Where the
world gathers for metal finishing
Q&As since 1989


-----

My pennies are floating

I have been doing a science fair project on the effects of liquids on pennies. Not what cleans them. I wanted to see if they would change if left in vinegar [in bulk on eBay or Amazon], salt water, oil. There is one penny in the vinegar that is now floating. I know what pennies are made of and I was just wondering if you could tell me if it is the zinc that is leaving or what would make it float? I have left it in the solutions for about 3 weeks. I am 10 and could really use your help.

Thanks,

Mason
student - Bowling Green, Ohio
2004

---

If you look at a chemistry book, you will see that zinc is soluble in acids. vinegar is a dilute acetic acid [on eBay or Amazon], so it will slowly "eat" the zinc. One of the products is hydrogen gas. Since it is formed so slowly in your case, it will be in very tiny bubbles which do not agglomerate into a big enough bubble to break the surface tension holding it to the penny. This is like attaching thousands of micro life preservers to the penny. Now, if you take a forcep and shake the penny violently in the solution, much of the hydrogen will leave and the penny will sink. Also, remember that some of the attack on the zinc center is happening under the copper. This has hydrogen replacing the zinc but yet the volume of the penny remains relatively the same. When the weight of the penny is less than the weight of the solution that it displaces, it will float.

James Watts
- Navarre, Florida
2004