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4th Grade Science Fair...Fire from a potato

April 19, 2009

Hello, I am the parent of a 4th grader who is interested in doing his science fair project based on a video he saw on a potato that is cut in two pieces, you then make a hole in the middle, fill it up with salt and toothpaste, close the potato up, attach two wires, you wait for 5 minutes and then you attach a piece of cotton to one of the wires, have the two wires touch and...voila..you got fire started!

The experiment itself I can help him do, but can you guide me to understand what is it that causes the electricity that causes the fire, is it the combination of the toothpaste and salt? What role does the potato play in this? Help me explain the statement of the problem and which key words do we need to do our research?

I would very much appreciate your answer ASAP. Thank you very much!

Maria S.
Substitute Teacher - Fort Lauderdale, Florida, USA

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Hi, Maria. I've seen the YouTube videos but how do you know if they are real or not (there are also fictitious videos where pointing three cell phones in the same direction starts a fire). So I think you are trying to start at step 3 instead of step 1.

Step 1 is to make it work, and reliably. Step 2 is to start changing things and determining exactly what changes keep it from working. For example, if you leave out the salt, does it still work; if you leave out the toothpaste, does it still work; if you put the salt and toothpaste into a hollowed out styrofoam ball or plastic Easter egg, does it still work? Step 3, trying to explain why it works, can't come until you know that it works and understand which elements are necessary and which if any is superfluous. Good luck!

Note to parents and teachers: there are many different versions of this video on YouTube; some are R rated, starting off with a young woman parading in a thong; get the addresses for G rated versions ahead of time so you don't embarrass yourself searching for it in front of the kids.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 23, 2009

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April 27, 2009

From one FL sub to another:

In a conventional potato battery, the potato only supplies the electrolyte which is a very dilute acid. A green potato will have more acid-IE: it is bitter. You need a potato with a good bit of liquid, not an old dried up one.
It uses dissimilar metals as in zinc and copper. The zinc forms a hydrogen half cell where the acid takes an electron from the zinc and yields 1/2 H2 and has 0 volts. The copper metal gives up 2 electrons and has a positive 1.5 volts.

Now with the fire potato, both wires appear to be copper, but it does not say that. Salt on a potato will make it sweeter which would mean to me that it is reacting with the acid and neutralizing it.
The toothpaste appears to be a fluoride toothpaste. I think that it is releasing a fluoride ion which will replace a salt chloride ion so may cause a faster reaction and possibly a higher voltage.
Since 1 wire was bent to be basically on the potato and the other one was in the salt/paste mixture it appears that there is an electron flow from the one area to the other. I do not know what the half reaction is.
The potato provides a fixture and the electrolyte.
There is a very tiny possibility that the salt/paste mixture is reacting with the starch in the potato.
I look forward to smarter people than me having a correct answer.

James Watts
- Navarre, Florida