HOW DOES ACID AFFECT DIRTY PENNIES?

A. Pennies are coated with copper oxide when they are made. The copper oxide makes the pennies look dirty, but they are really not. When the pennies are placed in the acid the copper oxide gets dissolved in the acids, making the pennies clean.

A. Hi, Students. Yes, Linsey is right that the tarnish is copper oxide, and the acid dissolves the tarnish. But actually the tarnish is not from when the pennies are made (brand new pennies are shiny); the tarnish is from the copper of the penny reacting with the oxygen in the air over time, people handling them, etc.

There are thousands of students around the world with similar questions, so we've written a Cleaning Pennies FAQ which will hopefully answer many of your questions on this project. Good luck!

Regards,

A. Hi Liz. Bleach is very alkaline, the opposite of acidic, but it does corrode pennies rapidly. That's because bleach is super active chlorine and oxygen with great oxidizing power. The alkalinity is incidental, it probably doesn't cause the corrosion; it's put there because the chlorine would all instantly escape as chlorine gas if the solution was acidic. That's one reason you are warned never to mix other stuff (except highly alkaline laundry detergent) with bleach.

It is true that the greater the acidity, the more power something has towards dissolving metal & tarnish (all other things being equal, which is not necessarily the case). But again that doesn't necessarily mean that all of the other possible ingredients in, and characteristics of, apple cider vinegar are irrelevant to the results. I'm not sure why the pennies turn black, but have you considered trying distilled white vinegar in one cup, and apple cider in another, to see if either gets the same black coloration? It might be interesting to observe whether or not the remains of the cider, with its complex apple proteins, seem to be responsible for the black coloration.

Chemistry is complex, and even most college students understand it only trivially, so I don't think the point of the experiment is for 8th-graders to deeply understand pH and tarnish and all the things that might drive the formation of tarnish or dissolve it -- but to make interesting (but documentable) observations for themselves.

Regards,

A. Hi again. His conclusion would be that acidic liquids turned the pennies darker. You can call the darkest "a darkness of 8" and the lightest "a darkness of 2" and assign intermediate values to other shades, then attempt to graph the pH vs. the darkness, and see if he can conclude that the darkness appeared to be proportional to the pH.

You don't even know what the dark coating is, nor what all of the chemicals in lemon juice and apple cider vinegar are, or their effects, so it might be a stretch to "conclude" much more. But certainly you can conclude that the reported correlation between pH and shininess has been disproven. Good luck.

Regards,