Mass of Pennies (Erica Brauer)

Time-lapse of newer penny in hydrochloric acid

Student prior knowledge: Student prior knowledge assumes that they believe pennies are made of copper. Students know copper is a metal from the periodic table. Students are familiar with how to use a scale.

Explanation:

In the study of matter, one of the first things chemistry students learn is how to find the mass of items on a scale. Many Americans do not know that pennies are actually not 100% copper, especially after 1982. This is a fun demonstration for students to make observations about an item (the penny) that think they know, but may have been making some assumptions about. In this experiment, students find out the mass of pennies before and after 1982, and they should notice a significant difference in the masses. Then students are able to make some observations of different ways the pennies react to Hydrochloric acid, different sounds the pennies make when dropped, and different appearance inside. After further observations, students should be able to make inferences about the differences, and some will begin to infer that the old and new pennies must be made of different things. In fact, the newer pennies are about 97.5% zinc composition and only copper plated. This is why the hydrochloric acid dissolves the inside of the newer pennies and not the old

Here are a few key things to help make your demo smoother:

1. Make sure you file slits into the pennies that you drop into the Hydrochloric acid to allow it to react. 6 M HCl is a very strong acid, so please be careful!

2. A time-lapse video is very easy to make and is a great way to show how the pennies react in HCl if you are not able to have a strong acid in your lab.

3. There will be some students who already know new pennies are not made of copper, but often times they will keep quiet and not ruin the fun for there peers.

Questions & Answers:

1. Why do you think the U.S. government changed the composition of the pennies?

In the late 1970s, the value of copper increased significantly. Pennies used to be made of 95% copper and 5% zinc, and now they are made of 97.5% zinc and 2.5% copper. Pennies became more expensive to make than they were worth. An pre-1982 penny is worth about 2.2 cents.

2. Why don't Americans melted down the old pennies and make a profit?

It is actually against the law to melt the pennies down in order to make a profit. The government thought it would lead to a penny shortage and it would have to spend more money to make more pennies, so they outlawed it in 2007.

3. Why did the government not use aluminum as the core of the penny, since it is a very abundant metal?

There were two main reasons that aluminum was decided against. First, vending machine companies thought that the aluminum would mess with the mechanics of the machine. Second, aluminum is difficult to detect in X-ray imaging and pediatricians were concerned about being able to detect swallowed pennies.

Applications to Everyday Life:

One application to everyday life is that many people feel we should eliminate pennies since it costs more to make than it is worth. Considering our governments current debt, what do the students feel about getting rid of the penny and how much will it effect the current economy?

A second application is of this phenomenon can be applied to processed food. Chicken Nuggets, for example are often times not 100% chicken, having many additive that we do not know about. Just because something looks and acts like a duck, doesn't mean it's a duck.

A third application could be to tie this into the economy. The reason the composition of pennies was change was largely due to inflation. How much can our fluctuating economy effect changes in how are government operates? How can inflation of the value of pennies compare to the inflation of house prices? Understanding inflation can help you to make decisions about what you buy and when.

Photographs: