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) |
| Color |
Phenolphthalein |
Thymolphthalein |
Nitrophenol |
| -----------------------------------------------------
|
| Red |
8 |
0 |
2 |
| Orange |
1.5 |
0 |
8.5 |
| Yellow |
0 |
0 |
10 |
| Green |
0 |
1.5 |
8.5 |
| Blue |
0 |
10 |
0 |
| Violet |
8 |
2 |
0 |
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 with water according to the following equation.
2Na(s) + 2H2O(l) -->
H2(g) + 2Na+(aq) + 2OH-(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.
Hazard and Disposal
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.
Literature Cited
1. Alexander, M. D.
J. Chem. Educ. 1992, 69, 418-419.
2. Ciparlck, J. D. J. Chem.
Educ. 1989, 66, 438.
3. Clare, S. I. J. Chem.
Educ. 1984, 61, 635.
4. Hutton, B. J. Chem.
Educ. 1984, 61, 172-;173.
5. 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.
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Citation
