The Science of Fireworks

[Music] hello and welcome to this lecture on the science of fireworks a firework are very popular but they can also illustrate some very interesting ideas in science this is called analyse fountain they looked as if it's white-hot but actually I can pass my hand through the flame something of quarters should never do with a regular fire way and the reason is that this is based on a different kind of chemistry and we'll see how that chemistry was discovered in an accident in the 19th century fiction you'll also see how an ancient Chinese recipe involving honey led to the development of gunpowder and we'll see how the bangs made by pilots have their origin in photography now of course you should never pass your hand through the flame of a firework and indeed in this lecture please do not try to copy any of the demonstrations that you see at home afterwards if you do that's the sort of thing that might happen so please do not try to copy any of the demonstrations if you look at all these explosions and bangs and flashes and you think that looks like jolly good fun you're right it is it's jolly good fun I have to say and the way to do that particularly you know if you're a youngster work hard at school study science work hard at your science work hard at your mathematics and become a professional scientist and then like me you can actually be paid to have fun right and that's the best thing about being a scientist being paid to have fun okay so with that we all will make a start and we're going to look at gunpowder because gunpowder is used in many kinds of fireworks as we'll see and also of course at this time of the year we celebrate Guy Fawkes night we celebrate one of the most famous non explosions in history and that also is based on gunpowder so gunpowder is a good place to start as I'm sure you know gunpowder has been around a very long time it has its origins in China and this is a part of a script from about 900 AD it's called essentials of the mysterious way of a true origin and in this script it says some have combined sulfur with salt pizza and mix them with honey and heated them so that flames have burst forth even to the point of reducing their houses to cinders so even in 918 people were saying don't try this at home okay now we're not at home we're in the chemistry department so I thought we would and give this a go so in this pot I have a mixture of saltpeter which the chemical name of saltpeter is potassium nitrate as a potassium nitrate mixed together with honey and sulfur so it's a lot of sticky yellow stuff and I'm going to follow the instructions from the Chinese script and we're going to heat this and we'll see what happens I'm going to put that in the stand and we'll we'll light the Bunsen burner I'll have the light stand for this and we're going to start to heat this mixture and we'll see what happens as we heat it so nothing much happening just yet but but inside that pot the honey will be melting and will be sort of running together with the the saltpeter and the sulfur and as the temperature starts to build up something interesting is happening a sort of very primitive kind of firework so it seems some combustion some chemistry going on also seeing some smoke being produced and we'll talk about that later and so this sort of very early kind of firework if you like but it certainly wasn't an explosion we think of gunpowder as an explosive we think of gunpowder is making a bang and that certainly didn't make a bang not only that we actually had to heat it we had to heat it for 10 or 20 seconds to get a reaction going and that's not very good for making fireworks and the problem of course is that that contains honey and honey contains water and usually water is on a very good thing to put into your pyrotechnic mixtures so the real breakthrough came when somebody discovered that you could replace the honey with a different ingredient in particular with charcoal and so this leads us to the three ingredients of gunpowder we can just have a look at these here so we have the the saltpeter or the potassium nitrate and that's just this white crystalline powder we have charcoal which is this black powder you're familiar with charcoal it's the sort of stuff you get on bits of partly burned wood the black stuff and this lovely yellow powder this is sulfur okay so those are the three ingredients then of gunpowder and let's just have a little look at those ingredients in turn will start with potassium nitrate snow VAT aciem nitrate is a chemical compound of potassium and nitrogen and oxygen we can think of it as a very concentrated form of oxygen so when a piece of paper burns in the air it's undergoing a chemical reaction with the oxygen which is about one-fifth of the air but the air is very thin and potassium nitrate contains a very concentrated form of oxygen in fact we call it an oxidizer and so what I've done is to dissolve some potassium nitrate in water and I've painted a shape on this piece of paper I've allowed it to dry and if I just like this right we can see the shape beginning to emerge there what's happening is that where I've painted the potassium nitrate it's reacting with the paper it's undergoing combustion but the oxygen is coming not from the air but from the decomposition of the potassium nitrate so the potassium nitrate is acting as a source of oxygen now can anybody see what shape what is it Kirt right curse the letter K why have I chosen the letter K who knows so definitely to do with chemistry yes any other thoughts laughter of fireworks a great answer there's a there's a there's a more sort of chemical answer brilliant it's the symbol for potassium K is the symbol for potassium and has potassium nitrate so potassium nitrate then is the source of oxygen for the combustion processes that go on in fireworks so that's potassium potassium nitrate the second ingredient is charcoal now charcoal is made by taking wood and heating it in the absence of air and it decomposes into a sort of black material and that black material chemically is mostly carbon so let's just have a little look at carbon this is a model of a very interesting useful kind of carbon it's diamund so each of these black spheres represents an atom of carbon and in diamond they're arranged in this very rigid lattice structure so diamond is a very hard material here's another kind of diamond this is called sorry another kind of carbon this is called carbon 60 or a buckyball it has 60 carbon atoms in a sphere with these lovely Pentagon's and hexagons just like a football and suddenly won the Nobel Prize for discovering that but the most common and the most I first mundane form of carbon if called graphite so the LED the so called lead in your pencil it's not made of lead at all it's made of graphite and in graphite the atoms are arranged in these sheets and the sheets can kind of slide over each other so it's a very soft material very very different from from diamond-- and charcoal right down at the atomic level it's really mostly graphite but there's something else that's rather special about charcoal and we'll come back to that in a moment and the third ingredient is sulfur that lovely yellow powder and sulfur has the property that it has quite a low melting point and sulfur is there to help the combustion so the Gunpowder burns much better if we include some sulfur okay so we know the ingredients of gunpowder then we've got potassium nitrate charcoal and sulfur but before we can make some gunpowder we need to know the proportions what proportions should we mix these ingredients together in order to make really good gunpowder well to understand