As this is the Friday before Halloween, here is a spooooooky explosion! I promise it won’t frighten yo–what is that behind you?! – Oh it’s just a chemist with a bunch of extremely flammable looking chemicals. Actually wait, that is pretty scary.

Yes: an exploding-self-carving jack-o-lantern! The best part is hearing the children scream [sound of evil laughter]. I did this experiment a few years ago during my teaching stint at the Vermont Commons School, and as I said then and will repeat now: don’t try this at home! Read on to learn the details…

The Chemistry

The key substance in this explosion is called calcium carbide, has the molecular formula of CaC2 and the structure looks like so:

Calcium Carbide

It looks jumbled up but what it means is that the molecule is actually made up of two distinct parts (note 1):

  1. The Acetylide Anion: [C≡C]2-
  2. The Calcium Cation: Ca2+

The calcium is boring and not spooky at all, you can safely ignore it. The interesting part is the acetylide anion. Carbon is most stable when it makes four bonds and as you can see from the picture above in the acetylide anion it makes only three bonds. That means that those carbons are in the market to make some bonds. So, let’s do what we did last week and drop it in some water to see what will happen…

Reaction of acetyline and water

Each carbon will rapidly pull a hydrogen off of a water molecule, and the products are some hydroxide anions (OH; super boring) and this other curious substance C2H2, also called acetylene gas. Hmm acetylene gas, that sounds familiar… oh yes, that’s because it is used as a fuel for welding. This is a good sign.

The Explosion

To recap: Drop some calcium carbide into water, and it will start to bubble out extremely flammable acetylene gas. Let’s use this on our pumpkin. In the video above, what is not shown is that the pumpkin is carved first and then the pieces are put back in. A cup of water is placed inside, the calcium carbide is dropped in the water and then the top is placed back on the pumpkin. The pumpkin rapidly fills up with the extremely explosive acetylene gas. Insert a lighter into a small hole in the back, and watch the spooooooky explosion.

But wait – the really spooky thing is that the teacher was actually dead the whole time, the cameraman is actually Dracula, and the children screaming is actually the sound produced when a portal to the undead realms are opened by this unholy ritual. Anyways, like I said don’t try this at home and have a happy Halloween. [Sound of bats flying]

– PV

Notes

  1. A molecule that is made up of a positively charged piece and a negatively charged piece is called an ionic molecule. The positively charged piece is called the cation and the negatively charged piece is called the anion. These molecules are held together by the electrostatic force between the cation and the anion. Ionic compounds tend to dissolve well in water, like table salt (sodium choloride, Na+ and Cl).

5 responses »

  1. krooks says:

    Yet another excellent start to a Friday! Can’t wait for next week.

  2. KMurph says:

    KABOOM!

  3. iampotassium says:

    lol but wait there’s more! The pumpkin is actually the head from an undead guy who unfortunately had a face that looks just like a jack o lantern!
    Also, Nice work, as usual. My favorite part is your description of Calcium in its spectator ion-ness… Poor Calcium…🙂

  4. Sweet site! Continue the informative posts.

  5. […] it’s different from the other two substances we looked at previously (alkali metals and acetylene gas). Most people are aware that flames need oxygen to continue to burn. The classic experiment is to […]

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