Why Do Bleach and vinegar Corrode Pennies?

A. Hi, Stephanie. First off, I hope your teacher and your parents warned you at least 5 times not to mix anything in with the bleach. Adding vinegar to it can release poisonous chlorine gas.

Bleach is highly alkaline and the zinc core of pennies dissolves readily in alkaline materials. Also, bleach is very high in chlorides, which are devastating corrosion-wise for most metals. vinegar is an acid which cannot dissolve copper but can dissolve the zinc core of pennies. But, moreover, vinegar can dissolve the brown oxide that is present on most older pennies, leaving a bright surface on the penny. If the vinegar evaporates, the copper that it dissolved from the tarnish will dry out as a blue green copper salt. Good luck wrapping up the project.

This is great info! THANKS SO MUCH! My 5th grader did this same experiment and the bleach corroded the penny greatly! I love the internet. I typed in this question and someone was nice enough to not only already have asked this question, but another had it answered perfectly! A million thanks from a 5th grader's mom!

A. Hi Vita.

Who says it does corrode in any of those things? I'm quite sure it doesn't. Do the experiment first with Coca-cola and vinegar and other household stuff. I'm not sure of your age and whether you can safely experiment with muriatic acid, but I would strongly discourage it except under the supervision of your science teacher.

Regards,

A. Hi Vita. You have probably seen that brand new pennies are bright reddish copper color, but they eventually become dark brown. That brown coloration is tarnish or corrosion. So, yes, copper pennies corrode. In fact, all metals except gold, platinum, and similar precious metals corrode.

The basic driving force behind such corrosion is that the reaction product, corroded copper, is at a lower energy state than the uncorroded copper. If you touch a lit match to a piece of newspaper, you know what happens: the paper catches on fire, takes oxygen from the air, and gets hot; smoke floats off, and ashes are left behind. But if you touch a lit match to the ashes, nothing happens. While the burning newspaper was hot, it gave off heat, so it ended up in a lower energy state. There is no way to get from ashes back to newspaper without adding that heat back in.

In a somewhat similar but much slower way, copper combines with oxygen from the air, very slowly releases a small amount of heat, and you end up with copper oxide on the surface of the penny. You can't convert that copper oxide back to uncorroded copper except by adding heat back in while excluding oxygen. If you look into how copper is made (or other non-precious metals like iron), ores are melted (heat is added back in) while oxygen is excluded.

So, yes, the oxygen in air will slowly corrode copper by combining with it and releasing heat.

Your situation is to determine by experiment whether you can speed up or worsen that corrosion with coca-cola or vinegar. Don't ask why they speed it up until you know from experiment whether they do :-)

Please note that you must find pennies from 1982 or earlier because after that they are not solid copper, but copper coated zinc. Using a copper coated zinc penny changes everything because there may be pinholes or porosity in the copper plating, so your ingredients will then be working against zinc instead of copper. Good luck.

Regards,

Hi Alex. Why do polar bears frolic in yellow & blue canoes on the Amazon ... if they do?
Don't even think about trying to get an answer to why zinc pennies dissolve in vinegar until your experiment determines whether they do or not. It's silly. Plus, if we tell you that they do or don't, then you won't trust your own experimental data and will be tempted to fudge it -- that's called "junk science" and should only be practiced by starving climatologists. Please describe your experimental method. Thanks.

Regards,

A. Hi Brad. We didn't get the pic ... you can email it to if you wish, but we don't have experience in that anyway. I suggest that you look at letter 25530 regarding how to attempt salvage or where to send the coins. People in the coin salvaging business will want to save as much of the coinage as possible because they get a cut of the salvageable coins. Good luck.

Regards,