Redox reaction demonstrated by fizzing
  Chemical polishing of a penny
 

Redox-with-fizz

Procedure:
1. Use a shallow battery jar or petri dish to contain about 0.73 grams of KMnO₄.  Fill the dish about half full of warm water to speed up the reaction and place the dish on an overhead projector.  The solution will be dark purple and very little light will pass through the solution.  Add 0.35 grams of Na₂C₂O₄ (sodium oxylate) incrementally with a small spatula.  If the dish is shallow you may see short-lived "clear spots" where the crystals enter the solution.  The students should be instructed to look across the top of the solution (From at least three feet away without eye protection!) to see "fizzing action".  If the solution is sufficiently warm (about 70 degrees Celsius) the CO₂ bubbles that form will fizz and "jump out of the solution". Using 0.35g of sodium oxylate should just reduce 0.73g of KMnO₄. Add a few more grains of sodium oxylate and the purple solution should become "clear water" with brown MnO on the bottom of the container.

What's Happening?
 
 2KMnO₄ + 5Na₂C₂O₄ + 6H₂0 --> 2MnO + 10CO₂ + 2KOH + 10 NaOH
 
 

50-40-10 Penny Polish

(Note: This experiment only works with pennies made *before* 1982. Pennies after 1982 are merely copper-coated zinc.)
 
    This is an example of "chemical polishing" that is sometimes used in the electronics industry to clean copper connections.  Note that some copper is actually dissolved, so the treatment should be very brief and followed by thorough rinsing with plenty of water.  Wearing eye protection, make up a solution in a 100 mL graduated clyinder of "50-40-10" by first placing 50 mL of water, then 40 mL of glacial acetic acid and finally 10 mL of concentrated nitric acid (HNO₃) into the cylinder (ALWAYS add acid to water; A --> W for safety). CAREFULLY pour this solution into a 400 mL beaker and place a dirty penny into the solution for about 15 seconds.  Retrieving the penny with tongs and immediately rinse the penny with copious amounts of water.  The penny should look like "new".  If several pennies are cleaned, the solution will gradually turn blue-green due to the formation of Cu(NO₃)₂.  For disposal of the solution, add at least 10 liters of water (more than two gallons) before flushing down the drain.
 

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