that what we're going to do is first of all look at some much simpler chemistry we would simply look at the reaction between hydrogen and oxygen so does anybody know what happens when hydrogen reacts with oxygen any yes excellent so in hydrogen reacts with oxygen it forms water something else is also released when that happens anybody anybody oh no any other guesses not quite something else something else happens when we combine hydrogen and oxygen I tell you what we're going to do it and we'll find out okay we'll find out what happens when we combine hydrogen with oxygen but let's just have a look first of all what happens to those molecules so on the left here is a molecule of hydrogen it has two hydrogen atoms joined together bound together in the molecule another molecule of hydrogen at the right and in the middle is a molecule of oxygen that has two atoms of oxygen now if we give a bit of energy to this we can break those bonds that bit of energy is called the activation energy so we provide a bit of energy it breaks the bonds and then the atoms can rearrange themselves and they can form new bonds and as somebody said earlier they form water so water is h2o and when those new bonds form that release is a great deal of energy much more than that little bit of activation energy that we put in so the other thing that happens when hydrogen combines with oxygen it makes water and it releases energy and so here is the the way we'd write this is a chemical equation it says so H - H of the middle - underneath is that molecule of hydrogen with two atoms of hydrogen so two molecules of hydrogen combine with one molecule of oxygen to make two molecules of water so the prediction then is that the best reaction between hydrogen and oxygen will happen when we have twice as much hydrogen as oxygen so what we're going to do is have an experiment to test that theory so Chris has been filling some balloons with various mixtures of hydrogen and oxygen what we're going to do is set fire to these and then we'll compare them to see how fast they react and you'll notice that I've got a pair of ear defenders and that's because my prediction is they'll react orally rather well and we'll get quite a loud bang from some of these balloons now what I want you to do is to listen to see how loud the explosions are okay the loud of the bang the better the reaction and our prediction is that the loudest bang the best reaction will happen when we have twice as much hydrogen as oxygen now I'm going to wear a defenders you might like either to put your fingers in your ears or just cover your ears with your palms so this first balloon is mostly oxygen just a little bit of hydrogen mostly oxygen so let's see what this sounds like okay so here we go you ready okay that wasn't very loud I think you'll agree okay let's try a balloon now which is mostly hydrogen mostly hydrogen just a little bit of oxygen so we'll see if this is any louder than the last one thank you Chris you ready okay yes all right a little bit better all right thank you Chris so that was mostly hydrogen in just a little bit of oxygen and what we're going to do now is to try to parts of hydrogen and one part of oxygen so the prediction is that this should be the loudest bang so we'll see if the prediction is correct okay so here we go okay so I think it's pretty clear then that the the two-to-one mixture it was definitely the loudest of those three so what that says is that to get the fastest reaction we should have the the the fuel in that case the hydrogen and the oxygen in just the right balance there's just enough fuel to match up with the oxygen in that case it was two parts hydrogen one part oxygen okay what about a gunpowder or black powder then so in the in the world of pyrotechnics we often call gunpowder of black powder so if I say black powder it means exactly the same thing as gunpowder so for gunpowder or black powder then what should the proportions be well the long before people understood the chemistry they had worked out the best proportions just by trial and error and it's pretty pretty well accepted that the best proportions for for making really good gunpowder is to have 75 percent of potassium nitrate 15 percent of charcoal and 10 percent of sulfur and then they will react in the fastest possible way and here is the chemistry of the reaction of gunpowder and I'll test you all on this later the the one thing we can apart from the fact this is very complicated much more complicated than the hydrogen and oxygen if we look at this we can also see that certain things are formed things like potassium carbonate calcium sulfate ammonium carbonate these are substances which are solids at room temperature and that means this reaction produces smoke so all the things will say is that we when we burn gunpowder we get smoke and that's why we've got this special safety screen that also sucks away the smoke for us so we know the ingredients of gunpowder we know their proportions so now we're ready to make some gunpowder so what we'll do is we'll make 10 grams of of gunpowder so to make 10 grams of gunpowder we need to start with seven and a half grams of potassium nitrate so there's seven and a half grams of potassium nitrate that's 75% we now need 15% or one and a half grams of charcoal that's the the black powder tip that in and now we need last of all 10% sulfur so one gram of sulfur so those are the ingredients of gunpowder I've put them in a beaker and I'm now going to stir these around and get them thoroughly mixed and at this point we're hoping aren't we that this is really really good gunpowder so I've used really top-quality potassium nitrate really good quality sulfur and the charcoal is willow charcoal and that's reckoned to be the best for making gunpowder so we've got the exactly the right ingredients we've mix them in exactly the right proportions I'm stirring the round really thoroughly so this really ought to be really really top quality gunpowder so we'll we'll see how well this this burns now I could just sort of set fire to this and see what happens but I want to be a little bit more scientific I want to try to measure how good this gunpowder is so to do that I'm going to spread the powder in this track and then we're going to do is to light the powder at one end and watch the powder burn along the track now we're after a fast reaction so the faster the powder burns along the track the better so what I want you to do when I like this once you see the Gunpowder start to burn I want you to start counting just in your heads your cells start counting see how many seconds it takes for the Gunpowder to burn from one end of the track to the other remember the faster the better now at the end there we've got a little fuse so I'm going to light the fuse the fuse will burn for a few seconds and then you'll see the Gunpowder light and just count yourselves how long it takes to burn along the track I think we have the lights down for this apiece okay so there's the fuse burning there's the gunpowder burning okay I made that about sort of twelve or thirteen seconds something like that so you know maybe if kind of disappointing this is supposed to be an explosive and it took about 13 seconds to get along the track there so something is wrong there's something wrong with our gunpowder we haven't made really good gunpowder even though we thought we were going to and what's the problem well this could be one of the problems imagine looking at that down a donor microscope what sort of thing would we see well we'd see something like this those fine particles are really on it on an atomic scale they're very large we see a huge white