Puking pumpkins: more hydrogen peroxide

It was Halloween yesterday and, unusually for the UK, it fell in school term time. As it turned out, I was teaching chemistry to a group of 12-13 year olds on that day which was too good an opportunity to miss.

Time for the puking pumpkin!

A side note: there’s loads of great chemistry here, and the pumpkin isn’t essential – you could easily do this same experiment during a less pumpkin-prolific month with something else. Puking watermelon, anyone?

Carve a large mouth, draw the eyes and nose with marker pen.

First things first, prepare your pumpkin! Choose a large one – you need room to put a conical flask inside and put the pumpkin’s “lid” securely back in place.

Carve the mouth in the any shape you like, but make it generous. Draw the eyes and nose (and any other decoration) in waterproof marker – unless you want your pumpkin to “puke” out of its nose and eyes as well!

Rest the pumpkin on something wipe-clean (it might leak from the bottom) and put a deep tray in front of it.

To make the “puke” you will need:

  • 35% hydrogen peroxide (corrosive)
  • a stock solution of KI, potassium iodide (low hazard)
  • washing up liquid

The puking pumpkin!

You can also add food colouring or dye, but be aware that the reaction can completely change or even destroy the colours you started with. If colour matters to you, test it first.


  1. Place about 50 ml (use more if it’s not so fresh) of the hydrogen peroxide into the conical flask, add a few drops of washing up liquid (and dye, if you’re using it).
  2. Add some KI solution and quickly put the pumpkin’s lid back in place.
  3. Enjoy the show!

Check out some video of all this here.

What’s happening? Hydrogen peroxide readily decomposes into oxygen and water, but at room temperature this reaction is slow. KI catalyses the reaction, i.e. speeds it up. (There are other catalysts you could also try if you want to experiment; potassium permanganate for example.) The washing up liquid traps the oxygen gas in foam to produce the “puke”.

The word and symbol equations are:

hydrogen peroxide –> water + oxygen
2H2O2 –> 2H2O + O2

There are several teaching points here:

  • Evidence for chemical change.
  • Compounds vs. elements.
  • Breaking the chemical bonds in a compound to form an element and another compound.
  • Balanced equations / conservation of mass.
  • The idea that when it comes to chemical processes, it’s not just whether a reaction happens that matters, but also how fast it happens…
  • … which of course leads to catalysis. A-level students can look at the relevant equations (see below).

Once the pumpkin has finished puking, demonstrate the test for oxygen gas.

Some health and safety points: the hydrogen peroxide is corrosive so avoid skin contact. Safety goggles are essential, gloves are a Good Idea(™). The reaction is exothermic and steam is produced. A heavy pumpkin lid will almost certainly stay in place but still, stand well back. 

But we’re not done, oh no! What you have at the end of this reaction is essentially a pumpkin full of oxygen gas. Time to crack out the splints and demonstrate/remind your students of the test for oxygen. It’s endlessly fun to put a glowing splint into the pumpkin’s mouth and watch it catch fire, and you’ll be able to do it several times.

And we’re still not done! Once the pumpkin has completely finished “puking”, open it up (carefully) and look inside. Check out that colour! Why is it bluish-black in there?

The inside of the pumpkin is blue-black: iodine is produced which complexes with starch.

It turns out that you also get some iodine produced, and there’s starch in pumpkins. It’s the classic, blue-black starch complex.

Finally, give the outside of the pumpkin a good wipe, take it home, carve out the eyes and nose and pop it outside for the trick or treaters – it’s completely safe to use.

Brace yourselves, more equations coming…

The KI catalyses the reaction because the iodide ions provide an alternative, lower-energy pathway for the decomposition reaction. The iodide reacts with the hydrogen peroxide to form hypoiodite ions (OI). These react with more hydrogen peroxide to form water, oxygen and more iodide ions – so the iodide is regenerated, and hence is acting as a catalyst.

