When Electroplating pennies, what is the silvery substance that occurs on the penny's surface?
Electroplating is the process of running current through a metal to attract ions from another metal source. The attracted ions attach to the Host Metal Object and produce a smooth surface. This is commonly seen in high-school science experiments to coat pennies with "silver" or "gold." The coatings are in quotations because they are usually not using actual gold and silver sources, but instead coating with metals that approximate the colors. In this case, one of the most common coatings is Zinc.
Simple electroplating can be done with a battery, a metal object, and a solution of Metal Salts, vinegar, and water. The vinegar helps with the reaction process. Attaching the Host Metal to the negative pole of the battery and immersing both poles in the solution (a wire from the Positive side, not the actual battery) allows the positively charged dissolved salts -- which are now agitated by the current from the battery -- to attach to the negatively charged Host Metal. This produces a very thin, but uniform, plating.
Plating a penny with Zinc Salts produces a shiny silver color; Zinc is naturally silver in color, and the penny already is composed mainly of Zinc, but electroplated with copper in the U.S. Mint. The color of the plated Zinc penny will be silver until the penny is heated over a flame, at which point the Zinc reacts with the heat and turns gold in color.