Shiny penny or 1 cent piece
Granular or powdered zinc
Tongs or forceps
6M sodium hydroxide
Beaker filled with cold water
Graduated measuring cylinder
1. Wear goggles and take CAUTION in performing this experiment.
2. Clean a penny with steel wool. Make it as shiny as possible.
3. Place 5g of powdered zinc into an evaporating dish. Add enough 6M NaOH solution to cover the zinc and fill the dish to about one third. Heat the mixture until it just starts to boil. This should take about five minutes.
4. Use tongs to place the penny in the dish. Carefully stir the mixture with the tongs and turn the penny. Continue to heat and stir gently until the penny becomes covered with zinc and turns ‘silver’. This usually takes 3-4 min.
5. Use the tongs to remove the penny. Rinse the penny in cold tap water, and pat it dry.
6. Using tongs to hold the penny, place the ‘silver’ penny on the hot surface of the hotplate. The penny should turn ‘gold’ almost immediately.
7. Immerse the penny into a fresh beaker of cold water. After the penny has been cooled for over a minute, pat it dry and record your final observations.
The chemistry explained:
1. Zinc dissolves in concentrated sodium hydroxide solution to form sodium zincate. When the copper penny is placed in this solution, the zinc plates out on the surface, displacing copper. The penny thus turns ‘silver’.
2. When the zinc-coated copper penny is heated on the hotplate, the zinc and copper atoms inter-diffuse to form brass at the surface, an alloy of zinc and copper which has a gold colour.
Safety: disposal of the Zinc-NaOH solution. This mixture and the waste zinc should not be disposed of in a bin.
Rinse off the liquid with several portions of water. Add the remaining zinc to a beaker containing 200mL of 1M sulfuric acid. This dissolves the zinc to give zinc sulfate and neutralises any remaining alkali.
The waste can now be washed down the drain with plenty of water.