H2O2 + I –> H2O + OI
H2O2 + OI –> H2O + O2 + I

The iodine I mentioned comes about because some of the iodide is oxidised to iodine by the oxygen. At this point we have both iodine and iodide ions – these combine to form triiodide, and this forms the familiar blue-black complex.

Phew. That’s enough tricky chemistry for one year. Enjoy your chocolate!

Trick or treat!




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Creepy combustion chemistry…

Halloween pumpkins

We’re burning!

So it’s October and I’m trying to think of a blog post topic. Hmm.

Well, the Nobel prize for Chemistry was announced earlier this month. But it went to some guys who’d developed a microscopy technique for seeing single molecules, specifically molecules involved in cell interactions. All very nice, but that’s biophysics isn’t it? Why did it get the Chemistry Nobel? (Biology famously doesn’t have it’s own Nobel prize, so maybe the committee just had to sneak it in somewhere?)

What else happens in October? Halloween of course! I love Halloween. But I’ve done pumpkins before. And I’ve written about sugar and chocolate, so that’s tick or treating more or less covered… hmmmm… candles, vampires, ghosts, the paranormal…


Is anyone else hot? (The Human Torch, art by Adi Granov)

Ahah! Inspiration! Spontaneous human combustion. What else?

If there’s any paranormal topic that touches on the edges of chemistry, it has to be this one. If you’ve never heard of it, spontaneous human combustion refers to the idea that humans can (or, er, maybe not – bear with me) suddenly and unexpectedly burst into flame and be reduced to ashes in a matter of moments. There is apparently no external source of this flame – it seems to come from nowhere.

It’s a creepy idea. I remember one of my chemistry professors at university, who had turned up to lecture us in his chemical-stained lab coat, with bushy white hair and too-dark eyebrows sticking out in all directions, pausing on his way out to tell us that we should think carefully when deciding whether chemical reactions would happen spontaneously or not under real world conditions. “After all,” he said cheerfully, “spontaneous human combustion has a negative Gibbs free energy, and you haven’t all burst into flame. Yet.”† And with that he gave us all an ever-so-slightly crazed grin and sauntered out of the room, leaving us looking around uneasily for traces of smoke.

Gibbs free energy change is a measure of how energy changes during a chemical reaction. It’s linked to couple of very important physical laws that pretty much describe how the world works. In short, do a bit of maths and, if you get a negative number, it tells you whether a chemical reaction can occur spontaneously but, and this was my lecturer’s point, not necessarily whether they actually will. It’s a subtle distinction, and one that’s easily forgotten. (Crucially, activation energy needs to be considered as well – if you want to know more about these terms, follow the links.)

Theatrically-minded chemistry lecturers aside for a moment, the idea that people, and things, might unexpectedly start burning is an old one. You can track it right back to the Old Testament, where there was quite a lot of suddenly bursting into flames going on, for example the angel of the Lord appearing to Moses in flames of fire from within a bush. Mind you, that was an angel rather than a human being, and they might be flame retardant of course. But you get the point. Fire has always been important to humans as a source of vital light and heat – indeed many would argue that the ability to control fire was a key turning point in human evolution – but at the same time it can be horrifyingly destructive. It’s hardly surprising that fire has found its way into so much of our history and mythology.

Let’s think about what the combustion part of ‘spontaneous human combustion’ means. The definition of combustion is a chemical reaction between a fuel and an oxidant (commonly oxygen) that gives out heat.


This applies to you, unless you’re a silicon-based lifeform.

There is more than one type of fuel, but the most familiar ones (coal, oil, gas, fats, wood and so on) are made of largely of carbon, hydrogen and oxygen. You are made up of the same elements (assuming you’re not some kind of alien life-form who’s stumbled over my blog – in which case, welcome). Of course you do have some other elements thrown in as well, notably nitrogen, calcium and phosphorous, but most of you is carbon, oxygen and hydrogen.

