Turning Copper Coins Silver and Gold
Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★★ Requires a more experienced teacher
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff
Categories: Chemical Reactions, Materials, Oxidation and Reduction
Alternative titles:
Summary
Copper coins can be plated with zinc, giving them a silver appearance. Heating the zinc-coated coins in a flame causes zinc and copper to alloy, forming brass, which gives the coins a gold color.
Procedure
Links
Variations
Try with different copper-containing coins to compare plating results.
Compare heating times to see how the depth of gold color changes.
Reuse the sodium zincate solution to coat multiple coins, noting how results diminish with each reuse.
Safety Precautions
Wear safety goggles, gloves, and a lab coat at all times.
Sodium hydroxide solution is extremely corrosive and can cause severe burns and blindness; handle with great care.
Zinc powder is flammable; spilled powder must be cleaned up with a damp towel to prevent ignition.
Hydrogen gas is flammable; add zinc to hot sodium hydroxide away from open flames.
Use tongs when handling hot coins and keep a heat mat under hot equipment.
Dispose of leftover sodium hydroxide solution down the drain with plenty of water only after cooling; solid zinc residue must be collected as hazardous waste.
Questions to Consider
What chemical reaction causes zinc to dissolve in sodium hydroxide? (Zn + 2NaOH + 2H₂O → Na₂[Zn(OH)₄] + H₂)
Why must the coins contact zinc powder in the solution? (Direct contact establishes the electrochemical cell needed for zinc plating.)
Why does the coin appear silver after plating but gold after heating? (Silver appearance is metallic zinc; heating forms brass, a gold-colored alloy of copper and zinc.)
What type of alloy is brass, and how does its composition affect its properties? (Brass is a substitutional alloy of copper and zinc; higher zinc content increases strength and hardness.)
How is this plating process similar to industrial galvanization? (Both involve zinc coating for corrosion resistance, but industry uses cyanide-based complexes rather than hydroxide.)