Boulder of potassium nitrate and in some distance away they're a huge sort of yellow Boulder of sulfur and these chemicals have to come into contact with it with each other in order to react and there are very few points of contact so it could be that the size of the particles has something to do with the rate of reaction if we could somehow make these particles very much smaller and get them really mixed up well together perhaps this would burn a lot faster okay so that's the theory we should now test the theory and we'll test the theory using this yellow powder it's called Lycopodium so it's a natural material and it contains a lot of fats and it should be quite flammable so I'm going to do is to put a little bit of this like a podium on the dish on the plate here and well see if we can set fire to this okay so I'm just going to try and set fire to the powder and sort of burn you were getting a bit of a flame I've got a little flame there it's Jeff burning don't know if you can see there's a tiny little flame not very spectacular I think you'll agree the reason of course is that the burning of the Lycopodium is a chemical reaction between the peridot and the oxygen from the air and that reaction can only occur where the the air comes into contact with the peridot and that's only at the surface so a theory is that if we can get a much better mixing between the fuel and the oxygen we'll get a much faster combustion so we're going to test that now by burning some all Lycopodium but this time we're first of all going to mix like a podium thoroughly with the air and then we're going to light it and we'll see if it burns a bit faster so in this tube is some like a podium and I'm going to puff some air through the tube and we'll see if we got a slightly faster reaction this time okay thank you okay so that suggests then that to make your gunpowder burn a bit faster what we need to do is to mix the ingredients together rather more thoroughly so let me show you then how commercial gunpowder is made and this is a picture of a piece of equipment from a catalogue from about 1900 from a company that manufactured a gunpowder making machines and this is a key step in the manufacture of gunpowder this is called an incorporating machine and I want you to get some idea of the scale of this those rollers are six-and-a-half feet in diameter they each weigh ten tons and this is used to grind the powder for about three hours now that's going to do a much more thorough job than I did with the beaker and the stick in about half a minute in fact not only does it make the particles very small and mix them together very thoroughly it does something else as well that's rather special and this comes back to the the nature of charcoal it turns out that if you make gunpowder using chemically very pure carbon like graphite powder it doesn't work very well there's something special about charcoal which is this natural material made from wood and this is a picture of some charcoal taken with an electron microscope and the top left image is about a millimeter from one side to the other and then we zoom in and zoom in and what we see is this sponge-like structure right this is a natural structure because the natural material and the process of incorporation the grinding for hours and hours with those massive rollers is actually forcing potassium nitrate and sulfur into all these little nooks and crannies in that sponge-like structure and that's one of the reasons why gunpowder burns as fast as it does so that's how commercial gunpowder is made so let's have a look at some commercial gunpowder and see if it burns a little bit faster than the than the gunpowder we made earlier so again we're going to measure this using the track so again I have 10 grams of gunpowder since the same quantity as last time but now this is a very top quality commercial gunpowder so this is the best gunpowder that you can get and again I'm going to sprinkle it along the track over similar length to last time and again we've got a little fuse to light this when the fuse burns to the end you'll see the Gunpowder burn and again I want you to count yourselves how long it takes to burn from one end to the other and I think last time the the handmade gunpowder took about something like 13 seconds so what we'll see if this is any faster we're hoping this will go a little bit faster than that 13 seconds that we had before so again we'll pop the lights down okay so there's the fuse and start counting once you see the powder burning okay so just a little bit faster okay so that was a lot better that was maybe half a second or so so there's at least 10 times faster or maybe 20 times faster than my handmade gunpowder so much much better and that's as good as gunpowder gets but it's still not an explosion it still took a half a second or so to burn in an explosion we have a very rapid release of energy in a tiny fraction of a second so how can we get our gunpowder to burn even faster than in that track well to understand how to do that what we're going to do is have a look again at that simpler chemistry the reaction between hydrogen and oxygen so this is what we might see if we imagine being able to look down an incredibly powerful microscope at those molecules the white molecules here are molecules of hydrogen remember two hydrogen atoms stuck together to make a hydrogen molecule and the red ones are oxygen molecules two oxygen atoms stuck together to make a molecule of oxygen now you see they're all moving around now that's heat heat is just the motion of molecules and atoms all the atoms and molecules in the world are all moving around in solids they sort of jiggle around in gases they're moving around like this and the higher the temperature the faster they're moving and from time to time they bump into each other and if they bump into each other with enough energy remember that activation energy we talked about before enough energy to break the bonds we get a reaction and that's shown here by little explosion so every now and again you'll see like there you'll see a little yellow explosion okay that's when they bump into each other with enough energy to reach that activation energy so what we want to do is to speed up this reaction how can we make this reaction go faster any ideas on how to make this go faster yes he's it brilliant right so if we heat it up what will happen well as we raise the temperature as I'm doing here we increase the speed and now a lot more of those collisions have enough energy to reach that activation energy and so we get a higher rate of reaction so heating it out should increase the rate of the chemical reaction there's something else we could do as well instead of heating it up we could increase the density so if we increase the density each hydrogen atom sees a lot more oxygen atoms and vice versa and so again we get an increase in reaction rate so our second prediction is increasing the density or the concentration should also speed up reaction and of course we could do both we could increase the density and increase the temperature and then we should get a really good reaction okay so those are our two predictions then it's what we need to do now is to test out the predictions so our first prediction is that a chemical reaction should go faster when it's hotter so to test that this what I would like our two volunteers or you're very keen you come on down and just wait there at the bottom it says let's have somebody from this side who would like to volunteer let's have would you like someone down yes go on down there's a big hand for all volunteers come this way we'd like to stand just about there and if you just want to pop on a pair of safety goggles that's good if our other volunteer would like to come and stand if you'd like to come and stand over here with a pair of safety goggles for you as well and what's your name Corinna there we go there pop those on come and stand just there what's your name then alright then you hold this this is a light stick and you have a light stick as well have you used light sticks before we use the lights it before don't worry I'll show you how to use it right well we've got a two identical light sticks and we've got two beakers of water now this beaker of water has got ice in it so it's very cold and this beaker of water is pretty hot so these are different temperatures what we're going to do is we're going to start the light sticks and we're going to put them in the beakers change their temperatures and we'll see what effect that has on the rates of reaction now the reaction the chemical reaction and light stick of course is producing light so the faster the reaction the more light it should produce so let's test this out or don't you do is take your light stick like this we go like this right in the middle like that snap it okay I'll go three two one give it a big card snap three two one snap really hard snap oh go for it yes well done and you big hard because now I'm sure you can do it so give a little bit of a little bit of a help well done okay give it a good shake and pop it in just give yours a little bit of a shake that's it pop it in the water okay so we're going to leave that there for a few minutes while while they adjust to their temperatures and we'll we'll see then whether temperature has an effect on reaction rate so you two just wait there a moment and our other prediction was that increasing the density should increase the reaction rate and for this I like to have three volunteers please and you were fast there yes you come on down you're very keen and let's have somebody from over here yes why don't you come on down so the handful volunteers please yes if you'd like to come and stand there please you just wait there a second and we've got some safety goggles so if you pop those on if you like to come and stand there popping a pair of safety goggles we need a pair for you don't we here we go those on right what's your name well your will what's your name Agnes and you are Matthew right so in these cylinders at the front we've got solutions of sodium hypochlorite and sodium hypochlorite is the active ingredient in bleach and this is really like a Solutions of bleach and in this cylinder we've got 400 milliliters of sodium hypochlorite solution in this cylinder we've got 200 milliliters of sodium hypochlorite plus 200 milliliters of water so the concentration in this cylinder is just half the concentration in this cylinder and finally in the third cylinder we've got 100 milliliters of sodium hypochlorite plus 300 milliliters of water so this is half again so as you go from here to here the concentration or the density of bleach goes down by two and going from here to here it goes down by another factor of two each of our three volunteers has a beaker containing water with blue food coloring in and these three beakers are identical so in a moment we're going to pour the the water containing the blue food coloring into the cylinders and what will happen is the sodium hypochlorite should bleach the blue food coloring and we'll look at the rate at which that happens so I don't you to do is to take take a beaker each okay if you pick up your beaker and then more or less the same time if you come and stand just about there around the front now can you reach over the top and you reach over the top there and pour it in do you think if you stand a bit closer okay everybody ready so you're going to pour yours into there okay and you manage good okay off you go then start pouring that's it pour it all in super job all right pop the beakers down now what's happening now is that the sodium hypochlorite should be bleaching the time you should start to see some changes in the colors of these three solutions and I think already this one at the end here has pretty much been bleached that's gone pretty clear the one in the middle is losing its color it's not quite as blue as it was that's getting bleached quite nicely and the one at the end is starting to lose its colour but what you can see is the bleaching is happening fastest in the solution of the highest concentration and is happening slowest in the solution of lowest concentration so again that tells us that if we increase the density or the concentration that increases the rate of chemical reaction okay let's go and have a look at our light stick so would you two like to fetch your life sticks light sticks out and just hold them up and we'll have a look just dim the lights a little bit hold up nice and high I think you can see the one that's been in the hot water is glowing very brightly the one that's in the cold water is not glowing quite so brightly just fill them up together like that okay good okay pop them back in the beakers and if you take the glasses off and go back to your seats and I'll have a big hand for all volunteers okay so we've got two predictions there which were confirmed by experiments which tell us that a reaction will go faster if we raise the temperature and it will go faster if we increase the density so let's apply that those scientific principles to the combustion of gunpowder and see if we can make our gunpowder burn a bit faster so let's think about what was happening when the gunpowder was burning in that long track as the Gunpowder burns it releases heat it releases energy it produces hot gases and those gases are simply expanding they're just pushing back against the atmosphere they can expand easily and so as they expand the temperature can drop the temperature can fall the density can fall and so the reaction rate remains relatively slow what we can do instead is to take some gunpowder and confine it in a very tight container now as the gunpowder reacts the energy is released these hot gases are produced but they can't escape and because there can find the density can't fall and the temperature can increase the reaction rate can increase and as the reaction rate increases it increases the rate of energy production which increases the temperature which increases the reaction rates even more and we call this thermal runaway and our prediction then is that if we can find the Gunpowder we should get this rapidly increasing reaction rate leading to a fast reaction so again we're going to try that out in order to do that I've actually reduced the quantity of gunpowder from 10 grams down to just one gram for reasons which will become apparent and this time the 1 gram of gunpowder is in this cardboard tube which is tightly bound up so it's in a very tight strong container and I'm going to put my ear defenders on for this and again you might wish to cover your ears at this point as we test out this theory so this is one gram of gunpowder tightly confined [Applause] so I think you'll agree then that confining the gunpowder greatly increase the rate of reaction and this is a very important concept in pyrotechnics and in fireworks that when we confine pyrotechnic compositions they burn very much faster and in some cases we'd get an explosion as we've seen so there we had a bang we had a bang from confined gunpowder now if we actually want to make bangs deliberately as often we do in fireworks using gunpowder and confining it is not very convenient we have to use quite a lot of gunpowder we have to have big strong containers it's not a very good way