In the Classroom edited by
tested demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
The Sodium Rainbow Submitted by:
Miroslav Prokˇsa Department of Education in Natural Sciences, Comenius University, 842 15 Bratislava, Slovakia
Checked by:
Cole McWherter and George Gilbert Department of Chemistry, Denison University, Granville, OH 43023
The reaction of sodium with water provides for some of the most popular demonstrations (1–3). The following is a new approach to an older demonstration. We would like to report our use of this simple, but impressive, reaction in the “rainbow” demonstration (4–6). Materials Sodium metal Paint thinner (or toluene or hexane) 0.5% solution of phenolphthalein in 95% ethanol 0.5% solution of thymolphthalein in 95% ethanol 1% solution of nitrophenol in 95% ethanol 9 M solution of sulfuric acid Water 100-mL glass cylinders (6) 5-mL pipets Dropping bottle Knife Crucible tongs Stirring rod White background
The combinations of indicators shown in the table give the “rainbow colors” indicated. Indicator (mL)
2 Na (s) + 2 H2O (l) → H2 (g) + 2 Na+(aq) + 2 OH {(aq) The evolution of hydrogen gas is evident, and the hydrogen evolved carries the sodium into the paint thinner layer temporarily stopping the reaction. The piece of sodium repeatedly drops back down to the aqueous layer until the reaction is complete. The aqueous layers turn to one of the rainbow colors due to the production of sodium hydroxide.1 From the dropping bottle, add 3–4 drops of sulfuric acid solution to each cylinder to return the aqueous layer to the original color. Stir the solutions carefully, starting with the first cylinder. As each solution is stirred, the color disappears. The addition first of sodium and then of sulfuric acid can be repeated if desired.
Color
Phenolphthalein
Thymolphthalein
Nitrophenol
Red
8
0
2
Orange
1.5
0
Yellow
0
0
Green
0
Blue
0
10
0
Hazard and Disposal
Violet
8
2
0
Sulfuric acid, sodium, and sodium hydroxide are extremely corrosive to eyes, skin, and other tissue. Wear suitable eye and skin protection. Paint thinner (toluene, hexane) is highly flammable. Extreme care must be taken to avoid fire hazards.
1.5
8.5 10 8.5
Procedure Arrange six glass cylinders in a single row extending from left to right along the demonstration table before a white background. Place 0.5 mL of the red indicator in the center of the first cylinder. In a like fashion, place 0.5 mL of the orange indicator into the second cylinder. Continue adding 0.5 mL of the appropriately colored indicator into a cylinder (from third to sixth) in the proper “rainbow” sequence (from yellow to violet). Put approximately 50 mL of water in each cylinder. Repeat with approximately 50 mL of paint thinner. Place a piece of sodium (ca. 0.1 g) into each cylinder. The sodium sinks in the paint thinner until it reaches the interface between the two layers. The sodium reacts
942
with water according to the following equation.
Note 1. A color version of this figure is shown on page 873.
Literature Cited 1. 2. 3. 4. 5.
Alexander, M. D. J. Chem. Educ. 1992, 69, 418–419. Ciparlck, J. D. J. Chem. Educ. 1989, 66, 438. Clare, S. I. J. Chem. Educ. 1984, 61, 635. Hutton, B. J. Chem. Educ. 1984, 61, 172–173. MacBeath, M. E.; Richardson, A. L. J. Chem. Educ. 1986, 63, 1092–1093. 6. Loffredo, R. E.; Crookston, D. J. Chem. Educ. 1993, 70, 774–775.
Journal of Chemical Education • Vol. 74 No. 8 August 1997
