of here for a moment, that metal can be seen.

00:11:48 There's a layer of mercury.

00:11:50 On top of this mercury, there is a half-inch thick layer of broken blue glass, dark glass,

00:11:58 chips, lying on top of the mercury.

00:11:59 I'll just put this back in here.

00:12:06 The glass has been emptied of all the air and is sealed off at the end, so there's nothing

00:12:11 in here but mercury and glass.

00:12:14 If I heat this, we can have an illustration of what the idea of molecular motion is like.

00:12:29 The mercury is now beginning to move along in a little current, there it goes, starting

00:12:36 to boil, and as it boils, it kicks the glass beads up to the top.

00:12:40 Now the glass beads start here, go as high as here and here, and there are even some

00:12:46 near the top.

00:12:47 If I take this away, as it comes to rest, you can see that the glass beads fall back

00:12:54 down to the surface of the mercury.

00:12:55 Now just touching it with heat causes the glass beads to jump way up here.

00:13:01 The ones that are bouncing around right about where my finger is, about an inch or so above

00:13:04 the mercury, are bouncing around because of the tiny little particles and atoms of

00:13:11 mercury that are hitting them.

00:13:13 There is mercury vapor being formed underneath the beads, that's bouncing up against the

00:13:18 beads, kicking the beads up to the top of this little glass cylinder.

00:13:24 Now this illustrates that by applying a little bit of heat, I can make the molecules, and

00:13:29 in this case, I can make molecules and move around faster, in this case there are atoms

00:13:33 of mercury moving around.

00:13:34 In general, as far as motion is concerned, atoms and molecules of a gas, for example,

00:13:40 move around in about the same manner.

00:13:43 Well you see, so instead of talking about temperature, we can talk about molecular motion.

00:13:48 That is, instead of saying something is hotter or colder, we can say, well it has either

00:13:53 more or less molecular motion.

00:13:56 This explains why a gas, for example, will expand when it's heated.

00:14:00 If a gas is composed of molecules or atoms, and we know that they are all gases, then

00:14:06 when we heat them, this increases the amount of molecular motion, causing the molecules

00:14:10 or atoms to bounce on the sides of their container, on the top, on the bottom, so that this increases

00:14:16 the pressure.

00:14:17 Then from this we get the principle that increasing the temperature of a gas increases its pressure.

00:14:22 You see, it increases the motion and then the number of collisions.

00:14:26 If the top of the given box that we're talking about is made a sliding top so it can move

00:14:30 up and down, then as more molecules bounce on the top of the box, they can even cause

00:14:35 the top to move.

00:14:37 This then would be an example of the expansion of a gas by increasing its temperature.

00:14:43 Now the chemist isn't content to let the situation remain where it is, because he doesn't want

00:14:49 to say, well, temperature increases the motion of molecules.

00:14:52 He says, well, the reason that this happens is because it obeys the kinetic molecular

00:14:56 theory, the kinetic molecular theory.

00:15:00 The word kinetic means motion, K-I-N-E-T-I-C, molecular, you see, molecules, and theory

00:15:08 because originally this was a way of explaining the motion of molecules.

00:15:14 By using this pan in the next experiment, we can show a little bit more about molecular

00:15:19 motion.

00:15:22 I'll sprinkle a collection of atoms on the water in the pan.

00:15:34 This happens to be charcoal.

00:15:37 We'll get enough spread out around here.

00:16:07 Now, to this I'll add just a drop or so of some molecules.

00:16:17 These happen to be benzene, a compound which is composed of carbon and hydrogen atoms.

00:16:27 Watch carefully.

00:16:43 The film of charcoal was spread apart by just the molecules in a few drops of benzene.

00:16:50 The benzene spread itself out over the surface of the charcoal, over the surface of the water

00:16:54 on which was placed the charcoal, and it pushed the charcoal ahead of it.

00:16:58 And even though I only added a few drops of benzene, there were enough molecules in that

00:17:04 pan, on the surface of the water, to spread out over the surface and push the charcoal

00:17:10 ahead of it.

00:17:11 Now, of course, what's happened now is as the benzene has evaporated, the charcoal is

00:17:15 slowly coming back in to occupy the place where it was.

00:17:19 We can push them around just a little bit more by adding some more benzene here and

00:17:24 There it goes again.

00:17:26 The film of benzene molecules pushing against the charcoal atoms.

00:17:31 Well, even though these benzene molecules are a little bit larger than water molecules

00:17:39 and in the class of slightly larger molecules in some gases, like oxygen and nitrogen, they

00:17:45 are still quite tiny.

00:17:47 In fact, it takes about a hundred million of them lined up alongside of one another

00:17:53 to make an inch.

00:17:54 So for every inch of space, we would have about a hundred million benzene molecules,

00:17:59 for example, in the surface of the water.