When you burn these kinds of fuels, this happens:

fuel + oxygen –> carbon dioxide + water (+ lots of energy)

Fuels give out lots of energy when they burn, and so, in theory, would you. Particularly if you have plenty of fat, because fats burn really nicely. After all, what were candles made of before paraffin wax? Largely tallow – which is a processed form of animal fat, usually from cows or sheep. And we all know that candles burn really well, that’s sort of the point.

The idea that you can burn a human isn’t surprising, after all people have been using fire to dispose of human remains for thousands of years. But spontaneous human combustion (SHC) is something different. In these cases, the person burns without any (obvious) source of ignition. At this point, you might be imagining a person suddenly bursting into flame right in front of shocked witnesses, but in truth reliable eyewitness accounts are pretty rare. Instead, what generally seems to happen is that a body is discovered, badly burnt but usually with very little damage to the surrounding furniture or even, sometimes, parts of the victim’s clothes. Observers of the scene then draw their own conclusions, some more rational than others, as to how the burning occurred.

Possibly one of the most famous cases like this is that of Henry Thomas. He was a 73 year-old man whose remains were discovered in the living room of his council house in South Wales in 1980. His entire body had been incinerated, leaving only his skull and a section of each leg. Bizarrely, sections of his socks and trousers were relatively unscathed, as was half of the chair he’d been sitting in, and most of the rest of the room except for some smoke damage.

Could ball lightning cause people to catch fire?

Could ball lightning cause people to catch fire?

There are various theories to explain this kind of gruesome discovery, from ball lightening, to flammable intestinal gases (namely methane, which is the same gas in your kitchen cooker), to acetone building up in the body. The most famous, and probably best accepted of the more scientific theories, is ‘the wick effect‘, popularised in a BBC QED documentary in 1998. This idea likens a clothed human body to a candle, but with the wick (clothes) on the outside. The person’s fat is the fuel source, and the theory goes that the person’s fat melts and burns slowly, like a candle, over a period of several hours. The burning is very localised, which explains the lack of damage to the surroundings. Police forensic officers decided that Henry Thomas’s death was most likely an example of the wick effect in action.

It is often the case that apparent SHC victims are elderly, have low mobility due to illness or obesity, and are smokers (in other words, had a source of ignition in the vicinity). The logic goes that they are somehow incapacitated, perhaps a heart attack or stroke, perhaps excessive alcohol consumption, drop their cigarette and burn slowly.

But there are cases where the burning seemed to be a lot more sudden, and even a few where someone else was on the scene. For example, the most recent (suspected) case of spontaneous human combustion in the UK was that of Jeannie Saffin, who died in 1982. She was a 61 year-old woman, but had the mental capacity of a child due to birth defects. She was sitting with her father in the kitchen of their family home. He wasn’t looking directly at her when she caught fire but, according to his account, something caught his eye and he turned to find her suddenly ablaze. He and his son-in-law put out the fire using water, and then called an ambulance. She eventually died in hospital despite treatment. The coroner refused to accept the suggestion of spontaneous human combustion saying there was “no such thing”, and recorded an open verdict.

Jeannie Saffin’s case clearly wasn’t an example of the wick effect; it happened too fast. As far as I can find out, no one has ever really been able to explain why she caught fire so suddenly. She was in a kitchen, and kitchens do typically contain sources of ignition. Perhaps something went unrecorded: matches, alcohol, use of a gas oven. But even if it did, why did she burn so quickly and so violently? Flammable clothing perhaps? The truth is, we will probably never know.

Not too much now.

Not too much now.

Searching around I found other examples, but in every ‘sudden’ case I found the victim was in close proximity to something flammable or something that could, conceivably, provide a source of ignition. Or both. In particular, there are several cases of apparent SHC happening in cars. Usually a fire crew has investigated and found no traces of petrol in the wrong place. But… this seems like too much of a coincidence to me. Petrol is extremely flammable – could a small trace be present, perhaps from filling up the tank? If something were to ignite it, it could cause other things to burn, like synthetic fibres or, an even more likely culprit, hair products like gel or hairspray. Hair coated in product can burn really quickly. It doesn’t entirely explain every detail, but then it’s hard to know what is and isn’t an accurate account in these cases.

