This fun chemistry trick turn a copper penny into a silver penny and then a gold penny. Of course, the coin won’t be real silver and gold. The silver color is zinc, while the gold color is brass. It’s a great way to make a pot of “gold” for St. Patrick’s Day or to learn about galvanization and alloys.
Did You Know: Modern pennies are zinc with copper plating. They haven’t been made out of copper since 1982!
Gold and Silver Penny Materials
- Clean pennies (or any small copper item)
- Zinc (either metal or powder or galvanized nails)
- Sodium hydroxide (NaOH) or sodium hydroxide solution (or lye drain cleaner)
- Tongs or tweezer
- Source of heat or flame
If your pennies are grimy, clean them with salt and vinegar to make them shiny.
If you have access to a lab, sodium hydroxide and zinc are easy to obtain. If you’re doing this chemistry trick at home, you can buy sodium hydroxide or lye online or else use drain cleaner based on the chemical. If you can’t find pure zinc, use any galvanized metal object.
Substitution: Instead of sodium hydroxide, use zinc sulfate or zinc chloride. Dissolve 30 grams of zinc sulfate 100 milliliters of hot water. Or, prepare a 1M ZnCl2 solution.
Turn Copper Pennies Into Silver Pennies
- Pour a bit of zinc (1 to 2 grams, or else add your galvanized item) into a small dish of water.
- Add a few sodium hydroxide granules. Alternatively, just add zinc or a galvanized item to a 3M NaOH solution. Metal and sodium hydroxide undergo an exothermic reaction, which makes the water hot and aids the color change. [If you use zinc sulfate or zinc chloride in water, add 1 gram of zinc metal to either solution and gently heat the liquid over medium heat until it boils.]
- Drop clean pennies into the hot solution. Use tweezers or tongs to arrange the pennies so they aren’t touching each other.
- The pennies change from copper to silver within 5 to 10 minutes. Once they change color, use tweezers to remove the hot pennies from the liquid.
- Rinse the pennies with water and set them on a towel to dry. It’s safe to handle the pennies once you’ve rinsed them with water.
The pennies change from copper to silver-colored because the chemical reaction plates the copper with zinc. This reaction is an example of galvanization. Zinc reacts with hot sodium hydroxide, forming soluble sodium zincate (Na2ZnO2) that deposits zinc metal upon contact with copper.
Turn Silver Pennies Into Gold Pennies
- Grasp a silver penny with tweezers.
- Gently heat the penny until it changes from silver to gold. You can heat it in a lighter, candle, or burner flame or else set it on a hotplate or heated frying pan.
- Remove the penny from heat once it changes color and rinse it under water to cool it.
Heating the penny fuses zinc and copper to form the alloy called brass. Brass is a homogenous metal that ranges in composition from 60-80% copper and 18-40% zinc. Brass has a low melting point for a metal. The copper plating on pennies is thin, so don’t heat the penny too long or the brass will melt away and you’ll be left with the silver-colored zinc inside the coin.
Sodium hydroxide or lye is caustic, so it’s a good idea to wear eye protection and gloves. If skin contact occurs, immediately rinse the affected area with water. If you spill the liquid, you can neutralize it with vinegar or lemon juice. Sodium hydroxide finds use as a drain cleaner, so it’s safe to rinse it down the sink with water.