A little reminder for anyone who's forgotten the Season 4 finale of Breaking Bad:
So, how did Walter White build a wheelchair bomb detonated by a bell?
Have a closer look at the benchtop during the scene where Walter is making his bomb:
Those are instant cold packs - cold packs that are activated by a chemical reaction. They contain a small bag of material inside a larger bag of water. Once you break the inner bag, it mixes with the water and cools to near-freezing. This is because the dissolution of the substance in the bag is endothermic: that is, it requires energy to happen, and it takes that energy in the form of heat from the surrounding water. The amount of energy required for the volume of fluid in the bag ends up being about 23 degrees Celcius, so if you're starting at room temperature (around 25 degrees) they cool to just above freezing temperature. (As an aside - don’t store instant cold packs in the fridge! If you do that, they’ll get so cold upon activation that they can cause frostbite.)
So, what’s the stuff inside the bag? Why, it's ammonium nitrate!
Ammonium nitrate (often abbreviated to AN, with the chemical formula NH4NO3) is made by combining nitric acid and ammonia. It's commonly used as a fertiliser because of its nitrogen content. But when AN is mixed with fuel to form ammonium nitrate fuel oil (ANFO) and then detonated, the AN reacts with the hydrogen and carbon in the fuel to form nitrogen, carbon dioxide, water, and a very large explosion.
AN explosives have a legitimate use in the mining industry, as shown in the video above. They also make for popular improvised explosive devices - they were the explosive used in the Oklahoma City bombing. Occasionally AN explodes completely by accident, and Wikipedia has this excellent list of all the industrial disasters caused by AN in the last century.
If you’re wondering “why on Earth am I allowed to buy this and keep it in my first aid kit?”, it’s because it requires a very large amount of AN to make a useful explosive. (For this reason, many countries require a permit to buy AN fertiliser.) The Oklahoma City bombing involved 2,200 kg of AN, and by the time you've ripped open that many cold packs you might as well have just hijacked a fertiliser truck.
(Note: DON'T HIJACK A FERTILISER TRUCK.)
So, Walt has taken his ice packs and mixed the AN inside with fuel. Let’s have a look underneath the wheelchair:
That looks like a pipe bomb. When ANFO - or any other explosive - detonates, it liberates a huge amount of energy. The energy builds up in the pipe until it reaches a point where the pipe’s structure can’t contain the pressure, at which point the pipe breaks and the energy is released. I'm guessing that Hector's bell was a signal for Walt to use the unreliable remote-controlled detonation he was practising earlier in his kitchen.
Pictured above: pipe, mobile phone, and the world's worst remote control
It’s possible to build a pipe bomb (or grenade, or mine, or shell) that won’t explode when it’s supposed to, but still contains either the explosives or the products of the explosion at high pressure. This is why unexploded ordnance is such a problem - there are weapons hanging around from WWII that are still dangerous. This is one of the many reasons why you should NEVER BUILD A PIPE BOMB. Again: DON’T DO THIS. There are actually loads of explosives that can be synthesised from everyday materials (the mercury fulminate used in an earlier episode is another example) but that doesn’t change the fact that making homemade explosives is incredibly dangerous (and illegal). Don’t be this guy. Or this guy. Or this guy.
(Also, chances are that you’ll be getting your recipe from the internet, and if my Fight Club post is anything to go by, the internet is full of people who aren’t great at chemistry.)
