The Copper Test Tube
Publication No. 12550
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.
(for each demonstration)
Fehling’s solution A, 2 mL*
Fehling’s solution B, 2 mL*
Glyoxal, HCOCHO, 40% in H2O, 1 mL*
Nitric acid, HNO3, conc., 2 mL*
Silver nitrate solution, AgNO3, 1%, 1 mL*
Tin(II) chloride solution, SnCl2,0.05 M in 6 M HCl, 1 mL*
Water, distilled or deionized
Beakers, 50-mL and 250-mL
Cylinder, graduated, 10-mL
Test tubes, small, 13 x100 mm, 7*
Test tube holder
*Materials included in kit.
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.
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.
Student Worksheet PDF
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesAnalyzing and interpreting data
Disciplinary Core IdeasMS-PS1.A: Structure and Properties of Matter
MS-PS1.B: Chemical Reactions
HS-PS1.A: Structure and Properties of Matter
MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
Answers to Questions
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.
Special thanks to Jim and Julie Ealy, The Peddie School, Hightstown, NJ, who provided us with the instructions for this activity.