Soaking pennies in vinegar is a popular science experiment often done in classrooms or at home. It allows you to observe a chemical reaction and see firsthand how acids affect copper metal. But an important question is, how long should you soak pennies to get the full effect?
What Happens When Pennies Are Soaked in Vinegar?
When a penny is soaked in vinegar, a chemical reaction called an oxidation reduction reaction occurs. The acetic acid in the vinegar reacts with the copper in the penny.
Here’s a simplified explanation of the reaction:
- The acetic acid transfers electrons to the copper atoms in the penny.
- This causes the copper atoms to break away from the zinc atoms in the penny.
- The copper atoms dissolve into the vinegar, leaving behind just the zinc.
As this reaction occurs, the penny will begin to slowly corrode away. Over time, it will turn green and eventually to black as the copper exterior wears off completely.
Factors that Affect the Reaction Time
Several factors affect how long the oxidation reduction reaction will take to fully run its course on a penny:
- Vinegar Acidity – Vinegar contains acetic acid, typically about 5% acidity. Higher acidity vinegar will react faster.
- Vinegar Temperature – Warmer vinegar will react faster than colder vinegar.
- Copper Content – Pure copper pennies will react faster than newer pennies coated in zinc.
- Exposed Surface Area – More exposed copper surface area will allow for faster reaction time.
By understanding these factors, we can determine about how long we need to soak pennies to fully observe the chemical reaction.
How Long Should You Soak Pennies for the Experiment?
Most sources recommend soaking pennies in vinegar for at least 4-6 hours to fully observe the reaction. However, longer soaking times of 12-24 hours are needed for complete corrosion and disintegration of the penny’s surface.
Here are some soaking time guidelines based on the pennies used:
Pre-1982 Copper Pennies
- 4-6 hours: penny will turn dark brown/orange and show some wearing away of the surface
- 12 hours: penny surface will turn green as oxidation occurs
- 24+ hours: oxidation fully runs its course, penny surface degraded to black
Post-1982 Zinc-Coated Pennies
- 12-24 hours: see slow wearing away of the zinc coating
- 1 week: zinc coating completely worn off, copper underneath exposed
- 2+ weeks: oxidation reaction begins attacking the newly exposed copper
Keep in mind that time varies greatly depending on factors like vinegar temperature and acidity. Warmer, more acidic vinegar will react much faster.
Experiment Setup
To set up the penny and vinegar experiment:
- Gather a few pennies from different years – try to get pre-1982 all copper pennies and post-1982 zinc-coated ones.
- Pour undiluted white vinegar into a plastic or glass bowl until there is enough to fully submerge the pennies.
- Record the date and time you start soaking the pennies.
- Place the pennies in the vinegar and leave them to soak undisturbed.
- Observe the pennies periodically, recording any changes you see. Carefully remove a penny at timed intervals for closer observation.
- Take photos over time showing the progression of the corrosion.
For the full effect, soak at least one penny for 24 hours or longer. Be sure to track the time accurately.
Experiment Results and Observations
Here are some of the effects you should observe over time as the pennies soak:
| Time Elapsed | Observations |
|---|---|
| 1 hour | Penny lightly tarnished but no other visible change |
| 4 hours | Pre-1982 penny darkened, post-1982 penny unchanged |
| 8 hours | Pre-1982 penny shows green oxidation, post-1982 penny lightly tarnished |
| 12 hours | Pre-1982 penny heavily oxidized with green surface, post-1982 penny beginning to show wear of zinc coating |
| 24 hours | Pre-1982 penny completely blackened with severe corrosion, post-1982 penny zinc coating completely worn off |
You should notice that the all-copper pennies react much faster than the newer zinc pennies. But given enough time, the zinc coating wears away to expose the copper underneath.
Experiment Conclusion
Soaking pennies in vinegar provides visual evidence of the chemical reaction between acetic acid and copper. A minimum 4-6 hour soak is recommended, but soaking for 24 hours or longer provides more dramatic and complete results.
Key findings include:
- Acidic vinegar causes the copper in pennies to oxidize and corrode away over time.
- Warmer temperatures and higher acidity speed up the reaction.
- Pure copper pennies react much quicker than zinc-coated pennies. But zinc coatings wear away over 1-2 weeks.
- Complete corrosion and disintegration of the penny’s surface takes approximately 24 hours.
This simple experiment provides an engaging visual of the effects of oxidation. It’s a great science demonstration for classrooms and homes.