Key concepts
Chemistry
Chemic reactions
Acerbity
Oxidation

Introduction
"Encounter a penny, pick it upward—all day long you lot'll have good luck!" Peradventure yous've heard this phrase earlier, and maybe y'all've even stopped to pick upward a lucky penny off the sidewalk. But sometimes those pennies you see on the ground expect anything but lucky. They appear brown or black, and sometimes they're so dirty looking you lot can't even tell whether they're pennies!

In this activity we're going to explore why pennies don't stay bright and shiny, and test different methods to bring their polish back!

Background
All pennies showtime out the aforementioned color—bright copper. But somewhere forth the way, those pennies lose their luster. What happens?

The answer is pretty elementary: Although copper is a shiny metallic, it is also reactive. The negatively charged oxygen atoms in our air are attracted to the positively charged copper atoms in the penny. When oxygen binds with copper, they course a new molecule known as copper oxide. Copper oxide is brownish or sometimes blackness in color (depending on other things in the penny's environs). This is why almost pennies yous see look dirty or tarnished—it's not really clay only copper oxide that makes them expect so dull.

Luckily, at that place are a number of ways to clean pennies and make them look vivid and shiny again—using chemical science! Nosotros're going to examination a few dissimilar methods for cleaning pennies and determine which gets our pennies the cleanest.

Materials

  • Set of measuring spoons
  • 10 very tarnished pennies (The more tarnished the better—and the activity works all-time with pennies that are about equally tarnished.)
  • Ii tablespoons of baking soda
  • 4 teaspoons of salt
  • Two tablespoons of white vinegar
  • Two tablespoons of freshly squeezed lemon juice
  • Two tablespoons of ketchup
  • Two tablespoons of Tabasco sauce (or other hot sauce)
  • xi plastic cups (Minor Tupperware containers or bowls volition work as well.)
  • Paper
  • Water
  • Pen or pencil
  • Dishwashing gloves
  • Permanent marker
  • Articulate tape
  • A timer or clock
  • Paper towels
  • Access to a sink
  • Camera (optional)

Preparation

  • Make a paste from your baking soda and water: Pour one quarter cup of baking soda into one of your plastic cups. Slowly add together water, 1 tablespoon at fourth dimension, until you've created a paste.
  • Use your permanent marker to label your plastic cups equally described below. If using bowls or Tupperware, write your characterization on articulate plastic record and gently adhere it.

Cup labels—on each of the remaining cups place a label as follows:
Ketchup i
Ketchup v
Tabasco 1
Tabasco 5
Vinegar 1
Vinegar 5
Lemon 1
Lemon v
Control 1
Control five

  • Put one tablespoon of ketchup into each "Ketchup" cup. Put one tablespoon of Tabasco into each "Tabasco" cup. Put one tablespoon of vinegar and ane teaspoon of table salt into each "Vinegar" cup. Put one tablespoon of lemon juice and ane teaspoon of salt into each "Lemon" cup. Leave the "Control" cups empty.
  • Use your paper and pencil to create a table with iii columns and 6 rows that lists your 5 test substances (ketchup, Tabasco sauce, vinegar, lemon juice and command) in a column on the left. Label this column "Cleaning Method." The next column, label "ane Infinitesimal." And label the third column "5 Minutes." Make each cell (square) of the table big enough to place your penny in it after your testing is complete. Practice you lot have a hypothesis near which substance will best clean a tarnished penny? Why?