00:18:03 And the pushing aside, as I say, of the charcoal was due to the motion of the benzene in the

00:18:07 surface of the water.

00:18:11 It also happens that benzene is not soluble in water, and for that reason, it stayed on

00:18:17 the surface.

00:18:19 If it were soluble, it would have mixed in with the water as it was that fell on the

00:18:23 surface and wouldn't show this effect nearly so strikingly.

00:18:27 Well, to prove further that molecules are indeed small, let's look at this next experiment.

00:18:39 Here's some zinc, which I'll put in this flask, and add a little bit of hydrochloric

00:18:51 acid so we can generate some hydrogen.

00:18:54 You get hydrogen almost immediately.

00:18:58 The fumes that you see are not hydrogen, but are fumes of hydrochloric acid, HCl, hydrogen

00:19:05 chloride, I should say, because this is a concentrated form of the acid.

00:19:09 Well, at the same time, we're getting hydrogen, but these fumes, the fog that's caused by

00:19:15 the hydrogen chloride, actually shows us where the gases are going.

00:19:19 And if I hold the beaker over this way, it will slowly fill up with hydrogen chloride

00:19:23 and also with hydrogen.

00:19:30 The fog is due to the fact that the hydrogen chloride reacts with the water vapor in the

00:19:34 air and creates little fog droplets of hydrochloric acid, which are not particularly pleasant

00:19:40 to breathe, but a small amount won't do any harm.

00:19:49 Now while this is generating a little bit more hydrogen to make sure that that beaker

00:19:53 is full, let me explain what I have here in this companion apparatus.

00:20:00 Here is a cup made of clay, a very hard material, but porous, with very, very tiny holes in

00:20:07 it.

00:20:08 It's a solid, very glassy-like, it's a hollow cup.

00:20:13 In the bottom is a stopper to which is attached a tube, a hollow glass tube.

00:20:18 This leads down into a flask of water, which is completely full, so that the level of water

00:20:24 can be seen right here in the tube, the little tube itself.

00:20:28 In another hole in the stopper, there is a little capillary tip, which is also full

00:20:33 of the same liquid.

00:20:34 See, if I just squeeze on the stopper, I can change the level in the tube, showing that

00:20:38 this is completely full.

00:20:39 Now this ought to have enough hydrogen in it.

00:20:42 If I pick this up upside down, the hydrogen, being lighter than air, will stay in the flask

00:20:48 while, in the beaker, while actually some of the HCl will go out.

00:20:54 Now watch the level of the liquid and this capillary tip as I put the beaker down over

00:21:02 this cup.

00:21:07 There's water squirting out of the little capillary tip.

00:21:12 Notice the bubbles of gas that are being pushed down here, going down into this flask, pushing

00:21:18 out water ahead of it, and water squirting out.

00:21:26 Let me stop this generator here, since it's still making hydrogen, by pouring in a little

00:21:30 bit of sodium hydroxide to neutralize the acid, and that stops the chemical action,

00:21:35 at least it slows it down.

00:21:38 Well here is an illustration then of the fact that even though this cup is solid, or to

00:21:43 all, as far as the eye can see, solid, it is not solid with respect to hydrogen.

00:21:48 Now hydrogen is such a small molecule that it can sneak right through the holes of the

00:21:52 cup and run on into the cup.

00:21:56 Well why does the water get pushed out?

00:22:00 Well this is due to the diffusion of hydrogen being greater than the diffusion of air.

00:22:05 Now let me explain these terms.

00:22:07 By diffusion I mean the passage of the hydrogen through these pores and through the molecules

00:22:11 of the gas inside.

00:22:13 Since hydrogen is smaller, is a smaller molecule than air, than nitrogen or oxygen, the hydrogen

00:22:19 gets into the cup faster than the air, that is the nitrogen or the oxygen, can get out

00:22:25 through these same holes.

00:22:26 The result is that pressure is built up on the inside of the cup.

00:22:29 This pressure then is transmitted through the gas of the hollow tube and down into the

00:22:33 liquid and pushes out the water, displaces the water.

00:22:38 Well how small are these holes?

00:22:43 Let me illustrate this.

00:22:46 Let me just erase some of these numbers here, we don't need them now.

00:22:52 If I make the hydrogen molecule about the size of a dot, maybe a quarter of an inch

00:22:59 or so, that's a hydrogen molecule on the blackboard.

00:23:03 Then the hole in the porous cup that this little dot has to sneak through to scale must

00:23:10 be something like 20 feet in diameter, that is almost as high as the ceiling and as far

00:23:15 out as the background here and way out as far as this.

00:23:19 Great big tunnel.

00:23:20 This is how the pore of the cup looks to the hydrogen molecule, almost 200 times as large.

00:23:27 So even though this looks solid, it's not at all solid to the hydrogen and the hydrogen

00:23:31 goes right through quite quickly.

