Pyrotechnic Delight!

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:

Nitrate Flash and Pyrotechnics round #1

Redox reactions are the most important type of chemical reactions in pyrotechnics. Redox reactions are all about moving electrons that are fixed around one chemical to a different chemical – chemistry is all about understanding this ‘dance’ of electrons.

This week we examined one reaction in detail: The redox reaction is commonly called refered to as ‘Nitrate Flash’. Ammonium Nitrate was our ‘oxidiser’ it is strong enough to take electrons out of other chemicals. Zinc powder was the reducer, it gave up electrons to the oxidiser. The whole dance of electrons resulted in us producing Zinc Oxide, Nitrogen gas and water in the following net chemical equation:

Zn(s) + NH4NO3(s) → N2(g) + ZnO(s) + 2 H2O(g)

Notice the sub script (g) on the water molecule, that tells us that the reaction gave off enough heat to boil the water – so we actually made water in a gaseous state i.e. steam. Steam was the main gas you could see coming out of the reaction as Nitrogen gas is colourless (it makes up 78% of the air we breathe).

Notice though that in the ‘net’ chemical reaction we haven’t mentioned the water we added to start the reaction, or the Ammonium Chloride that was also included… they have a very special role called a ‘catalyst’ – look it up if you want to know more!

Here’s what we did:

To answer the question during the video, yes we did manage to set off the internal fire alarms :).

We managed to break the bin…

 

Exothermic Reactions

An exothermic reaction is any reaction that gives off energy – typically in the form of heat. The two reactions we examined today utilise the reactive properties of metal powders. In chemicals energy is stored in the bonds between atoms, when we re-arrange these bonds sometimes that energy can be released.

This week we showed how common rust (Iron Oxide) can react with Aluminum. When this reaction occurs the Oxygen atoms that were joined to the Iron, switch and join onto the Aluminum instead. As this reaction occurs a lot of heat energy is released, enough to melt the Iron. You can see clearly in this thermite reaction the molten Iron forming and the solid Aluminum oxide (rusted Aluminum) forming as a white powder. After allowing the Iron to cool and solidify we collected the solid remains, interestingly some of the Iron was very colourful and some took on a crystalline form – I wonder why that might be?

Here’s what we did:

Cold Explosions

An explosive is any material that contains a large amount of stored energy that can be released rapidly. This energy is stored as potential energy and when released transforms into a combination of heat, light, sound and kinetic energy.

This week in our first science club meeting we examined simple pressure explosives. When matter transforms into a gas the particles are trying to push away from each other, if you constrain these particles inside a container the pressure builds up. Increasing pressure is used to inflate tyres, balloons and to push water through pipes. If the pressure in a container is increased too much and the container breaks, just like popping a balloon, you can make a pressure explosion.

Here’s what we did:

Year 11 2013 Subject Selection

Our current Year 10 students are selecting subjects for their senior studies in 2013 – 2014. This is often a daunting task.
Below is a link to a University of Sydney resource that provides information concerning a range of science related careers, including plausible employers and salary ranges.
This is not meant as a specific endorsement of the University of Sydney, rather providing access to a helpful resource.

http://sydney.edu.au/vetscience/future_students/undergraduate/documents/courses_careers_guide_2013.pdf

Remember to chat with your Science teacher if you have any questions about Science subject selection.

Science Club 2012!

Applications are now open to join the 2012 science club at WCCS. This club is open to year 7 and 8 students who are interested in getting some more hands on experience with science. We meet together once a fortnight after school for an hour of investigations involving experiments and demonstrations that high school students typically don’t get to see.

All students in years 7 and 8 should receive an email with application forms and more details. Get your application into Mr Jackson today as members will be selected on Monday week 3!