of producing bands solo gunpowder is very important in fireworks and you'll find out why at a moment we don't use gunpowder to make bangs we use a different kind of chemistry and the chemistry for this it goes back a long way in fact it goes back to the world of photography the the very early days of photography before we had digital cameras and so on when chemicals were used to do photography and also chemistry was used to produce lights because often you want to take a photograph in indoors or at nighttime and you need some artificial source of lights lives in the days before electric light and so people had to find an alternative and they discovered that if you burn certain metals they'll burn with a very bright light and so this is a piece of magnesium a little strip of magnesium I'm gonna set fire for this now and it'll burn with the oxygen in the air you'll see it produces a bright light in fact you may not wish to look directly at this because it really is extremely bright so this just takes a moment or two to get going and there it is burning away so there's the magnesium burning with the oxygen in the air and that white smoke is magnesium oxide that's being produced okay and you see that very bright light and so that could be used for taking photographs now well the light is very bright that was burning quite slowly and usually when we want to take a photograph we want the photograph to be very fast because we want to stop any sort of motion we don't any blurring so we want to get that light released much more quickly and so photographers devotes on or flash powder and flash powder again is based on burning of metals but now we use that idea of an oxidizer that is a solid material containing concentrated oxygen that can burn with the metals and so I'm going to show you now is quite an old recipe for flash powder that was used in the early days of photography and it's based on powdered magnesium you just saw magnesium burning and also another metal aluminium so an aluminium is in the form of a very fine powder it can also burn it also produces a very bright light and this time we're using a more powerful oxidizer it's called potassium perchlorate but again it's a compound that contains a lot of oxygen and so the oxygen from the potassium perchlorate will react with the magnesium and the aluminium and what we'll get is a bright flash now again this is extremely bright I recommend that you do not look directly at this instead just look at your friends or look at some other part of the auditorium we'll put the lights down and we'll see if we can get a nice flash from this okay so when the lights are down here we go okay [Applause] okay so that was flash as used for photography in the early days of photography now the chemistry of that was rather different from the the chemistry you were used with the balloons with the balloons we found that the fastest reaction happened when we had just enough hydrogen to balance the amount of oxygen in the flash powder there there was a huge excess of fuel there was lots and lots of magnesium lots and lots of aluminium and so as it burned the oxidizer was used up very quickly and most of that magnesium aluminium was thrown up into the air and actually reacted with the oxygen in the air to produce light because the goal of the photographic flash powder is to produce light so Brisa flash well we can use that same kind of chemistry to produce our bangs for fireworks and that's done by adjusting the proportions to make them balanced so we have just the right amount of the metals to combine with the oxidizer and so this leads to I think really quite a dramatic demonstration of the effect of confinement now when we confined the gram of gunpowder we got a nice bang but you might have been thinking well all that was really happening is the pressure is building up slowly and building up and building up and then the container burst did the reaction rate really increase as a result of confinement or was it just a container bursting that made the bang well this demonstration I think will convince you that the effect of confinement really is to increase reaction rate so what I've got is one gram of pyrotechnic flash powder and we're going to burn this on an open surface and then in a moment we're going to burn exactly the same quantity of exactly the same powder with a little bit of confinement and we'll see the difference so we'll leave the lights up for this but again this is quite bright so you may prefer not to look at it directly so this is one gram of flash powder just burning on an open surface okay so pretty similar to what we saw before we had a bright flash a little puff of smoke not very much noise what we're going to do now is take exactly the same quantity of exactly the kind of flash powder but this time we've put the powder in the bottom of a small cardboard tube so this cue is closed off at the bottom but it's open at the top so this is very very gentle form of confinement what we're going to do is to just lay a business card across the top so I'm gonna if somebody in the audience has a business card that I could I'll say borrow and you may not get it back we'll see anybody have a business card somebody must have a business card somebody must have a business card yes lovely thank you very much okay so what I'm gonna do is to take this business card and just rest it gently on the top of that tube so this is not a sealed container this container can't withstand any pressure it's just the business card laid laid on the top so there we go so the same quantity of flash powder at this time with some very gentle confinement but this time I'm gonna put my ear defenders on and I would recommend very much that you cover your ears for this so this is how we make the bangs in fireworks okay here we go [Music] [Applause] okay so I think that convinces you that a little bit of confinement really does accelerate the reaction and this is that's pretty much all that's left of that cardboard tube and afraid your business card didn't really do very well but I'll let you have that back as a souvenir there we go all right so that's how we produce the bangs in in our fireworks now we want to produce lots of other effects in fireworks of course and one of the most important effects in fireworks is color far as to be very boring if we didn't have colors so how are we going to make colored fireworks well the secret again comes from chemistry and it comes from adding compounds to our pyrotechnic mixture compounds which are made from specific metals so again metals play an important role and depending upon the particular metal we use so we get a different color so to get red we use strontium deer Orange we use calcium for yellow we use sodium for green we use barium for blue we use copper and to get a white flame what we've seen how to do that we can use magnesium or aluminium and there's another metal called titanium which is also very good in fact the ice fountain that you saw at the beginning had little particles of titanium and that gave those lovely branching white sparks so let's look at an example of this we'll look at the color red and we'll see how to make a a simple red flare so when it combined three ingredients the first is that oxidize a potassium perchlorate so the 8ee at the end the AIDS that tells you this has got some oxygen in it so this is our source of oxygen oxidizer the fuel is something called akroy these resin now this is sort of sticky gum that oozes out of the bark of trees in Australia and it sort of gathered up and dried and ground up into a powder and it turns out it makes a very good fuel for pyrotechnics and then the third