The Two-Cent Colorful Demonstration
Publication No. 13060
A myriad of colors is produced by several reactions occurring in one apparatus, and it all starts with two pennies.
Nitric acid, HNO3, 15.8 M, 500 mL*
Sodium hydroxide solution, NaOH, 0.1 M, 20 mL*
Universal indicator solution, 35 mL*
Water, distilled, 800 mL
Boiling flask (Florence flask), 1000-mL
Erlenmeyer flask, 500-mL
Glass tubing, 5 mm o.d., 3 pieces*†
Plastic tubing, 3/16" i.d., 1/16" wall thickness, 12 ft. length*
Pre-1982 pennies, 2
Stopper, 1-hole, #7*
Stopper, 2-hole, #8*
*Materials included in kit.
†See Prelab Preparation.
Nitric acid is corrosive to eyes, skin and other tissue; strong oxidant; toxic by inhalation; avoid contact with readily oxidized substances. Sodium hydroxide solutions are corrosive to eyes, skin and other tissue. Universal indicator solution is an alcohol-based solution and therefore flammable. Cupric nitrate solution is toxic. Nitrogen dioxide produced in the reaction is an irritant and toxic by inhalation. This demonstration must be done in a hood or well-ventilated area. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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 goverening the disposal of laboratory waste. The resulting product is an acidic soltion of cupric nitrate. It can be neutralized using sodium carbonate, then flushed down the drain with excess water according to Flinn Suggested Disposal Method #24b.
Assemble the tubing/stopper/glass elbows apparatus as shown in Figure 1. Lubrication of the holes of the stoppers with mineral oil will help with the insertion of the glass tubing elbows. Insert slowly and carefully, with a twisting motion. Wear heavy cloth gloves to help avoid cuts should the glass elbows break. The 12" piece of glass tubing should be pushed down so it is within one inch of the bottom of the flask.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesDeveloping and using models
Planning and carrying out investigations
Asking questions and defining problems
Disciplinary Core IdeasMS-PS1.A: Structure and Properties of Matter
MS-PS1.B: Chemical Reactions
HS-PS1.A: Structure and Properties of Matter
HS-PS1.B: Chemical Reactions
Cause and effect
Structure and function
Stability and change
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
The copper pennies react with the nitric acid to produce nitrogen(II) oxide.
3Cu(s) + 2NO3–(aq) + 8H+(aq) → 3Cu2+(aq) + 4H2O(l) + 2NO(g)The nitrogen(II) oxide quickly reacts with oxygen to form nitrogen dioxide, a red-brown gas.
2NO(g) + O2(g) → 2NO2(g)As the NO2 gas pressure increases, it bubbles through the basic (blue) solution in the boiling flask. Nitrogen dioxide is very soluble in water. As it dissolves, it reacts with the water to form nitric acid, which lowers the pH of the solution as it is formed.
2NO2(g) + H2O(aq) → 2HNO3(aq)As the solution changes from basic to acidic, the color of the universal indicator changes from blue to pink/yellow.
When the copper pennies are completely dissolved, the reaction stops. As the Erlenmeyer flask cools, the pressure inside the flask decreases. This lower pressure draws the solution from the boiling flask into the Erlenmeyer flask. As the liquid flows into the Erlenmeyer flask, it dilutes the cupric nitrate solution that was formed by the initial reaction of copper with nitric acid. The Cu2+ ions color this solution a bright blue.
Special thanks to Pat Funk of Watkins Memorial High School in Pataskala, OH, for bringing this demo to our attention.