Experiment 5

"Gold" Penny Lab

Forming Brass from Zinc and Copper

Objective: The objective of this lab is to use a post-1983 penny to produce a thin layer brass alloy and a pre-1983 penny to make a bronze alloy.

Scientific Principles:

In order to save expensive copper, penny coins, starting in 1983, were made of zinc with a thin layer of copper plated on the surface. If these coins are heated, the zinc will diffuse into the copper layer, producing a surface alloy of zinc and copper. These alloys are brasses. Not only does the zinc change the properties of copper, but also the color of the brasses changes with zinc content - reaching a golden yellow color at around 20% zinc and golden at 35-40% zinc. Copper also oxidizes when heated in air, producing a black layer of copper oxide (CuO). Thus when heated, there is a competition between the rate of oxidation (making the surface black) and the rate of diffusion (making the surface a golden-yellow color). Bronzes are alloys containing tin and copper.


Brasses are used in many industries because they are fairly corrosion resistant but harder and cheaper than pure copper. Bronzes are sometimes used for the same purposes and are also used to make bearings. Bronzes are generally harder and more corrosion resistant than brasses.

Time: 40 minutes

Materials and Supplies:

3, pre-1982 penny

5, post-1983 pennies

powdered tin (Sn)

steel wool

hot plate or Bunsen burner

wire gauze

forceps or tongs

General Safety Guidelines:

Procedure I:

  1. Obtain five post-1983 pennies. Thoroughly clean them using the steel wool.

  2. Pre-heat the hot plate using the setting which is 75% of the maximum value. For Bunsen and wire gauze, place the Bunsen to produce maximum heating and pre-heat.

  3. Start timing and place four of the post-1983 pennies on the hot surface in a ring around the center.

  4. FOR HOT PLATE: Using forceps, remove one of the pennies at each of the following time intervals:

    FOR BUNSEN AND WIRE GAUZE: Use the following time intervals:

  5. While these pennies cool, place the pre-1982 penny on the hot plate for 10 minutes or wire gauze for 40 seconds.

Video Clip


Record the color and anything else you observe about the pennies.

Post-1983, no heat

Post-1983, 1 min/15 sec

Post-1983, 5 min/25 sec

Post-1983, 10 min/35 sec

Post-1983, 20 min/45 sec

Pre-1982, 10 min/40 sec

What causes the color variations among the coins?

Procedure II:


  1. Place 3 or 4 small grains of tin on the two remaining pre-1983 pennies. Place these pennies on the pre-heated surface.

  2. When one of the pennies develops a silver color in the area of the grains remove it from the surface. Remove the second penny in a time period equal to twice that of the first penny.


1. Describe how the zinc alloy (brass) differs from the tin alloy (bronze).


1. What is an alloy? What distinguishes an alloy from a compound?

2. Why did the color of the post-1983 penny change as you heated it longer?

3. What would happen if you heated other coins?

Teacher Notes:

Answers to Questions:

1. An alloy is a mixture (solution) of different types of atoms, generally metals. An alloy can have a variable composition, but a compound has a specific composition.

2. The color continued to change as more zinc diffused into the copper. Also, the longer it was heated, the more oxidation occurred.

3. If the coins were made of pure metal, oxide might form on the surface. If the metals were a mixture (dime or quarter), an alloy might form.

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