The Copper Test Tube

Introduction

Take a test tube that appears to be absolutely clear, add a few milliliters of solutions, swirl the test tube, and place it in hot water. Before the students’ very eyes, they will see a beautiful film of copper deposit on the test tube glass.

Concepts

• Oxidation–reduction
• Classification test

Materials

Safety Precautions

Concentrated nitric acid is corrosive, a strong oxidant and toxic by inhalation. Avoid contact with acetic acid and readily oxidized substances; especially glyoxal. Tin(II) chloride solution is an acidified solution. It is a corrosive, moderately toxic solution and should be treated with care since the solution contains hydrochloric acid. Silver nitrate solution is moderately toxic and irritating to body tissue. Avoid contact with eyes and skin. Fehling’s Solution A (copper(II) sulfate solution) is slightly toxic by ingestion. Fehling’s Solution B (potassium sodium tartrate solution) is caustic; avoid eye and skin exposure. Glyoxal solution, 40%, is irritating to skin and mucous membranes. A precipitate may form upon prolonged storage but can be redissolved by warming to 50–60 °C. Practice strict hygiene when using these materials. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Work in an efficient fume hood. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Fehling’s solution B may be neutralized according to Flinn Suggested Disposal Method #10. The silver nitrate may be precipitated according to Flinn Suggested Disposal Method #11. For disposal of the glyoxal solution see Flinn Suggested Disposal Method #18b. The nitric acid may be neutralized according to Flinn Suggested Disposal Method #24b. Fehling’s solution A and the tin(II) chloride solution may both be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Procedure

1. Using steel wool, thoroughly roughen the inside of a small test tube.
2. Wash the inside of the test tube with a few mL of concentrated nitric acid. Rinse the test tube well with distilled water.
3. Rinse the test tube with about 1 mL of the acidified tin(II) chloride solution. Pour this solution out.
4. Rinse the test tube with about 1 mL of the silver nitrate solution. Pour this solution out.
5. Add 15 mL of the Fehling’s Solution A (copper(II) sulfate solution) to 15 mL of the Fehling’s Solution B (potassium sodium tartrate solution) in a small beaker. Mix the two solutions. (See Tips.)
6. Place 4 mL of the resulting mixture into the test tube. Save the rest of the Fehling’s solution for demos later in the day. (See Tips.)
7. Add 1 mL of the 40% glyoxal solution to the test tube.
8. Place the test tube in a warm water bath (about 50 °C) and observe as the copper mirror develops in a few minutes. Keep the water bath below the boiling point (100 °C).
9. Pour out the reaction mixture and rinse with distilled water to observe copper mirror.

Student Worksheet PDF

12550_Student1.pdf

Teacher Tips

• Though it is somewhat difficult to do, it is important to roughen the inside of the test tube in order to get good results. One method is to scour the inside of the test tube using a wooden splint to manipulate the steel wool.z
• A precipitate will form when silver nitrate is added. The suspension will be poured out before the Fehling’s solution is added.
• If you are going to do all seven demonstrations on the same day, mix all of the Fehling’s Solution A (copper(II) sulfate solution) and Fehling’s Solution B (potassium sodium tartrate solution) in the morning. It will last all day, but no longer.Otherwise, mix only 2.0 mL of each solution to make the 4 mL needed for each demo.
• If the copper mirror forms quickly, the water bath may be too hot. Better results are achieved when the mirror forms slowly.

Answers to Questions

1. Describe what happened in this demonstration.
   

   A solution was placed in a clean, clear test tube. A small amount of another solution was then added to it. The test tube was heated in a warm water bath, and a copper, mirror-like film deposited on the glass.

2. In this demonstration, copper from the copper(II) sulfate solution (Fehling’s Solution A) is reduced to copper metal. Write a chemical equation for the reduction half-reaction.
   

   Cu²⁺ + 2e^– → Cu(s)

3. The inside of the test tube was scoured with steel wool and rinsed with nitric acid, acidified stannous chloride solution and silver nitrate solution. The steel wool and the nitric acid were used to make sure the test tube was completely clean. What do you think the purpose of the tin and silver solutions was?
   

   The purpose of the tin and the silver was to deposit a thin, silvery layer on the glass, where the copper deposited.

4. What is an oxidation/reduction reaction?
   

   An oxidation/reduction (or “redox”) reaction occurs when one or more electrons are transferred between molecules. Oxidation refers to a loss of electrons (and rise in oxidation state), and reduction refers to a gain of electrons (and subsequent decrease in oxidation state).

Discussion

In this demonstration, glyoxal, an aldehyde, is oxidized to glyoxylic acid, a carboxylic acid. Copper ions in the Fehling’s solution are reduced.

Fehling’s solution is used as a classification test to identify aldehydes. Normally, the reaction of an aldehyde with Fehling’s solution reduces Cu²⁺ to Cu(I), as in Cu₂O(s). However, at high concentrations of glyoxal further reduction occurs, producing the metallic copper. The steel wool and nitric acid are used to thoroughly clean the glass surface. The tin and silver solutions are then added to create a thin layer of silver on the glass. Copper then deposits on the silver layer, creating the copper mirror surface.

References

Special thanks to Jim and Julie Ealy, The Peddie School, Hightstown, NJ, who provided us with the instructions for this activity.

Hill, J. W., D. L. Foss, and L. W. Scott J. Chem. Educ., 1979, 56, 752.

Summerlin, L. R., C. L. Borford, J. B. Ealy, Chemical Demonstrations: A Sourcebook for Teachers. American Chemical Society:Washington, D.C., Vol. 2, 1988, pp 187–188.