Procedure

  • Place a penny on each square of your tabular array, heads side upwardly. If you accept a camera, take a motion picture of the pennies. If yous don't have a camera, look closely at the colour of each penny. This will permit you to make a better judgment of how well each method cleaned, by looking at the pennies before and after their treatment. What do you find about each penny? Tin you clearly see the date on the penny or is it too dirty? Can you clearly meet Abraham Lincoln's contour?
  • Start with the cups labeled "i." Line these cups up in a row. Have the penny from the "Ketchup, 1 Infinitesimal" square of your table, and place it in the "Ketchup 1" cup. Gently swirl the cup so the penny is completely covered with ketchup.
  • Start a one-infinitesimal timer.
  • Put on your dishwashing gloves.
  • When one infinitesimal has passed, rinse the cup with water until its within and the penny is clean. (Be careful not to lose your penny.) Keep the penny in the loving cup.
  • Repeat this procedure with each of the other pennies in the "ane Minute" column, in plough placing each into their corresponding cup. In each example, brand sure the penny is completely covered by the contents of the loving cup.
  • For the "Control, 1 Minute" penny exercise not add together anything to the cup. This is your control. Complete the full process.
  • One time all of the "1 Minute" pennies take been rinsed, use your fingers to gently scrub the pennies one at a time with your baking soda paste. Keep your gloves on for this part, existence conscientious to continue each penny with its labeled loving cup. Scrub each penny for 10 seconds, then rinse it again with water and place it back into its cup.
  • Fix these cups aside for now.
  • Echo these steps with the pennies in your "5 Minutes" column. Start by placing the "Ketchup, 5 Minutes" penny into the loving cup labeled "Ketchup 5." Once again, make sure the penny is completely covered by ketchup.
  • Offset a timer for five minutes.
  • Continue to identify each penny in the "5 Minutes" column in its corresponding cup.
  • Once all the pennies are in their correct cups, employ the remaining time to discover the pennies and the contents of the cups. Do you lot notice any changes in the appearance of the pennies? What is irresolute? Do y'all observe any changes in the contents of the cups? Is anything bubbling or changing color?
  • Once five minutes take passed, rinse the cups with water until their insides and the pennies are clean. (Be conscientious non to lose your pennies). Keep the pennies in the cups.
  • In one case all of the pennies take been rinsed, use your fingers to gently scrub the pennies one at a fourth dimension with your baking soda paste. (Once more, continue your gloves on for this part.) Scrub each penny for 10 seconds, so rinse it once again with h2o and identify it back into its loving cup.
  • Become back to the table you drew. Apply your newspaper towels to gently dry off each penny then place it in the corresponding square of your tabular array.
  • Compare the "1 Minute" and "v Minutes" pennies in each row. How does time touch the cleanliness of the pennies? Was i or 5 minutes more than effective in cleaning the pennies overall? Can you lot think of why time would be important? Were there whatever cleaning methods where the time didn't affair, so the "one Infinitesimal" and "5 Minutes" pennies are equally clean? Why do yous retrieve this might exist?
  • Compare all of the cleaned pennies with the command ones. Which penny looks the cleanest compared with the control? Which is still the most tarnished?
  • If you took a picture at the starting time, go back and compare each penny'south cleanliness earlier and after being cleaned. Which penny is the cleanest compared with its earlier motion picture? Which is the least make clean?
  • Extra: Test fifty-fifty longer periods of time with each of the cleaning methods. What is the optimal time to exit the pennies in the cleaning solution?
  • Extra: Test whether these cleaning solutions work on other types of coins. What do your results tell yous about the special properties of copper pennies?
  • Extra: Can you think of other safe household acids you could exam? What near soda? Orange juice?


Observations and results
In this activity yous tested four different cleaning methods to decide which was the virtually effective at cleaning pennies. Y'all should have observed that each cleaning method fabricated the pennies cleaner but some methods may accept been more successful than others. Which method works best is determined past the amount of acid and the presence of salt. Dissimilar brands of ketchup and hot sauce will have different amounts of table salt and acid in their ingredients, and so your results might be unlike than someone else doing this activity at their house. In each case, even so, the cleaning method that should accept worked best for you is the ane that has the highest concentration of acid.

As they are exposed to the environment, pennies become coated with a layer of copper oxide, making them look tarnished (with a irksome, brownish, dirty advent). Copper oxide dissolves in a mixture of weak acid and table salt. Vinegar and lemon juice are both acids. Bank check the ingredient list for your ketchup and Tabasco sauce—some of the labels might listing vinegar or citric acid whereas others might say love apple puree. Tomatoes contain some naturally occurring citric acid, some other acrid that is generally slightly weaker than vinegar or the amount of citric acrid in lemon juice. Therefore, each cleaning method you tested contained acid and common salt simply the ketchup and Tabasco sauce may accept had a slightly weaker acid. In this case, you might have noticed the ketchup and Tabasco pennies were not quite as well cleaned every bit the pennies that were in the vinegar and lemon juice.

You likewise might accept as well noticed the longer the pennies stayed in the cleaning solution, the cleaner-looking they became! The chemical reaction that dissolves the copper oxide on your pennies is an ongoing process. Leaving your pennies in the cleaning solution longer gives the reaction more than time to go on, and gives yous shinier pennies!

More to explore
From Dull to Dazzling: Using Pennies to Test How pH Affects Copper Corrosion, from Science Buddies
Rusting Out: How Acids Touch on the Charge per unit of Corrosion, from Science Buddies
Cabbage Chemistry—Finding Acids and Bases, from Scientific American
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies