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.

Method:

  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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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!