The truth is that spontaneous (if that really is an appropriate adjective) human combustion remains a bit of a mystery.

Just be careful around those jack-o-lanterns.

† I may be misquoting, it was a long time ago, but I’m sure I’ll be forgiven if I am.

A scary Halloween tale…

Food additives, E-numbers – they’re scary aren’t they? Everyone knows they’re horrible, toxic things that make kids jump around, refuse to go to bed, go purple in the face and generally drive their parents around the bend (do kids really need chemical help with any of those things?)

pumpkin eating

Be careful what you eat…

It’s Halloween, a day when children traditionally stuff their faces fully of sugary, brightly-coloured sweeties. But never mind those, let’s give some thought to the humble pumpkin. Yes the orange things that grow in the ground. Did you know they’re stuffed full of additives too? Even ‘organic’ ones? They are, really! Here’s an ingredients list…

Water, carbohydrate, protein, E300, E375, E101, pyridoxine, thiamine, E470a, pteroyl-L-glutamic acid, E306, E160a, palmitic acid, linoleic acid, oleic acid, palmitic acid, stearic acid, purines, E621, potassium, calcium, phosphorous, magnesium, sodium, iron, zinc.

Scared yet?

Ok ok, don’t panic. Put down the baseball bat. It’s all right really, allow me to translate…

E300 is ascorbic acid, otherwise known as vitamin C. E375 is niacin (vitamin B3). E101 is riboflavin (vitamin B2). Pyridoxine is vitamin B6. Thiamine is vitamin B1 (seeing a pattern here? Pumpkin is good for B vitamins). E470a represents potassium salts of fatty acids. Pteroyl-L-glutamic acid is another name for folic acid. E306 is tocopherol, or vitamin E. E160a is beta-carotene (vitamin A).

Palmitic acid is the most common fatty acid found in animals and plants, and linoleic, oleic, palmitic and stearic acids are essential fatty acids particularly found in pumpkin seeds (very tasty roasted). Purines are some of the building blocks of DNA (the word purine comes from ‘pure urine’ because they were first synthesised from uric acid, isolated from kidney stones – ewwww). E621 is the dreaded monosodium glutamate. A lot of people fear this one, but actually it’s just a sodium salt of glutamic acid, which is another key amino acid. Totally natural. In fact, it’s one of the most abundant naturally occurring non-essential amino acids. (I will confess I’m improvising a wee bit here, but there’s no doubt that there’s glutamic acid in pumpkin – very abundant amino acid see – and there’s also sodium, so chances are there’s some monosodium glutamate knocking around in there.)

Potassium, calcium, phosphorous, magnesium, sodium, iron and zinc are all elements, and also important nutrients – you’ll find them all listed on the back of any good multivitamin and mineral supplement.

Food for thought? There’s a lot of nonsense spouted about additives and E numbers. For starters, that E? It means they’re regulated food additives that have been tested and approved for use with the European Union. They are, by definition, safe. Not only that, but quite a few of them are in your food to keep you safe by preventing harmful bacteria growing in it, for example. Lots and lots of them come from natural sources. Chemists like to extract and identify things, which is why lots of entirely natural substances have ended up with chemical names. An unfamiliar and complicated-sounding name doesn’t make something inherently dangerous.

On the other hand, there is something that’s been proven time and time again to cause numerous health issues from crashing energy levels to obesity, type two diabetes and dental problems. Yup. Sugar. E numbers have nothing on it.

I’m not suggesting anyone gives up sugar (where would be the fun in that, especially on Halloween?) but it’s always worth thinking about relative risk. If you’re going to accept a bit of sugar isn’t the end of the world, then give additives a break as well.

Right, I’m off to eat some Halloween biscuits – trick or treat!