Why Do Pickles Glow Orange When Plugged In? A Portland State Engineer Has an Answer
After burning more than 100 pickles, Josh Méndez says the classroom stunt is a hybrid: an electrical spark ignites hydrogen gas, and sodium in the brine supplies the color.
Mount a pickle on two metal forks, plug it into a wall socket, and one end will light up in a bright orange glow. It is a classic science demonstration, shown in classrooms for decades, but the physics behind it has never been settled. Josh Méndez, an assistant professor of electrical engineering at Portland State University, was surprised to learn that the glowing pickle was still an unsolved electrostatics problem. More than 100 burned pickles later, he says he is finally approaching an answer.
Pickles conduct electricity because they sit in salty brine. When current runs through one, two explanations have usually been offered for the glow. The first is a spark discharge. As current heats the pickle, liquid boils off and leaves pockets of gas, along with a pungent smell of burned pickle and hot metal. The gas pockets block the current, so the pickle cools, the pockets shrink and the conductive brine flows back toward the electrode. "As that water comes back in toward the electrode, a little arc crosses between the liquid and the electrode, just as when you reach for a doorknob and you get a zap," Méndez said. "That doesn't explain everything that happens, though."
What it does not explain, he notes, is why only one end glows. The second explanation is chemical ignition from electrolysis, in which the current splits the brine's water into oxygen and hydrogen. Hydrogen is produced only at the negative electrode, which could account for the one-sided glow. In that version, tiny hydrogen explosions caused by intense heat at the electrodes, and trapped inside the pickle, produce the light.
Méndez's experiments point to a hybrid. "There is hydrogen involved, but it's actually not heat that's igniting it, it's the spark," he said. "It's just like why you don't light a cigarette at a gas station. You're producing a spark that ignites the gas." In other words, the hydrogen explanation gets the fuel right, the spark explanation gets the ignition right, and neither alone is enough.
The color is the simplest part. Salt brine is full of sodium, and when sodium ions are excited by energy they emit a characteristic orange light, the same color seen in sodium street lamps and in a pinch of table salt tossed into a flame. "You're exciting sodium ions, which produce that characteristic orange glow," Méndez said. "If you were to pickle something in a different salt, you'd get a color unique to those elements."
The work, which Portland State described in a release, is more of a careful piece of lab detective work than a technological advance. It ties together plasma discharge, electrolysis and the chemistry of excited atoms in a single homely system. The key concepts listed with the study are oxidation and reduction, and plasma discharges, which are the same physics found in much larger systems such as lightning, arc welding and industrial plasma sources.
Méndez has described himself as a scientist who is drawn to obscure electrostatic problems, and the pickle fits that description. It also makes a point about science in general: a question that seems solved because everyone has seen the demonstration can turn out to be open, and a few dozen sacrificed pickles can be enough to close it. The secret behind this classroom experiment, in his account, is not a high-tech breakthrough. It is a slightly explosive pickle.
