Fun With Acid!

Step 3 Now for the bubbly! (and an explanation)

Put the penny in the glass with the exposed zinc facing up.

Next, pour in just enough acid to cover the top of the penny.

Leave it for several hours or overnight.

So what's going on in the glass?

The composition of U.S. pennies changed in 1982.

Before 1982 (for about 20 years), pennies were an alloy of about 94% copper and 6% zinc.

The increasing cost of copper made this impractical. Therefore, some 1982 pennies and all
pennies produced after that time have a pure copper shell with a zinc interior.

The new pennies contain only about 2.4% copper and weigh 2.5 grams as opposed to the 3.1-gram weight of the older type.

Zinc is a more active metal than copper. In this experiment, we see that zinc will react
with acid to produce hydrogen gas:

Zn(s) + 2 HCl(aq) => ZnCl 2(aq) + H 2(g)

The hydrogen gas generated by the reaction will bubble up because it is less dense
than the penny and the acid solution.

However, towards the end of the reaction some bubbles of hydrogen may adhere to the copper or be trapped inside the shell. If this occurs, the combination of copper and hydrogen gas may be less dense than the acid solution and what is left of the penny may float!