ingredient is where our color comes from there's a strontium carbonate there strontium carbonate is just a white powder it's not the color of the powder it's actually the strontium the strontium atoms we'll get into the flame and will emit this red light so let's just have a little look at a simple flare then a red flare made using that chemistry so a lovely red color caused by strontium and anytime that you see a firework producing a red color you can think to yourselves yes that's produced by strontium okay so we've seen how to make bangs we've seen how to produce colors what we really want to do to make fireworks so let's see how fireworks are made now there are hundreds or maybe thousands of different kinds of fireworks we don't have time to look at them all so we're just going to look at one kind of firework and it's the kind that's the most important for big professional displays and this firework is called a shell this is an example of a shell and the shell goes into a tube the tube is called a mortar so the shell is lowered into the mortar and when we're ready to 2phi the firework a gunpowder charge at the base of the shell is ignited and this then functions like a cannon it shoots the shell rather like a cannonball straight into the air and at the highest point the shell explodes I'm gonna get some lovely effect and to see what's going on let's just have a look at a cross-section through a shell so there's the cross-section of our shell and the first thing we have is a fuse so the fuse is used to conduct the flame down into the shell and the flame is is sent down to the charge of gunpowder at the bottom that's called the lifting charge so that explodes and that pushes the shell up the mortar and into the sky and this is where we use gunpowder black powder rather than flash powder flash powder makes a very sharp bang but gunpowder when it explodes is a softer explosion it's more like a push and it's good for pushing the shell up the mortar and into the sky now that lifting charge also does something else as well it sets fire to a delay fuse that produces a delay of a predetermined number of seconds and when the shell is at the highest point that delay fuse then lights another charge of gunpowder in the middle of the shell that's called a bursting and that burst in charge blows the shell apart also inside the shell we have some kind of an effect and often the effect takes the form of stars so stars are like cylinders or spheres of compress pyrotechnic mixture and they're ignited by the bursting charge and they might for example produce color so they might be a bit like our red flare you'll see the shell explode with all these points of red light spreading out from the shell okay so we look there at a 4-inch shell but if you really want to have some fun then what you need is one of these and this is called an eighth inch this is an eighth inch mortar and the eighth inch mortar is used to fire this which is an eighth inch shell now I don't know if anybody would like to and go like to just just get hold of that just tell us if it is it heavy seriously heavy all right so this is this is quite a serious firework in fact when an eight-inch shell leaves the top of the mortar it's traveling at 200 miles an hour and it travels to a height of about a thousand feet and when it explodes the diameter of the explosion is about 800 feet and so this is a picture of a ground test of an eight inch shell there's the explosion and if we zoom in on that small rectangle we see a little smudge that smudge is a 17 foot long one-ton truck just give you an idea of the scale okay so you're thinking these look alike a lot of fun where could I buy one of those and that leaves us on to a discussion of the categories of fireworks so there are four categories of fireworks category one indoor fireworks fireworks designed for use indoors category two garden fireworks if you buy a firework designed to you they used in the garden somewhere on the side of the firework will say category two and these can be viewed safely from a distance of at least 5 meters category three are called display fireworks this is if you're having a big display in a park somewhere these are larger fireworks you need to be further away you eat at least 20 five metres away from a category three firework but shells all shells of any size fall into category four in category four are professional fireworks these are not on sale to the general public and I think you'll understand the reason from that photograph there are no size restrictions but they're not available to the general public so all shells are our category four so shells are really the most important kind of firework for those big professional displays and that's why those displays can produce these tremendous effects in the sky that you really can't achieve with the kinds of fireworks that you can buy in the shops so the shell of course has to be ignited somehow and it's ignited by this fuse and fuses are very important in fireworks because we need to be able to set the firework off when we're some distance away so we either need a a delay or some way of lighting a firework at some distance away from us so let's just have a look at some different kinds of fuse so first of all we'll look at something called black match now black match is a very old fashioned very primitive kind of fuse it's no longer used today very much but it's just a piece of string that's been coated in a sort of paste of gunpowder and water and allowed to dry so when we like this the Gunpowder will burn along and that gives us a little bit of a delay we can light the fuse and then we can get some distance away and then the firework will go off so that's a very old-fashioned kind of fuse it's not a very good kind of fuse because Sparks can jump ahead and so you can't really tell how quickly it's going to burn that's not very good and if you've got two pieces near by each other the flame could jump from one to another and if it gets damp it will stop working completely so that's black match it's not really used today except in one application and that's in things like shells this leader on the shell or any situation in which we want to transmit a flame very rapidly so if we've got lots of fireworks not them all to go off at the same time we need a fast-burning fuse and the way we can make a fast-burning fuse is to use that idea of confinement I'm going to take the black match this time we put it inside a loose-fitting paper tube and now as the match burns the flame can't expand it pushed along the tube and that increases the speed of burning so what we've done here is to take a section of black match but instead of putting it inside a paper tube it's now inside a clear plastic tube so you can see what's happening so we'll put the lights down and I'm gonna light this section of black match that's outside the tube and as that black match burns along the flame in a moment we'll go inside the tube you should be able to see it speed up a little bit okay here it goes [Music] okay so that's quick batch that's how we get a flame from one place to another very rapidly now what about if we actually want to produce a delay what do we do if we want to light a fuse wait a while and then have the firework go off well black matches is not very good as we've seen in when I was a youngster fireworks used to have blue touch paper that's just like that fire writing we did earlier blue touch paper is paper that's been soaked in potassium nitrate solution allowed to dry and what are you lighted it smolders along again it's very primitive fuse it's not very good so these days if you look at a firework you'll see