Three interconnected flasks each containing colorless liquids yield red, white and blue solutions when a coin is added to the first flask.
This demonstration is particularly useful for practicing observation skills. It is appropriately performed during lectures when redox reactions are being introduced, but also involves acid base reactions as well as other concepts including solubilities of gases, ideal gas laws, indicators, neutralization, dilution effects, and the formation of a colored transition metal complex. Students should be encouraged to speculate on the contents of the various flasks and what reactions account for their observations.
Explanation of Experiment:
Three flasks are arranged side by side, clamped to ring stands, and connected with glass tubing. All three contain colorless liquids - the one on the left is half filled, the one in the middle is completely filled, and the one on the right contains only a small amount of liquid. When a pre 1982 copper penny is added to the flask on the right, a sequence of visible changes takes place. The sequence ends after ten to twenty minutes with each flask half filled, one with a blue liquid, one with a colorless liquid, and the last with a red liquid.
This procedure uses the reaction of copper with nitric acid to initiate a series of chemical and physical processes that last for nearly 20 minutes. When the copper penny is dropped into a flask containing nitric acid, the flask fills with a red-brown gas, nitrogen dioxide. As gas is produced by the reaction, the pressure in the flask builds. The increased pressure forces half of the 0.10 M HNO3 solution containing phenolphthalein into the flask containing 0.35 M NaOH. As the phenolphthalein mixes with the NaOH solution, it turns red. As the reaction of copper with nitric acid nears completion, the reaction slows and the flask cools. In addition, the very soluble NO2 produced by the reaction has entered the middle flask, where it dissolves in the 0.10 M HNO3. The combined effect of the cooling flask and the dissolving gas is to lower the pressure in the stoppered flasks. This causes the liquid from the open flask to be forced back into the middle flask. As the red liquid mixes with the acid, the phenolphthalein returns to colorless. When the middle flask is filled with liquid, the liquid flows into the flask that contained the copper penny. This liquid dilutes the solution of copper nitrate formed by the reaction of copper with nitric acid. The copper nitrate colors this solution blue.
Cu(s) + 4 HNO3(aq) ---> Cu(NO3)2(aq) + 2 NO2(g) + 2 H2O(l)
The solution formed when copper reacts with nitric acid is blue because of the blue [Cu(H2O)4]2+ ion in solution.
200 mL 0.35 M sodium hydroxide, NaOH (To prepare one liter of solution, dissolve 14 g of NaOH in 600 mL of distilled water and dilute the resulting solution to 1.0 L.
1 mL phenophthalein indicator solution
ca. 570 mL 0.10 M nitric acid, HNO3 (To prepare one liter of solution, pour 6.3 mL of concentrated (16 M) HNO3 into 600 mL of distilled water and dilute the resulting solution to 1.0 L.
15 mL concentrated (16 M) nitric acid, HNO3.
One U.S. copper cent dated prior to 1983 (ca. 3 g). Do not use a post-1982 copper-clad zinc cent, because it will cause the reaction to become too vigorous and difficult to control.
Three 500 mL Erlenmeyer flasks
Three ring stands with buret clamps.
Drop a pre 1982 copper penny into the flask containing the concentrated nitric acid (flask 3), and reseat the stopper firmly in its mouth. Immediately, a red-brown gas is formed in flask 3, and liquid flows from flask 2 into flask 1, turning red. After several minutes, red-brown gas fills flask 3 and liquid stops flowing from flask 2. As the red liquid from flask 1 enters flask 2, it becomes colorless. The solution in flask 3 is blue.
Concentrated nitric acid is both a strong acid and a powerful oxidizing agent. Contact with combustible materials can cause fires. Contact with the skin can result in severe burns. The vapor irritates the respiratory system, eyes, and other mucuous membranes.
Hydrochloric acid can cause severe burns. Hydrochloric acid vapors are extremely irritating to the respiratory system, eyes, and skin.
Sodium hydroxide can cause severe burns of the eyes, skin, and mucuous membranes. Dust from solid sodium hydroxide is very irritating to the eyes and respiratory system.
The brown gas produced in the reactions is nitrogen dioxide. Nitrogen dioxide is irritating to the respiratory system - inhaling it can result in severe pulmonary irritation which is not apparent until several hours after exposure. A concentration of 100 ppm is dangerous even for a short period of time, and exposure to concentrations of 200 ppm or more can be fatal.
The waste solutions should be neutralized by adding sodium bicarbonate (NaHCO3) until fizzing stops, and the neutralized solutions should be flushed down the drain with water.
- Acids and Bases
Type of Reaction:
- Acid Base
- Gas Evolution
- Shakhashiri, B.Z. 1983, Chemical Demonstrations – A Handbook for Teachers of Chemistry, vol. 3 pp. 83-91.