Breaking Bad is into its final few episodes, with fans already speculating how the story of a teacher-turned-drug-producing-criminal-mastermind will reach its denouement. But how many of the frequent science scenes reflect reality, asks chemist and physicist Dr Jonathan Hare.

Spoiler warning: Multiple plot details revealed below

"The chemistry must be respected." So says Walter White.

Walt is a brilliant research chemist who has to leave his work and take up a career teaching high school chemistry.

After discovering he has terminal cancer, he turns his skills to methamphetamine production in collaboration with former pupil Jesse Pinkman.

About the author Dr Jonathan Hare, of the Creative Science Centre at Sussex University, is a physicist and chemist. He is a fellow of the Royal Society of Chemistry. More of his Breaking Bad articles

With his background as a chemistry teacher, there are times when Walt instructs Jesse as if he is still back in the classroom. Jesse was a very poor science student at school, but while "cooking" meth with Walt, he starts to pick up and respect the chemistry that's so essential for a good product.

But how do Walt's "lessons" fare from a scientific point of view?

Can blue meth be pure meth?

The crystal meth Walt makes is understood to be unusually pure and also has a characteristic delicate blue colour. This is a useful device for the narrative but generally the colour of a crystal does not suggest a pure or impure chemical compound. Impurities in minerals such as quartz crystal can lead it to look pink (rose quartz) or violet (amethyst) but generally the colour is a result of the way the electrons in the substance absorb light and is not a specific indicator of purity.

Poisonous gas

In one scene, in their makeshift mobile meth lab out in the desert, Walt is being threatened by two gangsters. He improvises a method to gas them by throwing red phosphorus into hot water. Walt manages to run out, locking the gangsters in. He later explains to Jesse that this reaction produced poisonous phosphine gas. Red phosphorus can react with hydrogen to produce phosphine - but not with hot water. White phosphorus can react with sodium hydroxide (a chemical he would have had) but you can see he throws in a red powder, rather than a white substance. Nor is that what he describes to Jesse. I don't think this trick would work.

The dissolving bath

The gas only kills one of the gangsters. Walt summons up the courage to kill the other but now has the problem of getting rid of the body. In a gruesome scene, Jesse adds hydrofluoric acid (HF) to dissolve the body. It's a useful acid to have in any lab because of its unusual chemistry. It dissolves glass and so has to be stored in plastic (PTFE or Teflon) bottles.

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It is a powerful acid but it's the chemistry of HF that makes it dissolve glass (and body parts) and not its super "strength". Unfortunately, Jesse does not follow Walt's careful advice to use a specific type of plastic container (which would be HF-proof). He simply pours it into his bath. The remains of a partly dissolved body and bathtub crash through a partially dissolved ceiling.

The makeshift battery

In another desert scene, Walt and Jesse are "cooking" but when they need to drive home, they find the car battery is dead. Walt makes an improvised and very basic battery out of acid, different metals and wires and explains the chemistry to Jesse. If you put two different metals in an acid (or even electrolyte solutions such as sea water), the difference in chemical reactivity between the metals produces a voltage. It's a basic electrochemical cell. A number of these cells wired in series like a daisy chain is called a battery.

Making meth Many of the chemicals used to make methamphetamine are correctly named in the show

They include pseudoephedrine and methylamine

But the show's adviser organic chemist Prof Donna Nelson has explained how complete drug-making processes are avoided in the show

The end result is said to feel authentic without showing anyone how to actually make meth

Some bloggers have analysed the detailed chemistry of the meth-producing

Mythbusters recently had a one-off special on Breaking Bad's science

Anyone who had metal amalgam fillings as a child will recall the weird sensation of accidentally getting a piece of aluminium sweet wrapper in your mouth. The saliva was acting as the electrolyte solution. The metal filling and foil were acting as the two different metals, and we were being electrocuted by our very own mouth battery. Walt's explanation is fairly accurate but unfortunately such a simple battery would only provide a tiny amount of the power required to turn over an engine.

Fulminate of mercury

Jesse has been swindled and beaten up by psychopathic gangster Tuco. Walt confronts Tuco in his office, offering him more crystals but insisting on being paid immediately. Tuco starts to get nasty but Walt has a plan. The bag of meth crystals he has just given Tuco were in fact "fulminate of mercury". He throws a crystal on the ground which detonates, creating an almighty explosion. We see Walt walking victoriously from the smoking remains, clutching his bag of money. But could a small crystal really do so much damage?

Mercury fulminate is a very unstable and explosive compound that can only be safely made in very small crystals, but it is something that a high school teacher could make.

Crystals larger than a few millimetres in size are very tricky to handle. Snappits, the children's toy that you throw on the ground to create a small crack, contain small amounts of silver fulminate. Walt's crystals are rather large and a bag of them would not be stable enough to walk around with and handle as we see in the programme. They would, however, theoretically create a very powerful explosion. But the shockwave would no doubt have detonated the other crystals in the bag on Tuco's desk. If Walt and Tuco had miraculously survived the explosion, they would not have been able to hear much for a long while.

About Breaking Bad US TV series about a chemistry teacher who is diagnosed with lung cancer

He starts making methamphetamine, or crystal meth, to provide for his family

First screened in 2008, it is now in its fifth and final season Why did Breaking Bad use the poem Ozymandias?

Burning out the lock

Walt and Jesse burn out a lock in a heavy duty door to get access to an industrial chemical store. Walt describes the process they are using - the thermite reaction - to Jesse. Here you mix a metal oxide (for example iron oxide) with a reactive metal powder (such as aluminium) and it produces iron metal and aluminium oxide. The temperature of the reaction is extremely high and can be used to weld train tracks together or indeed burn out a lock. The science here is correct and the episode is made memorable as Jesse and Walt fumble and stumble as they try to carry the chemical barrels instead of rolling them.

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