Editing Penny Alchemy (section)

== Why This Works ==

===Short Explanation===

The pennies are being placed in a strong basic NaOH solution, which is being heated by the hot plate. ''Basic'' solutions can be thought of as the opposites of acids: they accept protons, add negative ions to a solution, and increase the pH of the solution. Acids accept electrons, add positive ions to a solution, and decrease the pH of the solution. Most soaps are weak bases, so one can think of our NaOH solution as being a really strong "soap". The copper on a penny can dissolve into a basic solution, but only if two things are met: The solution needs to be hot, and the copper needs to be in contact with the zinc in the beaker. The zinc in the bottom of the beaker can dissolve, but just barely. The zinc would rather take the place of the copper as it dissolves off the penny, so once the penny touches the zinc we see this happen! The penny starts bubbling rapidly, and in less than a minute the outside layer of copper has dissolved, with it's place taken by the zinc! This type of reaction is called a ''Single Displacement Reaction'', since a single element (Copper) switched places with another (Zinc).

The "silver" pennies have only a very thin layer of zinc on top of the copper underneath. These zinc-plated pennies can have their silver color rubbed off after repeated use, since the zinc is only sitting on top of the copper. When the zinc-plated pennies are heated up on the hot plate, the zinc layer and copper layer start to heat up together. This causes them to meld together, with the zinc atoms sliding between the copper atoms, and become brass! Brass is a ''Metal Alloy'', or a metal which comes from the combination of different metal elements. Brass is often used to make instruments such as tubas, trumpets and saxophones, and is recognized by its yellow-gold color. 

===Full Explanation===

The NaOH solution is strongly basic. ''Bases'' are effectively the opposite of acids: They accept protons, add negative ions to a solution, and increase the pH of the solution. NaOH is a strong base, and fully dissociates into two ions, Na<sup>+</sup> and OH<sup>-</sup>. The OH<sup>-</sup> ions can give electrons, which is what drives this reaction.

When the zinc is added to the solution, it will weakly dissolve while the solution is being heated, and react with the OH<sup>-</sup> ions:

{| class="wikitable" style="color:black; background-color:#ddd; text-align: left; margin-right: auto"
|Zn<sub>(s)</sub> ⇒ Zn<sup>2+</sup><sub>(aq)</sub> + 2 e<sup>-</sup>
|-
|Zn<sup>2+</sup><sub>(aq)</sub> + 2 OH<sup>-</sup><sub>(aq)</sub> + 2 H<sub>2</sub>O<sub>(l)</sub> ⇒ [Zn(OH<sub>4</sub>)]<sup>2-</sup><sub>(aq)</sub> + H<sub>2</sub><sub>(g)</sub>
|}

This only happens weakly, meaning that very little of the zinc actually dissolves. This, however, changes dramatically when the pennies are put in. The pennies are copper, or at least have copper on the outside. Copper gladly accepts electrons, and is commonly used as a conductor. It is used to drive this ''Electrochemistry'' reaction, or chemical reaction which is driven by an electrical charge or produces an electric charge as the reaction continues. The solution of free electrons was an ''Electrolyte'' solution, meaning it was ionic and could provide electrons. The copper, upon landing on the zinc in solution, was the ''Electrode'', meaning it could accept electrons. This results in the following reaction:

{| class="wikitable" style="color:black; background-color:#ddd; text-align: center; margin-right: auto"
|[Zn(OH<sub>4</sub>)]<sup>2-</sup><sub>(aq)</sub> + 2 e<sup>-</sup> ⇒ Zn<sub>(s)</sub> + 4 OH<sup>-</sup><sub>(aq)</sub>
|}

This reaction results in the zinc coming out of solution and plating onto the copper coin. This results in the coin looking to turn "silver", the first half of this interesting reaction. After the coin is plated, it can be removed and rinsed off to showcase the shine. If a coin does not appear to be plated enough, it can be placed back in to let the remainder of the coin be plated. The second half, when the coin is heated, is a surprisingly simple equation:

{| class="wikitable" style="color:black; background-color:#ddd; text-align: center; margin-right: auto"
|<sup> </sup>Zn<sub>(s)</sub> + Cu<sub>(s)</sub> ⇒ ZnCu<sub>(s)</sub>
|}

This reaction, although easy to write out, is not as easily explained. When the coin is heated, the atoms of the zinc and copper start to move. The movement isn't enough for either metal to melt, but it is enough for the zinc to start sliding into the copper surface underneath. As this happens, the metal atoms form a ''Alloy'', or a metal which isn't a unique element on the periodic table, but rather formed by the combination of metals. Brass is a common alloy, used to make instruments and largely as a decorative metal. Unlike copper, brass is a very soft metal and can be easily shaped and molded. The brass layer on the coin looks a bright yellow gold color, but over time it will tarnish and dull. It cannot be easily cleaned, as any attempt at cleaning the brass layer would remove it. It can, however, be re-plated with zinc, and then reheated to create a new brass layer on top.