a little piece of green fuse and this is called visco and this is much better and this is really just a tube containing compressed gunpowder if I just get a torch I'll just like the section of this go and you can see it burning along so this is just a tube containing compressed gunpowder so there it is burning long it burns at a nice steady rate and it makes a very good safety fuse okay so that's visco but the problem with things like visco visco they put a sort of lacquer on to try to make it waterproof but it if it gets wet it's gonna stop working because it just contains gunpowder gunpowder doesn't work when it gets wet what are we going to do if we're in sort of the middle of November November the fifth and it's a sort of a gray rainy day and the fields all muddy and so on we need some sort of fuse that isn't going to be affected by damp we can use this is called plastic igniter cord or pick and it actually has a waterproof plastic coating with a pyrotechnic composition down the middle and this piece of pic has actually been sitting in this bum off of water for a few hours and we gotta love it and we'll see if it can continue to burn even under water so there it is burning away in the air and now the flames going into the water and hopefully it will continue to burn and you can just see it's burning away underwater and in a moment we hope the flame will appear yes okay that's plastic igniter cord but the trouble with all of these things they all involve fire and flame and they're all a little bit primitive in some way if we want a nice modern firework display we need a lot more control over the setting off of the fireworks and so we don't use a pick and visco and that sort of thing anymore we do everything electrically in fact you saw a couple of demonstrations there which I set off by pressing a button and that was done using an electrical igniter so we're gonna show you an electric igniter now it looks just like the head of an ordinary match just the little red bead but it has these wires coming out of it and when we apply a voltage to the wires it produces a little burst of flame so if you just look carefully at that match I'll show you what that looks like when we apply a voltage okay maybe not the most spectacular demo of the lecture but it shows you how we can set fire to something electrically and that's how modern professional firework displays are fired they're all fired electrically so here are some photographs from a professional fireworks display this is one that I helped set up a couple of years ago what you see here are those mortars and they're all clamped into racks to keep them very rigid and this firework display was done in a very long field with the audience sort of at right angles to the field we had ten stations all the way along the field and this was one or part of one of those stations you can see a lot of different mortars now each mortar has its own shell and each shell has its own electrical igniter and what that means is that the field is also full of bundles of cables thousands and thousands of cables running from all of these ignite ease and these igniter cables all go into these kind of junction boxes which are placed in the field at various intervals and big cables from the junction boxes then go to this central controller and the central controller is plugged into a laptop computer now it's actually the computer that's going to fire the display so on the right hand side there each row is a separate igniter a separate effect and the timing of those is timed down to a hundredth of a second so now we can do very very precise honest affects the the display could even be set to music we could be very precise effects which depend upon very accurate timing of individual fireworks so once you've done all the hard work of setting this up all you have to do is press the red button and then sort of put your feet up and watch the display so that's how modern professional firework displays are fired okay so we've seen in this lecture a number of effects we've seen how to make bangs we've seen how to make colors but there are hundreds of other effects that we can produce using interesting chemistry to help us produce an interesting fireworks display and I'm going to show you three of my favorite effects and the the first effect is called crackle and crackle is a widely used effect it's when you get an explosion followed by a bang a shell followed by thousands of little cracks or explosions all going off at the same sort of time and that's produced using a particular type of star called a crackle star and I've got one here if we can bring the camera and have a look at this so it's just a little cylinder of material it's rather like compress gunpowder but embedded within the black powder is there are some little brown lumps and those are the crackle composition when it one of those lumps is ignited it attracts with a sharp bang and so when the star bird burns we'll get a series of little cracks and now if you imagine a shell with hundreds of these in we get thousands of little explosions all happening in very quick succession so let's see what happens when we set fire to this so but that in the hood I think might have the lights down for this all right so this is a crackle star okay so that's the first effect that's crackle that's very widely used you'll you'll hear crackle it most fireworks display is these days the second effect that I rather like is a more subtle one it's called strobing and a strobe shell you had an explosion and then lots of twinkling light little lights going on and off in the sky it's rather pretty and this is a picture of the explosion of a strobe shell and it's a long exposure this is a photograph taken over a period of several seconds and if we just zoom in you can see these trails in the sky what's happening is that each of the stars is actually blinking on and off it's flashing on and off now instead of showing you a strobe star I'm going to show you a ground firework that uses the same chemistry and has the same strobing effect and so will will put the lights down for this we see once it gets going it's starting to flash now what's very interesting here is the flashing is not caused by little particles little lumps of flash material this is just a single pyrotechnic composition the flash is caused by some very complex chemistry during the dark phase some reaction products are being produced or when they build up to a certain level they themselves undergo reaction to release the flash the exact mechanism is not really understood so even though fireworks are perhaps a thousand years old there are still aspects of the chemistry of fireworks which are not fully understood which are the subject of research and experiments and people are still publishing papers on various theories of why this strobing effect should occur okay so that's strobing pyrotechnics droving [Applause] so the third effect that I wanted to talk about is again a very familiar defect it's the the whistle the pyrotechnic whistling sound so how is that done we have some sound effects good right oh I'll make her para technical in a minute I bet it's louder than all of your whistles put together we'll see so how does it work first of all so a whistle is made from a cardboard tube which is sealed off at the bottom and then on top of the the base we put some pyrotechnic composition and that composition burns from the top downwards so as it burns the length of tubing between the top of the composition and the open end of the tubing that length will increase as the composition burns down now that tubing acts a bit like an organ pipe or or a musical instrument