This weekend will see bonfires and fireworks across the country, remembering and celebrating the gunpowder plot (or rather, the foiling there of) in 1605.
But what is the science of gunpowder, key to the original gunpowder plot and still used today as the propellant in fireworks?
Gunpowder as an explosive has been around for a really long time – well over 1000 years – and until the mid-19th century was the only known chemical explosive. It is a mixture of three different ingredients: saltpetre (potassium nitrate, KNO3), charcoal (broken down cellulose, approximately C7H4O), and sulphur. Each plays a very important role. If one was missing, or if the proportions were wrong, your gunpowder wouldn’t work. There are a few things we need to be clear on before we can explain what is happening though.
For fire, you need three things: oxygen (usually from the air), a fuel (a lot of the best ones contain carbon), and heat. Together, these form the three sides of the fire triangle, so called because if you take one side away you have neither triangle nor fire. This is the main reason why people are asked not to use their mobile phones at petrol stations – the air has relatively high quantities of fuel (petrol vapours) in it, as well as its usual oxygen content, and the heat from a mobile phone could be enough to trigger a fire.
Because the oxygen usually comes from the air, it is normally only the surface of the fuel that burns – the part of it in contact with the air. If you can get more oxygen in contact with your fuel, it will burn quicker, and there are multiple ways you can do this.
- Make your fuel be in really small particles. This increases the surface area a lot, so it is much easier to expose more of your fuel to oxygen in the air so it can react.
- Mix your fuel with something that will release oxygen. This means all of the fuel can have access to oxygen and so http://catthesciencetutor.co.uk/wp-admin/post.php?post=661&action=editcan burn, without needing to wait for contact with the air.
- Add something which will react at a fairly low temperature in a reaction which is exothermic (gives out heat, from the Greek exo meaning outside and therme meaning heat) to increase the heat of the mixture.
Gunpowder uses these three basic principles.
(I saw a really cool experiment once, in which a piece of wood was soaked in liquid oxygen for a short while, then set fire to. The oxygen had penetrated into the wood, and it burned so well that it could even carry on burning under water – it had its own supply of oxygen from the liquid oxygen which had soaked into it, and was already generating so much heat that the water couldn’t cool it enough to make it go out. It was awesome!)
To get an explosion, you need a lot of heat and a lot of gas produced very very quickly. The heat causes rapid expansion of the gas (discussed here and here), and this is the explosion. Explosions used to be quite common in flour mills and warehouses. Flour is a carbohydrate – it is has a lot of carbon in it – and this makes it an excellent fuel. This makes sense if you think about it: carbohydrates are the main source of energy in our diet, and things like bread, pasta, cakes, and biscuits, are all made of flour. In the same way that flour can be a fuel for us, it can also be a fuel for a fire. In flour mills and warehouses, a lot of the flour sits in the air – flour is, after all, a fine powder – mixed with the oxygen already present. It only took a tiny spark (for example, someone lighting a cigarette close to the door) for a huge explosion to happen.
So, back to gunpowder.
The main ingredient in gunpowder is saltpetre, which is potassium nitrate, KNO3. This accounts for about 75% (by weight) of gunpowder. Potassium nitrate decomposes (breaks down/apart, from Latin de- meaning opposite of [as a prefix], com meaning together, and ponere meaning to put or to place, so literally the opposite of putting together, i.e. taking apart) when it is heated, releasing oxygen. If your potassium nitrate is mixed in throughout your fuel, this means all your fuel has oxygen, and so can all burn, even if it isn’t all exposed to the air.
Next is charcoal, accounting for about 15% by weight of gunpowder. Charcoal is made from the slow charring of wood, and isn’t actually pure carbon. It is a broken down form of cellulose (what plants are made of), with the approximate formula C7H4O. When it combusts, it forms carbon dioxide and water, if there is enough oxygen.
2 C7H4O + 15 O2 -> 14 CO2 + 4 H2O
As you can see from the balanced equation, a lot of carbon dioxide is formed. Carbon dioxide is a gas, so the rapid combustion of charcoal could easily produce enough gas for an explosion.
