A pickle through which an electric current passes produces an orange glow. This phenomenon, which has been demonstrated for several decades in some science classes, is now much better understood. On the menu of the glowing pickle: electrical conduction, steam, sparks and tiny explosions.
A pickle contains a lot of salt water. The dissolved salt separates into ions, that is, electrically charged atoms or groups of atoms. This brine allows the current to flow. The flesh of the pickle then behaves like a conductive medium, but an imperfect one: it heats up rapidly near the electrodes.
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Until now, the most widespread explanation attributed the light to the sodium ions present in the salt. These can indeed produce a yellow-orange hue when excited. However, recent observations show that this mechanism is not enough to explain the spectacle. The glow also depends on more intense physical phenomena, localized near the electrode.
A first step seems to be the formation of a vapor pocket. The heat locally boils the water contained in the pickle. This gas bubble partially interrupts the passage of the current. Sparks can then cross the small space created between the electrode and the wet part of the vegetable, which further increases the temperature.
At the same time, the current decomposes part of the water through electrolysis. This reaction notably produces hydrogen and oxygen. Together, these two gases form a flammable mixture. The sparks then seem to ignite it in small quantities. The orange light would therefore come from successive mini-explosions, difficult to distinguish with the naked eye.
Researchers studied the phenomenon with a fast camera and a hydrogen sensor. They compared alternating current, which usually comes from outlets, and direct current. Hydrogen appeared in both cases. Yet, only the alternating current supply caused the characteristic glow.
Alternating current regularly reverses its direction. According to the hypothesis presented, this oscillation helps the vapor pocket stay open long enough. Sparks can then form and ignite the gas mixture. With direct current, the bubble would collapse more easily: hydrogen is still produced, but without a spectacular glow.
The light does not appear just anywhere either. When the pickle is placed vertically, it generally surges at its lower end. This is where the brine tends to accumulate. The most soaked part conducts the current better and favors the observed reactions.