Fireworks have many important components that come together to create the amazing effects we enjoy. To examine these components in detail we had an extended Science Club session this week.
Our first set of experiments was investigating how fireworks produce the colours that they do. Fire is actually a chemical reaction called combustion that gives off light and heat. So when we see burning wood, we’re actually seeing the light that is given off by hot gases reacting as they are boiled out of the wood. The colour of flames depends on the chemicals that are in the material being burnt. We made flammable mixtures of various salts. A salt is a simple chemical with a metal and non-metal bound together. Table salt (Sodium Chloride) is a very common salt, but is not the only kind of salt. While examining the coloured fire produced from different types of salt we were able to discover it was the metal component of the salt that determines the colour of the flame. For example, Sodium-based salts produce a strong yellow flame, Boron produced a bright green flame, Strontium was crimson red. Therefore, when we see wood burning yellow, we can conclude that it must contain Sodium-based salts (which is correct as most living tissue is rich in Sodium Chloride). Forensic scientists can use techniques based on this science to identify unknown chemicals. Pyrotechnicians use this science to mix chemicals to produce the brilliant colours in fireworks.
Combustion is a chemical reaction between a fuel and Oxygen (or more accurately, any oxidant) that produces heat and light. An explosive reaction is a sub-category of combustion reactions that occur fast enough to produce a sudden and large amount of pressure that expands outwards. We can hear this sudden change in pressure as sound. We explored the importance of Oxygen for explosive reactions in a series of reactions. Firstly we examined three balloons, one filled with Oxygen gas, one with Acetylene gas and the third with a mixture of both Oxygen and Acetylene. When we ignited the first balloon it didn’t react at all, there was no fuel and so the Oxygen merely floated away. The second balloon had plenty of fuel, but no Oxygen. However, when ignited it still reacted, as when the balloon popped the fuel could react with some of the Oxygen in the air. There wasn’t much Oxygen immediately available though and so not all of the fuel reacted. This ‘partial combustion’ produces a lot of sooty remains. The last balloon had a good mixture of Oxygen and fuel so when it was ignited all of the Acetylene reacted at once and produced a very loud explosion with no sooty remains. We demonstrated this concept again with Hydrogen gas in a Milo tin and blew the lid over the tree tops. Black powder (gun powder) and flash powder are two common explosive mixtures that rely on oxidizer-fuel reactions.
Fireworks though don’t use gaseous chemicals for their reactions. Instead they use various mixtures of powders. One powder is almost always a metal powder, this acts as the fuel for the reaction and provides the colours for the fireworks. The second powder typically contains Oxygen that is bound to other chemicals so that it stays a solid. There are many solid compounds that contain Oxygen, the ones used in fireworks are often Nitrates (the suffix ‘-ates’ tells us that this compound contains Oxygen) which is NO3. When they react the Oxygen leaves the Nitrogen and reacts with the fuel.
To demonstrate the explosive nature of some powders we produced Touch Powder. Touch powder is a solid compound of Nitrogen Tri-Iodide that can be triggered to explode by loud noises, small amounts of friction, heat from the air or even the soft touch of a feather. This compound is so easily detonated that even the Army won’t use it as the compound would explode if you tried to transport it anywhere – hence this explosive is classed as ‘sensitive’. This chemical reaction is actually quite different the typical reactions for fireworks and is NOT a combustion reaction. Instead, it is a decomposition:
2 NI3 (s) → N2 (g) + 3 I2 (g)
The Nitrogen Tri-Iodide literally blows apart producing very large amounts of gas. By rapidly producing large amounts of gas it produces a sudden increase in pressure and this is why we hear a loud explosion. Thus decomposition is a second reaction that can produce explosions (TNT, Nitroglycerin, C4 and RDX are common explosive materials that rely on decomposition reactions).
Here’s what we did: