Question

Penny Sandwiches

From 1864 until 1983, the usual composition of a penny (its actual name is "cent") was 95% copper and 5% zinc and tin. Because of the increasing costs of copper, pennies are now composed of zinc interior with a thin copper coating comprising only 2.4% of the penny. This activity allows you to remove the inside of the penny, leaving only the thin copper coating.

Materials

2 pennies (dated 1983 or later)

file or coarse sandpaper

2 cups lemon juice

2 cups vinegar

2 clear wide-mouthed containers (greater than 2-cup capacity)

cup measure

Exploration

Step 1 Using the file or coarse sandpaper, completely remove the copper coating from the edge of both pennies. Pour the lemon juice into one container. Position one penny in the container so that it is standing on its edge against the side of the container. Pour the vinegar into the second container and similarly position the second penny. Observe the pennies for 4 or 5 minutes. (a) What do you see? (b) What is occurring? (c) What is the identity of the observed substance?

Step 2 (a) Observe the pennies over the course of 1 week and describe any difference(s) in their behavior. (b) What accounts for the difference(s)? (c) What specific process is occurring in each container? (d) How do you know when the processes are complete? (e) Write an appropriate equation representing the processes. (f) Use your observations to rank the three elements hydrogen, copper, and zinc from most to least reactive.

Challenge

What is the chemistry-based reason for this peculiar construction of a penny?

Answer 1