The Secret of the Salsa
Teacher’s Instructions
Teaching objectives:
 Students will observe and describe how the physical
properties of a penny change when the penny combines
chemically with salsa
 Students will come up with hypotheses and interpret
class data to make further predictions about what
substances help to clean pennies
Recommended Grades: 3-5
Estimated Time: 1-2 hours
Key concepts and terms:
Chemical reaction: a process in which one substance is
converted into a new substance with new properties
Physical property: a characteristic of a substance that can
be observed. (ex. color, taste, texture, density).
Procedure:
1. Talk to your class about salsa and/or taco sauce. What
chemical properties does it have? What makes it taste
spicy? What are the main ingredients? Present the idea
of using salsa to clean pennies. Have students form
into groups of 4-5 at tables.
2. Distribute 16-20 tarnished pennies to each group (4
per student). Have each group designate one penny as a
“control” that will not be used to test anything.
Allow students to tape control pennies on Worksheet.
3. Tell groups to smear one penny with taco sauce in a
bowl. The entire penny should be covered. The taco
sauce should clean the penny sufficiently in 8
minutes. While waiting, ask the class whether a
chemical or physical change is taking place.
Brainstorm about what ingredient in the salsa cleans
the penny, and why. State that the essential
ingredients that make up salsa are tomatoes, salt, and
Created by Katy Harmon
January 21, 2008
vinegar. Each group should hypothesize which
ingredient helps to clean up the penny.
4. At the end of the 8 minutes, have students dry their
pennies and tape them to their worksheets. Ask each
group to test either salt, tomato paste, or vinegar,
making sure every ingredient is tested by at least one
group. In a new bowl, completely cover the
experimental penny with the ingredient of choice.
While waiting, talk about why old pennies get
tarnished (See Why it works section) .
5. Nothing worked very well! At the end of the 8 minutes,
have students dry their pennies and tape them to their
worksheets. Ask a member for each group to report what
happened to their penny. Make a table on the board
with results (see Sample Table).
6. Hypothesize with the class about what happened.
Suggest that a mixture of some of the ingredients
together might work. How can you mix together each of
the three ingredients? Ask each group to test either
salt and vinegar, salt and tomato paste, or tomato
paste and vinegar, making sure every ingredient is
tested by at least one group. While waiting, have
students write down their predictions on the
worksheet. Which of the combinations is going to work
the best?
7. After another 8 minutes have passed, have students
wipe off pennies and tape them to their worksheets.
Ask a member for each group to report what happened to
their penny. Fill in the table on the board (or draw
another one) with results.
8. Explain how salt and vinegar work together to remove
the tarnish on pennies. Tomato paste contains acid.
Ask the class if they can explain the results.
9. If time permits, have the class design an experiment
with a variety of hot sauces, Tabasco sauce, and
ketchup. Which do they predict will work best? What
actually happens? What does that mean about the
ingredients in those sauces?
Why it works:
Created by Katy Harmon
January 21, 2008
What is copper tarnish?
Copper tarnishes in a number of ways. Carbon dioxide in
moist air creates a layer of copper carbonate of a greenish
color, like we see on the Statue of Liberty. Household
copper, like a penny, tends to become coated with a black
or brown film of copper oxide (CuO) or copper sulfide
(CuS). We say this tarnished form of copper has an
oxidation state of 2. This copper does not react with salt.
The Secret of the Salsa
Vinegar, also known as acetic acid, can dissolve the CuO
and CuS typical of copper tarnish. However, this reaction
takes time. The secret of the salsa is that it contains
both vinegar and salt (NaCl, or sodium chloride). A small
amount of the tarnished copper on the surface is partially
dissolved by the vinegar. We say the copper that is
dissolved has an oxidation state of 1, meaning it can react
with the salt to become CuCl2-. This allows more tarnished
copper to react with vinegar, which in turn reacts with the
salt. The CuCl2- is aqueous and is in the solution, rather
than on the penny, leaving the bright copper metal beneath
it exposed. This means that a combination of vinegar and
salt can quickly, effectively remove the tarnish on
pennies.
Why does a mixture with a lot of salt and a little vinegar
make pennies look dull?
Salt is good at corroding things. Too much salt, or
leaving pennies in the salt and vinegar mixture for a week
or so, causes pennies to start corroding again, even if
initially they appear shiny. For this reason, you may want
to have students take pennies out of the salt/vinegar
solution as soon as you notice a change.
Why does a combination of tomato paste and salt clean
pennies?
Most tomato paste contains acid, which like vinegar is able
to begin dissolving the tarnished copper on the surface.
Sample Table:
Penny topping
Nothing (control
penny)
Salsa
Tomato Paste
Predicted Result
Actual Result
Created by Katy Harmon
January 21, 2008
Salt
Vinegar
Tomato paste + Salt
Tomato paste + Vinegar
Vinegar + Salt
Supplementary exercises:
1. Try leaving pennies in vinegar for a few hours or
overnight. The pennies should be cleaned just as
effectively as the salt/vinegar solution. Ask students
salt is necessary for the reaction. What does the
experiment suggest about vinegar?
if
2. Leave pennies in the vinegar/salt solution for a week or
so. You should start to see signs of corrosion or a general
dullness to the coin. Ask students what they think has
happened. Does the vinegar/salt solution clean pennies
effectively? Why might time make a difference in the
reaction?
REFERENCES
1.
Rosenhein, L. (2001). The Household Chemistry of
Cleaning Pennies.
J. Chem. Ed. 78, 513-515.
2.
Cleaning pennies with taco sauce. Online:
http://cruftbox.com/cruft/docs/cleaningcopper.html.
3.
Steve Spangler Science. Online:
http://www.stevespanglerscience.com.
The Secret of the Salsa
Materials List

~120 brown, tarnished pennies (about 4 for each
student, plus a few extra)

Mild Taco Bell Taco Sauce (others can be used, but
this works well)
Created by Katy Harmon
January 21, 2008

Tomato paste (without added salt)

A mixture of tomato paste and salt (2:1 ratio)

A mixture of tomato paste and vinegar (1:1 ratio)

A mixture of vinegar and salt (in solution)

White Vinegar

Salt

Ketchup (optional)

Tabasco sauce (optional)

A variety of other taco sauces/salsas (optional)

Tape

Plastic white plates

Plastic bowls

Timer

Water and paper towels for cleanup
Name __________________
Worksheet
The Secret of the Salsa
Created by Katy Harmon
January 21, 2008
Experimental
conditions
Sample
penny
What happened?
Control
Taco Sauce
Is this a chemical or physical change?
___________________________
Which salsa ingredient do you think turns the penny
shiny, and why?
___________________________________________________
___________________________________________________
____________________
___________________________________________________
__________
Experimental
conditions
Sample
penny
What happened?
Penny 1:
______________
1
The Secret of the Salsa
Worksheet
Created by Katy Harmon
January 21, 2008
Which mixture do you predict will clean the penny
best?
Experimental
conditions
Sample
penny
What happened?
Penny 2:
______________
Based on the class results, which two ingredients
form the best mixture to clean a penny?
___________________________________________________
___________________________________________________
___________________________________________________
______________________________
Bonus Question: You have a hundred dirty pennies.
You want to clean them up, but you don’t have any
salsa. Which of the following items would clean
your penny the best? Circle the best answer.
Italian Salad Dressing (contains vinegar)
Ketchup (contains salt, vinegar, and tomatoes)
Tomato sauce (contains tomatoes and salt)
Chicken noodle soup (contains salt)
2
Created by Katy Harmon
January 21, 2008