It is not always gold that glitters

Demonstration Experiment on Video

"It is not always gold that glitters!"

Objective: "Silver and gold plating" of copper coins

Peter Keusch

German version

Chemicals:
citric acid
ethanol 96 %
zinc powder
2 N NaOH

Apparatus and glass wares:
magnetic stirrer
magnetic stirring bar
stirring bar remover
pair of tweezers
beaker 100 mL
evaporating dish d = 8 cm
snap-cap vial 20 mL

Hazards and safety precautions:

ethanol is highly flammable.

Safety glasses and protection gloves should be worn.

Experimental procedure:

Three 5 cent coins are cleaned in a warm solution of citric acid. Two of the cleaned coins and 2 g of zink powder are placed into a 100 ml beaker containing 20 mL of 2N NaOH and a magnetic stirring bar. The mixture is heated on a magnetic stirrer to weak boiling whlie stirring. The coins are taken out of the liquid with a stirring bar remover. One of the two coins is heated in a ethanol flame until a color change on its surface can be observed. The coin is turned so that both sides are heated equally.

Results:

The two 5 cent coins are "silver plated" in the NaOH solution mixed with zink powder. Heating in the ethanol flame transforms the "silver coin" into a "gold coin".

Video clip (Download RealPlayer .rm file)

Discussion:

Some zinc dissolves in hot alkaline solution forming zincate:

When the copper coin is in contact with a piece of the remaining zinc powder, then an electrochemical cell is produced. Electrons flow from the piece of zinc to the copper coin. At the surface of the copper coin, these electrons reduce the zinc ions, present in solution. The copper coin is completely covered with a thin layer of zinc. Since zinc is "silver" in color, the coin now looks like silver.

The electrode reactions are

at the zinc electrode:

at the copper electrode:

When a "silver plated" coin is heated zinc migrates into the surface layer of the copper. Brass is formed, The alloy gives the coin the gold color.

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