Self Carving Pumpkin Halloween Science Demonstration

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Self Carving Pumpkin

The self carving pumpkin demonstration is a classic Halloween chemistry presentation! You place the pieces of a carved jack-o-lantern back into the pumpkin and then use the reaction between calcium carbide and water to produce acetylene gas. Igniting the gas forces the pieces back out of the pumpkin, so it carves itself. Set up and performing the demonstration is simple. Just be sure you follow the safety precautions for igniting a flammable gas. If you want a version without flames, try the safe self-carving pumpkin project instead.

Self Carving Pumpkin Materials

You only need a few materials for the self carving pumpkin demonstration:

  • 2 grams calcium carbide (CaC2)
  • 50 milliliters water
  • Small plastic cup
  • Medium to large pumpkin (approximately 10-inch diameter)
  • Piezoelectric lighter with at least an 8-inch cord

Calcium carbide is available for sale online or from some sporting or home supply stores.

You want a medium or large pumpkin because it gives you more working space for setting up the demonstration. Substituting other produce, like a watermelon or cantaloupe, is fine. Artificial paper or plastic jack-o-lanterns are not appropriate for this project.

Safety Precautions

  • Perform the demonstration in a well-ventilated area.
  • Make certain there are no open flames or heat sources near the project.
  • Keep a fire extinguisher handy in case of accidents.
  • Have the audience 15 feet away from the pumpkin. Ideally, have them wear safety goggles. The combustion reaction and its resulting pressure wave are loud. Warn the audience members in advance and advise them to cover their ears.
  • Do not scale up the project! Do not use a regular lighter!

Perform the Self Carving Pumpkin Demonstration

  1. Carve the pumpkin. Scoop out the seeds, make a simple face, and keep the pieces. Be tidy so that when you replace the pieces, the pumpkin appears “uncarved”. If desired, trim the backs of particularly thick pieces.
  2. Cut a hole a couple of inches wide in the back of the pumpkin for inserting the piezoelectric lighter wires.
  3. Place the carved pieces back into the pumpkin.
  4. Pour the water into the plastic cup and set it inside the pumpkin.
  5. Drop the the calcium carbide into the cup.
  6. Close the top of the pumpkin.
  7. Insert the two wires of the piezoelectric lighter into the back of the pumpkin.
  8. Allow 30-45 seconds before sparking the lighter. If the reaction does not occur, continue sparking the lighter every 10 seconds for up to two minutes. Acetylene build-up takes longer for larger pumpkins than for smaller ones.
  9. Upon ignition, the carved pieces fly out of the pumpkin, accompanied by flames, black smoke, and a loud sound.

If the reaction does not occur after two minutes, place the pumpkin inside a fume hood, open the top, and leave it there until all of the calcium carbide has reacted and the acetylene gas has dispersed. Remove the plastic cup, neutralize it as instructed in the “disposal” section, and repeat the experiment with fresh water and calcium carbide.

How the Self Carving Pumpkin Works

Calcium carbide (CaC2) reacts with water (H2O), producing acetylene gas (C2H2) and aqueous calcium hydroxide [Ca(OH)2 (aq)]:

CaC2(s) + 2H2O(l) → C2H2(g) + Ca(OH)2(aq)

Igniting the acetylene results in a combustion reaction:

2C2H2(g) + O2(g) → 4CO2 + 2H2O(g)

Breaking the carbon-carbon triple bond in acetylene releases a lot of energy. The heat of combustion of acetylene is 1300 kJ/mole. Acetylene burns with a very hot flame (up to to 2,300 °C or 4,172 °F).

See What to Expect

If you search online, there are lots of videos of this reaction. Here’s an example:

Disposal

Calcium hydroxide is a strong base, so the reaction between calcium carbide and water produces an alkaline solution. Neutralize the liquid with vinegar (weak acetic acid) or dilute hydrochloric acid. Test the pH of the liquid using pH paper until you achieve a neutral solution. Then, flush the liquid down the drain with water.

History

In 1862, German chemist Friedrich Wöhler discovered that calcium carbide reacted with water, forming acetylene and calcium carbide. The reaction led to the industrial manufacture of acetylene. Today, the compound primarily comes from partial combustion of methane or from hydrocarbon cracking.

For a time, producing acetylene using calcium carbide and igniting it was common in the headlamps of early automobiles and in miner’s lamps. While the lamps were safe in copper and tin mines, they posed a risk in flammable environments like coal mines.

References

  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 298. ISBN 978-0-08-037941-8.
  • Keen, Robin (2005). Buttner, Johannes (ed.). The Life and Work of Friedrich Wöhler. Bautz.
  • Renouf, Edward (1899). “The use of Acetylene“. Popular Science Monthly. 335–347.