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What juice or liquid cleans pennies best?

by Ted Mooney <>

pennyUnited States Mint image

Guidance for your Science Fair project

1. You will probably hear the words "hypothesis", "variable", "control", "observations", "conclusions", "demonstrate" vs. "prove".

2. Do your experiment first, the research later. Why? Ever heard the term "Junk Science"? Junk science happens when you know the answer you want to get, so you use stupid but nice sounding reasons to throw away your contradictory observations, pretend to yourself that you didn't see some things, and you just keep at it until you get the answer you wanted by making the "rules" for the experiment as wacky as necessary :-)

That's not science, that's poison! As a young person trying to please the teacher, you will find it very difficult to fight the temptation to practice junk science if you know the answer that you think you're "supposed to get".

3. Don't call the brown color on pennies "rust"! It's not. "Rust" means iron oxide -- the corrosion product of steel or iron. There is no steel or iron in pennies (with the exception of pennies from 1943, which were steel with a coating of zinc because of the shortage of copper during WWII), so pennies can't rust.

4. Pennies before 1982 were solid copper, about 95-97 percent pure and the composition can be found on the website of the U.S. Mint at Pennies from 1983 and later are a zinc core with a thin copper plating (pennies from 1982 can be either solid copper or zinc core). Post-1982 pennies will behave funny if a liquid gets through a scratch or pinhole and reaches the zinc, so you should not mix the two types in an experiment. Use pre-1982 pennies if you can.

5. You are not really "cleaning" the pennies, you are dissolving the copper oxide "tarnish" on them, allowing it to wash away, exposing the underlying copper metal. This is important to note because things that are good at removing soils, like soap, detergent, and shampoo will be of no use in dissolving the copper tarnish. But some things that are poor cleaners like lemon juice plus salt (a mild acid), vinegar plus salt (a mild acid), and Coke & Pepsi (mild acids) will be good at removing the tarnish.

6. The acidity of the juice has a bit to do with it, but salt has a bigger effect. You can "clean" a penny a little bit and very slowly with lemon juice or vinegar (mild acids), but put a dash of salt in the lemon juice and the penny will turn orange with a quick rub. People say that ketchup and taco sauce are good cleaners for pennies, but read the ingredients: "Tomatoes, Vinegar, Salt . . . "

7. Your teacher probably doesn't fully understand this subject. It is very complicated to understand why salt plays such an important part in dissolving the tarnish, yet the salt won't work without the acid. One explanation, which is not exactly correct nor completely wrong, is that salt plus acid makes hydrochloric acid, which is a quite powerful acid.

8. The purpose of this experiment is not to get the "right" answer, because there isn't one! The strength of juices varies by season, and the country where they were grown, and the ripeness of each individual fruit. Plus, fruit juices contain hundreds of different chemicals that complex, chelate, sequester, buffer, and otherwise make the results of your experiment variable. Coke & Pepsi are secret formulations; we don't even know what is in them! Someone may claim that the acid in soft drinks is what is doing the tarnish removal, but when they don't even know what else is in them, how can that be anything but a guess?

9. What you should learn from the experiment is a piece of "the scientific method". Before you do anything else, get a notebook or composition pad for the experiment and number the pages so you won't be tempted to rip a page out if you later don't like what you wrote earlier. This is called a lab book. Then use a pen, not a pencil, because you don't want to be able to erase anything. Then write down everything you do in setting up the experiment, and everything you see, smell, hear, or otherwise observe. Keep jotting down the date and time as you do this. If you accidentally drop your chewing gum into the vinegar bowl, write it down because it might affect the results and be a relevant observation (how are we to know?). If you have written something that you think is completely wrong and you should remove it, strike it through once but leave it legible.

10. Remember the difference between "observations" and theories / explanations / hypotheses / conclusions. What sets observations apart from the rest? You can't change them; they are not opinions or guesses, they are facts! If you saw that your penny in vinegar was covered with tiny gas bubbles, it doesn't matter whether your classmates' pennies were or not, yours were. Period! As you rethink your theories and conclusions to account for what you've seen, you never go back and change an observation. Your lab book will hopefully get you an A whether your answers were what the teacher expected or not.

Here are some Q&A threads on the subject if you wish to read still more:
  • letter 779c - tarnish removal and prevention
  • letter 1293 - Lab technique & safety, cream of tartar, toothpaste, Microscrub, vinegar + salt, hot sauce, ketchup.
  • letter 2650 - bleach.
  • letter 2867 - cola, Sprite, lemon juice.
  • letter 4267 - soy sauce, ketchup, lemon juice + salt, hydrochloric acid, hot sauce.
  • letter 4361 - Zud, ketchup, vinegar, copper polish.
  • Letter 6675 - citric acid, lemon juice
  • Letter 6998 - pineapple juice, lemon juice, etc.
  • Letter 7181 - same old same old, left unanswered
  • Letter 7854 - gasoline, ammonia
  • Letter 8018 - pennies from the '50s
  • Letter 8159 - acids
  • Letter 8399 - Pepsi, ketchup, milk
  • Letter 8614 - the standard questions, left unanswered so far.
  • Letter 11111 - same old same old
  • Letter 12249 - the standard questions, left unanswered so far.
  • Letter 12356 - same old same old
  • Letter 12498 - apple juice, lemon juice
  • Letter 12741 - humorous question
  • Letter 12848 - baking soda, acids & bases, Tide, dish washing liquid.
  • Letter 13102 - baking soda
  • Letter 13443 - lime juice
  • Letter 14036 - things that won't work
  • Letter 14065 - household products
  • Letter 16630 - cleaning without removing 'the finish'
  • Letter 17518 - vinegar
  • Letter 17592 - forming an hypothesis
  • Letter 17593 - baking soda, spaghetti sauce, but mostly juices--again & again & again :-)
  • Letter 17830 - a professional chemist cleans pennies
  • Letter 17976 - ammonia
  • letter 196 - this letter is primarily about electroplating as a school science project, but some info on cleaning pennies is homogenized into it.

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