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When concentrated nitric acid is dropped on a copper disk in the bottom of a glass cylinder, a reddish-brown cloud of nitrogen dioxide gas is rapidly evolved.

Curriculum Notes 

This demo illustrates the strong oxidizing power of nitric acid. It can be used when redox reactions are being introduced or later on when students are trying to predict reaction products. It illustrates that nitric acid not only reacts through the "acid" pathway, but it can also act as an oxidizing agent. Some instructors like to use this demo, along with the accompanying narrative by Ira Remsen, during the first lecture that accompanies a general chemistry lab course to illustrate the nature of chemical experimentation. Allow about 5 minutes for this demo.

One day of lead time is required for this project.

Discussion 

Nitric acid reacts with copper according to the reaction:

$\ce{4HNO3(l) + Cu(s) -> Cu(NO3)2(s \,and\,aq) + 2NO2(g) + 2H2O(l)}$

The copper nitrate salt that forms is a deep blue color. The nitrogen dioxide is a maroon vapor.

Materials 

• glass cylinder
• copper disk
• dropper bottle of concentrated nitric acid
• 100 mm watch glass

Procedure 

Place the copper disk in the bottom of the glass cylinder. Carefully drop a few drops of nitric acid onto the disk. Cover the cylinder with the watch glass to prevent the escape of nitrogen dioxide gas.

Safety Precautions 

"Concentrated nitric acid is a strong acid and a strongly oxidizing acid. Ingestion causes burning and corrosion of internal tissues."¹ If you spill concentrated nitric acid on your skin you should wash it off immediately with a large quantity of water. "Nitrogen dioxide forms acid on contact with moisture and is a poisonous, choking gas. Exposure to nitrogen dioxide may cause only slight pain or even pass unnoticed, but several days later may result in edema and cause death."¹

Practice this demo under a vent hood before performing it in front of a class. If too much nitric acid is added, nitrogen dioxide fumes will escape into the classroom.

Be sure to place the watch glass over the glass cylinder after adding the nitric acid to prevent the escape of nitrogen dioxide gas.

¹Division of Chemical Education, Inc., American Chemical Society, "Copper Penny with Concentrated Nitric Acid - Demonstration Notes, Warnings, Safety Information" Chemistry Comes Alive, 1999, 6 Jul. 2005,

Prep. Notes 

Be careful not to inhale the nitrogen dioxide fumes when breaking down this demo. Carefully prepare the dropper bottle for each demo and carefully return the unused nitric acid to the bottle after the demo. Concentrated nitric acid should not be stored in a dropper bottle for extended periods of time because it corrodes the rubber parts of the dropper.