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Generally, when people talk about a gold penny, they are talking about a normal penny that has been treated to give it a gold color. In some cases, however, a penny may actually have been plated with gold. And in some even rarer cases, a penny may have been struck accidentally on a blank of the wrong metal. The gold penny is also the name given to an ancient British coin, worth twenty pence.
Creating a gold penny is a common science experiment, especially at the primary and secondary school levels. Many teachers use it to discuss the ancient study of alchemy, and how that relates to modern chemistry. It also demonstrates some basic concepts of chemistry, while providing a concrete result that students can take home, and which can be quite exciting to younger students.
To make a gold penny at home, you’ll need a normal copper penny, a Bunsen burner, and a mixture of sodium hydroxide and zinc. You will also want to wear proper safety equipment, including goggles and gloves, and have a pair of tongs to handle the penny. Sodium hydroxide can be very dangerous if mishandled, so it’s important to take care at every step of the way.
The zinc and sodium hydroxide mixture should be placed in a vessel and put on top of the Bunsen burner. It should remain on the heat until it begins to boil, at which point the normal copper penny can be dropped into the mixture. It should stay in the mixture for two to three minutes, and then should be removed with tongs, and allowed to dry on a paper towel. The color at this point should be roughly silver, as the zinc has formed a thin layer on the outside of the copper.
Next, the silver penny should be held with tongs over the direct heat of the Bunsen burner until the color changes. Once the silver penny begins to transform into a gold penny, it should be removed from the heat and placed on a non-flammable surface to cool. The penny will now appear to be gold, though in fact all that has happened is that the heat has caused the zinc to mix with the copper. This essentially forms the metal known as brass, which is commonly mistaken for gold, although it is much less yellow and has an entirely different consistency. So the penny most people create can more accurately be described as a brass penny.
A gold penny may also refer to the coin used during the reign of King Henry III of England. Before his reign, most high value coins used in England were from either the Byzantine Empire or from Arabia, but during his reign, in 1257, he saw the need to have a local currency which could handle larger transactions. So he had a gold penny created, which on one side featured him on a throne, and on the other side featured a cross and flowers. King Henry III unfortunately set the value of this penny too low, relative to the actual price of gold, and so most of the coins were melted down to be sold as pure gold, making an actual gold penny from this era quite a rarity.
Good job on pointing out a common science experiment that has gotten more than few kids interested in a career in that field. While the experiment is simple, it is the type of "hands on neato" thing that has led to some students being nudged toward the sciences and becoming very successful in their fields.
It is fascinating how such seemingly minor things can lead to fantastic results. The better teachers are well aware of the impact of such experiments and are to be commended for using them effectively.