the tubing produces a note at a particular pitch or sick Euler frequency which depends upon the length of that tubing so as that length increases the pitch of the notes will fall and I can illustrate that using a toy whistle so this whistle has a transparent wall and it has a piston inside that I can move up and down what I'll do is I'll play the whistle and as I do it I'll move the piston down we should hear the change in the note okay so you hear the note falling so the same thing happens with the pyrotechnic whistle so here's a pyrotechnic whistle this is actually fairly loud and it lasts for about five or six seconds what I want you to see is if you can notice the pitch falling as it burns [Music] okay so that's the pyrotechnic whistle now one things you notice about the whistle and about a lot of the other demonstrations that we have this smoke and that's because the chemistry produces reaction products which are solids at room temperature and that's the cause of the smoke and the smoke can be a good thing because sometimes it can reflect the colors and help you see the colored light more clearly but it also be a problem in a long display with lots of effects the smoke can build up in the sky and it can start to obscure the effects and of course if you want to set off pyrotechnics indoors then smoke can be a particular problem so is there a different kind of chemistry that we could use that would do away with that smoke well there is and there's a very interesting story about how this was discovered so this is a chemist Christian Friedrich Schon bine who was quite a famous chemist he made some important discoveries it was also a very enthusiastic chemist because well as doing experiments in his laboratory he liked to do experiments at home on the kitchen table now this is the middle of the 19th century and in those days the kitchen would have been operated by his wife that was his wife's domain and she didn't like him doing these experiments so she banned him from doing experiments in the kitchen but one day she was out of the house and he couldn't resist so he started to do some experiments with a mixture of concentrated nitric and sulfuric acids it's a very very corrosive mixture and unfortunately he spills some of this mixture on the kitchen table so very quickly he reached out for a cloth to wipe this up but unfortunately the cloth that he reached for was his wife's cotton apron so he wiped up the nitric and sulfuric acids with the cotton apron and then he hung the apron up to dry in front of the stove and when it dried out he got a little surprise and let me show you what that surprise looked like so this is a piece of cotton cloth it's been treated with concentrated nitric and sulfuric acids it's been washed and dried and the end result is unchanged this looks and feels just like an ordinary piece of cotton cloth but I'm going to set fire to it and I want you to watch closely what happens let's see what happens when I burn this piece of treated cotton cloth so it seems to have disappeared what happened well it hasn't disappeared of course what's happened of course is that all of the reaction products are invisible gases they're things like carbon dioxide and nitrogen gas and water vapor things that are present in the air already and they're invisible gases you can't see them what was going on there is that when we treated the cotton with the nitric and sulfuric acids a special process happened we call it nitration in which some nitrogen and oxygen has been incorporated into the actual molecules of the cloth now cloth cotton is made out of a material called cellulose and when we treat it with these acids it becomes nitrocellulose and so whereas gunpowder is a mixture of the fuel and the oxidizer in nitrocellulose the actual molecules are change the oxygen is built into the molecule so we sort of have perfect mixing between the fuel and the oxidizer so that's a little piece of nitrocellulose what I thought we'd do is to just show you a slightly larger quantity of nitrocellulose I think we'll put the lights down for this one so again this is the combustion of nitrocellulose but this time a slightly larger quantity [Music] [Applause] okay so that's nitrocellulose now all the nice things about fireworks is that we can combine effects we can take to add a toy ideas we combine them together to produce a new effect and so you could take the idea of nitrocellulose and we could combine it with the chemistry we learned about colors how to produce colors so let's just remind ourselves of how we produce colors in pyrotechnic chemistry at this time instead of using the chemical names of the metals I'll use their symbols so for red remember we use strontium for orange we use calcium for yellow we use sodium for green we use barium for blue we use copper and if we want to make a complete rainbow then if we're careful with the chemistry we can get a purple color by using a combination of strontium and copper and that will give off a mixture of red and blue light and we'll see that as purple so let's just see what happens then when we combine nitrocellulose with this color chemistry so again I think we'll put the lights down for this so this is a combination of nitrocellulose and color chemistry okay well thank you very much we are we're almost at the end of the lecture we probably need to go out with a little bit of a flourish don't we but Sir before we do just a couple of things first of all I just like you all to join me in saying a very big thank you to someone special someone who's put in a huge amount of efforts helping to set up this lecture and also to deliver the lecture and that's Chris brac stone just before we finish I just want to go back and have a look at those light sticks there remember our prediction was that a higher temperature would produce a higher rate of reaction and so we saw the hot light stick was glowing more brightly than the cold light stick now it was about half an hour ago so let's see what's happened in the meantime so this is the cold light stick now and this is the hot light stick and if I put them together you can see well I think they're almost just about the same brightness now maybe the the cold ones even perhaps slightly brighter than the hot one what's actually happening is that although the rate of chemical reaction should be higher when we have a higher temperature because the reaction rate has been higher it's been using up the materials faster and so the density of materials has been falling and so that fall in density has been lowering the reaction rate in the hot light stick and that's been compensating for the higher temperature and if we came back at about another half an hour we find that the cold light stick was still glowing dimly and the hot light sticker sort of burnt itself out okay so that's the effect of temperature on reaction rate so to end the lecture then I thought well it's a it's a bonfire night of November the fifth and it's traditional on bonfire night to sacrifice a model of Guy Fawkes so we'll have a special model of Guy Fawkes made by Chris and here he is his Guy Fawkes [Applause] now this guy fawkes is a little bit unusual because he's made entirely out of nitrocellulose now we need to we need to set fire to guy forks and so I thought what we do is we'd use a piece of that old-fashioned black match because gummy forks would have known all about black match so we'll use a piece of black match to set fire to guy fawkes so here's your last chance just to say goodbye to guy Forks and I'll say goodbye to you and thank you all for coming and have a very enjoyable bonfire night thank you very much you you

Loading