Introduction

A copper penny appears to turn to silver after heating in an evaporating dish with a mixture.  The penny then appears to turn to gold when heated in a flame.

In evaporating dish:

Zn + NaOH → Na2ZnO2

                                                                                                            sodium zincate

Cu in penny reduces Na2ZnO2 to metallic zinc:

Cu + Zn2+ → Cu2+ + Zn

Heat causes a fusion of zinc and copper to form brass (gold-colored).  Brass is 60-82% Cu and 18-40% Zn.

Three pennies in various stages of transformation

To Conduct Demonstration

  1.  Heat NaOH/Zn mixture to near boiling in a crucible (3.5 in in diameter with handle)  on a hot plate.
  2. Place one or more cleaned pennies into the mixture and continue to warm for 3 or 4 minutes.
  3. Remove the pennies when they are completely and evenly coated with zinc.  Wash them and blot them dry.
  4. Using tongs, hold each penny in a Bunsen burner flame (3-4 sec.). Students in first few rows report being able to see the zinc turn grey while in the flame.
  5. Wash and dry the "gold" penny.

 Demo Time: ~ 5 minutes

Notes:

  1. Clean the pennies in concentrated HCl.  Watch zinc filled pennies (after 1983) to make sure the HCl doesn’t get to the zinc and dissolve it.
  2. Stir NaOH/Zn mixture with a glass rod.
  3. Zn will clump up on the bottom of the crucible in the NaOH.  The Zn compound can stick to the pennies, so experiment with the placement of the pennies to minimize this occurrence.
  4. This demonstration is not recommended for large audience viewing unless a video camera or document camera can be used to project the demonstration on the screen.

 Safety 

Safety:  Weigh the zinc dust in a hood.  Heat sodium hydroxide and zinc dust using the ventilation grating in Chem 140 or in a chemical fume hood. Use tongs to handle the pennies.  Use caution with hot sodium hydroxide and hot pennies.  Brass pennies are safe to handle (when cool) and may be given away to students.

Reference:

L.R. Summerlin and J.L. Ealy, Chemical Demonstrations: A Sourcebook for Teachers, 1985.