ALCHEMY LAB: THE GOLDEN DREAM

***Teacher note: Because most classrooms do not have enough fume hoods for all students to perform the lab, I generally have students bring in pennies and line up by the fume hood. I heat the pennies (a couple at a time) in the NaOH and zinc mixture for them. When the pennies look “silver”, I take them out and drop them in a beaker of water and give the beaker to the student. The student then takes the penny to the lab station and completes the rest of the procedure.

INTRODUCTION: METALS AND ALLOYS IN OUR LIVES
Metallic elements silver (Ag), gold (Au) and copper (Cu) are lustrous, malleable, ductile and conductive. For millennia, each has been used to make sculptures, jewelry and structures large and small. Artists often work with alloys like bronze, steel or brass because of their durability and color. Bronze is a mixture of copper and tin; brass, made in the activity below, is a mixture of zinc and copper. Steel is made of iron and carbon. Alloys are used to make coins, jewelry, sculptures and other items. Alchemists in the Middle Ages believed they could turn ordinary metals into gold. In this lab, you become a 20th-century alchemist and turn pennies into shiny "silver" and "gold".
OBJECTIVE: To observe how an alloy is made.

MATERIALS

3 M NaOH Zincbeakerhot plate

tongs 200 mL beaker of waterpenny Bunsen burner

Safety

NaOH should be handled with great care. It is corrosive and can burn skin. Zinc should not be inhaled. Safety goggles should be worn the entire time. Perform in a fume hood.

PROCEDURE

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  1. Place 5-6 pieces of zinc in one of your beakers
  2. Add 50 mL of NaOH solution to the dish, on top of the zinc.
  3. Set hot plate to medium heat (5) and place the beaker on top. If you are working with a hot plate that has no dial- you must immediately pull out the plug once your liquid starts to bubble even if the 5 minutes is not up!
  4. Heat for 5 minutes. Do not boil! When dish is hot, place a penny in it. Heat for two minutes or until the penny is coated and becomes silver in appearance.

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  1. Remove the penny from the dish with tongs and drop into water.
Gently wipe to remove excess water. CAREFUL! The penny will be hot!
/ 6. Using tongs, hold the penny in the flame of a Bunsen burner and gently heat. The penny should turn "gold" (brass). (Do not overheat the penny.)
/ 7. Dip the penny in the beaker of water and cool to touch. Be prepared for a sizzle sound.
If time permits, you may make more pennies.
When completely done, put the NaOH solution in the waste beaker.

REACTIONS:

  1. The first reaction involves the plating of the penny with a coating of zinc. The granular zinc reacts with the hot sodium hydroxide solution to form soluble sodium zincate, Na2ZnO2. This substance is then converted to metallic zinc when it contacts the surface of the penny. This results in a loosely held zinc coating on the surface of the copper.
  1. The second reaction involves the heating of the penny to fuse together the zinc and copper metals to give an alloy (homogeneous metal mixture) known as brass. Brass varies from 60-82% Cu and from 18-40% Zn.

POST-LAB QUESTIONS:

  1. Why did the penny turn silver after a few minutes in the hot solution?
  1. Why did the penny turn gold when heated in the burner flame?
  1. What is an alloy?
  1. What is the name of the alloy produced in this activity? What two metals are fused together in this gold-colored alloy?
  1. Is it possible to turn metals into gold? In other words, is alchemy possible?

Alchemy_Demo_or_lab

Updated On: 10/25/2018

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