Hundreds of years ago alchemists claimed to be able to transform base metals into gold. Gold was the noblest metal and all metals strove to become gold naturally. Alchemists used their mystic arts to bring about that natural transformation with a speed to match the greed of their rich patrons. Much of the imagery of sorcery—the potions, the incantations, the flashy robes—come from these charlatans who claimed to be able to do the impossible through the use of magic.
Or was it impossible? Light some candles and chant the ancient names of the elements and perhaps magic can still be worked by the adepts. A magic potion heated over a fire and the purifying fire itself can bring about an astounding transformation.
Don’t believe in magic? That’s wise, as there is no such thing. Copper (Cu) pennies can take on a silvery appearance when they are coated with the similarly-colored metal, zinc (Zn). In strongly basic solution the following reaction takes place between zinc and hydroxide ions:Zn + 2OH– ⇒ ZnO22– + H2
On the surface of the copper pennies the zincate ion (ZnO22–) is reduced to metallic zinc (Zn). A reduction half-reaction may be something like:ZnO22– + 2H2O + 2e– ⇒ Zn + 4OH–
The ‘silver’ penny turns gold when it is placed in the fire. This comes about because the zinc melts (mp 420°C) and dissolves some of the copper underneath. This forms the homogenous physical mixture, or alloy, known as brass. The attractive, corrosion resistant and hard alloy has a golden appearance even though no atoms of gold are in it.
This demonstration is based on demonstration 11.33 “Copper to Silver to Gold” pg. 263 of Chemical Demonstrations, Vol. 4 by Bassam Z. Shakhashiri.
The text is meant to be displayed on a projector screen while students witness the demontration.