00:23:35 And of course there's a further illustration here of the fact that the hydrogen has motion,

00:23:41 it can diffuse, the molecule has motion, can diffuse into the cup.

00:23:44 Notice how the liquid has begun to rise back up as we released the pressure, that is the

00:23:49 hydrogen now can get out.

00:23:52 Well since molecules are so small and move around through each other like the hydrogen

00:23:58 moved around through the nitrogen and the oxygen, it must be true that in general there

00:24:03 are spaces between molecules.

00:24:05 Well let's look at some evidence of spaces between molecules.

00:24:08 What do we mean by spaces between molecules?

00:24:11 Well this sort of thing.

00:24:12 Here's a beaker full of mothballs, about the size of marbles, and here's a beaker full

00:24:18 of sand.

00:24:19 They're about equal volume, the mothballs occupy about the same volume here as the sand

00:24:23 occupies in here.

00:24:24 Well if I pour these two together, I can add a lot of sand before there is even any increase

00:24:31 in volume of the beaker containing the mothballs.

00:24:34 In fact, I can darn near get in all the sand, got in about three quarters of it.

00:24:44 Well here's a case where one volume of sand plus one volume of mothballs is not quite

00:24:49 equal to the two volumes of the mixture, quite an awful mixture at that.

00:24:54 But you see the sand, which are small grains, sand particles which are small grains, fit

00:24:59 in around the mothballs which are large grains.

00:25:03 Well does this have any bearing in actual practice, in the actual case of liquids or

00:25:09 gases as we've demonstrated here with this illustration?

00:25:11 Yes it does.

00:25:13 Here is a hundred milliliters of potassium iodide powder, which is a solid compound.

00:25:19 Here is a hundred milliliters filled to the top of water.

00:25:23 I have marked this cylinder, this empty cylinder, at the hundred milliliter mark equal to one

00:25:28 volume of this, and at the two hundred milliliter mark equal to both of these put together.

00:25:33 Let's put them together and see what happens.

00:25:38 Trying to get it all in here without spilling it because it's important to our volume measurement

00:25:58 that we don't shortchange ourselves here in this experiment.

00:26:03 Some wants to stick in there, let's just scrape it down.

00:26:08 There, I don't want anybody to think that we're hunching here.

00:26:12 Now that about gets it all, there's a tiny bit.

00:26:19 I'm sure there's somebody that will say, well he didn't use it all, let's see if he can

00:26:23 use it all.

00:26:24 That's about it.

00:26:28 Even the most careful now, plain.

00:26:30 To this, you see, to this we'll add the water, this just about fills it up, in fact it does

00:26:34 to the mark which is a hundred milliliters.

00:26:37 Now, let's add a little water to this, or the equal volume of water.

00:26:44 So one hundred plus one hundred, let's see what happens.

00:26:51 Shake it up.

00:26:54 We now have both volumes of materials in here.

00:26:58 Most of the potassium iodide has dissolved, some is still in the bottom.

00:27:02 By the way, this is getting quite cold, potassium iodide dissolves in water and it's an endothermic,

00:27:07 means it takes up heat, a reaction which takes up heat.

00:27:11 There, one hundred plus one hundred by this kind of experiment equals a hundred and fifty.

00:27:17 Well this is odd arithmetic, the chemist isn't particularly surprised at this, let the mathematicians

00:27:23 worry about the fact that one hundred plus one hundred should equal two hundred.

00:27:27 Well, let's look back and see what was demonstrated.

00:27:30 First, we saw that in that little half ounce of water or so we had Avogadro's number of

00:27:39 molecules, six O two times ten to the twenty-first.

00:27:43 Sometimes this is commonly written as six point O two times ten to the twenty-third.

00:27:51 Add two more tens on because the decimal point has been moved over.

00:27:55 Either way is just as correct.

00:27:58 This number of molecules, if present as a gas, was shown to be twenty-two point four

00:28:04 liters at standard conditions.

00:28:07 By standard conditions I meant something like zero degrees centigrade and atmospheric pressure.

00:28:13 We then saw molecular motion as illustrated by the mercury in glass and then went on to

00:28:20 look to further molecular motion by the charcoal in the tray and the size of the molecule.

00:28:27 Again, this was illustrated by the hydrogen going right through the porous cup and finally

00:28:32 we showed that there are spaces between molecules because if you add one hundred plus one hundred,

00:28:39 in the case of the potassium iodide in the water you don't get two hundred as the volume,

00:28:43 but in this case we got a hundred and fifty.

00:28:47 Now the potassium iodide still hasn't all dissolved but the total volume adds up to

00:28:53 about a hundred and fifty milliliters and we started with a hundred each.

00:28:57 This was similar to the case as illustrated by the sand filling in the spaces between

00:29:02 the moth balls.

00:29:05 Molecules are indeed small, they cannot be seen, but they are certainly all around us.

00:29:10 Thank you.

00:29:57 This is National Educational Television.