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demonstrations:turning_copper_coins_silver_and_gold

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

  • See links below for full method.

Turn Pennies Silver and Gold (Chemistry Trick) - NurdRage:


Turning copper into 'silver' and 'gold' - Royal Society of Chemistry:


📄 Experiment - Turning Copper coins 'silver' and 'gold' - Periodic Table of Sustainable Elements: https://eschemistry.org/periodic-table/ExperimentTurningCoppercoinssilverandgold

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.)