- Some dirty old coins made of copper or coted with copper (Such as American pennies)
- Two glass jar or any kind of water container
- Fresh water
- Dish soap
- 1 tablespoon of salt
- ½ cup of lemon juice
- Put some water and dish soap into one transparent glass or container.
- Put the lemon juice and one tablespoon of salt in the other transparent or container.
- After that, put some dirty old copper coins (Such as American pennies) in both containers.
- Now let those copper coins soak for a few minutes (Five minutes should do it)
- After a few minutes take the copper coins out of the glasses or containers and wash them with fresh water.
- Now observe their appearances.
- The copper coins which were in the glass containing soap water looks as dirty and old as before.
- The copper coins which were in the salted lemon juice looks very shiny and bright after soaking and washing!
Copper can react with oxygen in the air and produces copper oxide. Copper oxide looks like dirt. So when copper coins or copper coted coins gets a bit old, then copper on the coin reacts with the oxygen in the air and produces copper oxide. This makes the coin look very dirty and old. This kind of things always happens to the American pennies because they are copper coted. That’s why we cannot wash the dirt of the copper or copper coted coins because - the dirt or the copper oxide is actually the result of a chemical reaction and it is stuck with the coin. But on the other hand, lemon juice is acidic. It has an excess hydrogen ion (H^+). The chemical reaction which takes place in the lemon juice with the copper coin is given bellow:
CuO (Solid) + H^+ ----------> Cu^2+ + H2O
Copper oxide + Hydrogen ion ----------> Copper ions + Water
The acid actually turns the solid copper oxide into copper ions and water. The salt added into the lemon juice acts as a catalyst and speeds up the chemical reaction caused in between the lemon juice and copper coins or copper coted coins.
MORE EXPERIMENTS FOR YOU:
- Try this experiment by not adding salt into the lemon juice.
- Try using vinegar to shine the copper coins.
- Try other kinds of less powerful acids to do the experiment.