Pennies
Materials:
April 26th and 27th, 2011
10 – 50+ pennies from before 1982 (have at least one per group)
Small bottle of white vinegar
Table salt
Clear plastic cups
Paper towels/napkins
Steel screw/nails/bolts
GLOVES!
Goals: To explore the properties of metals, specifically oxidation and surface interactions
between oppositely charged metal ions.
INTRODCUTION
Q: What are pennies made from?
A: Copper/copper-zinc
Select one clean penny and one dirty penny and then ask the following questions.
Q: Why do these pennies look different?
A: Regular dirt, but also oxidation (point out green points of the pennies); oxidation occurs when
metals are exposed to air.
Q: Do only metals oxidize?
A: No, but it is easy to see this process over time because of the color change to the metal that
results from losing one or more electrons.
EXPERIMENTS
Adapted from Chemistry Fun with Pennies – Learn about Metals by Anne Marie Helmenstine,
PH.D. (Chemistry Fun with Pennies)
CLEANING PENNIES
1. Add a teaspoon of salt into ~ ¼ cup of vinegar (all in plastic cup).
2. Stir until the salt dissolves.
3. Dip a penny halfway into the liquid and hold it there for 10-20 seconds.
4. Remove the penny from the liquid. What do you see?
DISCUSSION: What did you see? What was happening? Explain that the copper oxide on the
outside of the penny was removed and revealed a cleaner layer underneath. Did we reverse the
oxidation reaction? No, again when you place the pennies in the salt and vinegar solution, the
acetic acid from the vinegar dissolves the copper oxide, leaving behind shiny clean pennies. The
copper from the copper oxide stays in the liquid, there is no reaction here that is producing
copper.
INSTANT VERDIGRIS
1. Dump the rest of the pennies into the liquid. The cleaning action will be visible for
several seconds.
2. Leave the pennies in the liquid for ~5 minutes.
3. Pass out paper towels and have them write RINSE on one paper towel and NO RINSE on
the second paper towel.
4. Take half of the pennies out of the liquid and place them on the paper towel labeled NO
RINSE.
Pennies
April 26th and 27th, 2011
5. Remove the rest of the pennies and rinse them well under running water. Place these
pennies on the paper towel labeled RINSE.
6. Allow about an hour to pass and take a look at the pennies you have placed on the paper
towels; move on to last experiment and then come back to finish the discussion.
DISCUSSION: What happened? You should see some green verdigris on the ones that you did
not rinse with water. Rinsing the pennies with water stops the reaction between the salt/vinegar
and the pennies. They will slowly turn dull again over time, but not quickly enough for you to
watch! On the other hand, the salt/vinegar residue on the unrinsed pennies promotes a copper
oxidation, resulting in the blue-green copper oxide commonly called 'verdigris'.
COPPER PLATED NUTS AND BOLTS
1. Place a steel object so that it is half in and half out of the solution used to clean the
pennies; this will not work in clean vinegar because there are no copper ions available!
2. Completely immerse another steel object into solution used to clean pennies and an
identical object into new vinegar and/or one used to clean newer pennies
3. Allow 10 minutes to pass and then take a look at the nail/screw.
DISCUSSION: What did you see when you first put in the object into solution? Bubbles in
‘contaminated’ solution and nothing in vinegar only solution. What did you see when you took
out the object – is it two colors, why or why not? What is happening? The copper that coats the
nail/screw comes from the pennies cleaned in the first experiment. However, it exists in the
salt/vinegar solution as positively charged copper ions. Steel objects are mixed metals (consist of
different elements) containing iron. The salt/vinegar solution dissolves some of the iron and its
oxides on the surface of the nail, leaving a negative charge on the surface of the nail. Opposite
charges attract, but the copper ions are more strongly attracted to the nail than the iron ions, so a
copper coating forms on the nail. At the same time, the reactions involving the hydrogen ions
from the acid and the metal/oxides produce some hydrogen gas, which bubbles up from the site
of the reaction - the surface of the steel object.