The final component is sulphur, making up the final 10% by weight of gunpowder. Sulphur’s main role is to increase the temperature. Sulphur burns at a relatively low temperature in a highly exothermic reaction. This releases a lot of extra heat into the reaction, rapidly increasing the temperature.
Put these three together, and as you can probably see, what you get is a very fast combustion reaction, with a lot of gas produced very fast and at very high temperature – exactly what is needed for an explosion.
The full set of chemical reactions that happen is actually quite complex, but overall the total process can be summarised (a huge simplification, even calling the charcoal just carbon) as:
potassium nitrate + sulphur + carbon à potassium carbonate + potassium sulphate + carbon dioxide + nitrogen
10 KNO3 + 3 S + 8 C -> 2 K2CO3 + 3 K2SO4 + 6 CO2 + 5 N2
But why do we light bonfires and set off fireworks on November 5th? What was the gunpowder plot?
The full story of the gunpowder plot is actually a fairly complex one involving religion and politics, although it is also very interesting and well worth reading up on. In essence though, it revolves around a plot in the early 17th century to assassinate King James I. When James I first came to power, after Queen Elizabeth I died, he was far more sympathetic to the Catholics in the country than she had been. It didn’t take long for his tolerance to wane, however, especially after two small plots against him were uncovered in 1603. The Catholics – as you might imagine – were less than impressed. Most of them just got on with their lives, living mainly in secrecy, but some were not content to do this anymore.
There was a young man called Robert Catesby who, as well as being a devout Catholic, also had a bit of a knack for making people like him. Beginning in the first half of 1604, he began collecting a group of loyal allies who together planned to blow up Parliament. The original five, alongside Catesby, were Thomas Wintour, Jack Wright, Thomas Percy, and of course, Guy (or Guido) Fawkes. Working together, and using false names, they rented first a house near Parliament House, and then a cellar right underneath it into which they managed to stash 36 barrels of gunpowder.
Although originally from York, Guy Fawkes was recruited to the cause in Flanders. The plan, set for the Opening of Parliament on November 5th, was for him to light the fuse on the gunpowder and then escape through a window and away to mainland Europe. Unfortunately, someone managed to tip off Robert Cecil, Earl of Salisbury and first minister to James I, and he ordered a thorough search of Westminster. Guy Fawkes was caught and arrested in the cellar with his matches. Under torture, he eventually gave up the names of his co-conspirators. Those that weren’t shot during capture were taken to the Tower of London, where they were found guilty of high treason and sentenced to be hung, drawn, and quartered (a particularly cruel and unpleasant punishment).
In thanksgiving for the discovery of the plot and celebration of his survival, James I ordered the people of England to have a great bonfire on the night of November 5th. This tradition continues to this day, although over time we have replaced the effigies of the pope that were originally burnt on the bonfires with effigies of Guy Fawkes, and added fireworks displays to the mix.
Really, you could say it all goes back to when King Henry VIII broke from the Catholic church so he could divorce his first wife (this is a really interesting part of British history, and well worth reading a lot further on than what you are taught in school, as I have recently discovered).
Whatever you are doing this bonfire night, make sure you stay safe. Remember never to return to lit fireworks, as they can still go off. If you are having fun with sparklers, remember to wear gloves, and have a bucket of water or sand nearby to put dead sparklers into (believe me when I say burns from dead sparklers hurt an awful lot). Check bonfires for hedgehogs before lighting, and if you have pets make sure to keep them indoors. My cats will be spending the weekend indoors, with plenty of treats, and lots of extra hiding places for in case they get scared. And of course, have fun!
There are so many applications for this in the classroom:
- At KS3, discussions about combustion and the fire triangle, about hazard symbols, about general safety.
- At KS4, in addition to combustion, it can also be used for balancing equations (combustion of charcoal), rates of reaction (particle size leading to explosions), and exothermic reactions. You can also link it to NPK fertilisers.
- At KS5, you could even get your students to draw up Born-Haber cycles to show the energy changes taking place, and why sulphur needs to be present in the mixture.