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A Chemistry Puzzle to Be Solved

two beakers with liquid on a lab bench. Right is colored, left is not

Where did the color in the left beaker (which contained baking soda) go?

Adding baking soda to vinegar is a well-known, frequently used reaction that illustrates gas formation. Both ingredients are readily available from the grocery store, and both can be safely discarded down the sink. In order to jazz up this demonstration, colored gummy bears can be put into the vinegar before the baking soda is added. The idea is that the evolved gas bubbles will lift the gummy bears, enhancing the visual effect of the gas formation. Since gummy bears are heavy and large compared to the size of the gas bubbles, the gummy bears can be sliced into small pieces before being put into the vinegar.

The suitable demonstration procedure is as follows:

  • Cut 24 gummy bear candies (must be colored) with a knife or scissors into very small pieces.
  • Fill two 600 ml beakers halfway with water.
  • Put equal amounts of gummy bear pieces into each beaker.
  • Pour 100 ml of vinegar into each beaker.
  • Put approximately 1 tablespoon of baking soda into one of the beakers but not the other.
  • Label the beakers to indicate their contents.

After a short amount of time, the gummy bear pieces in the beaker with the baking soda will start floating to the surface and then fall down in a cascade-like effect, due to carbon dioxide gas being produced by the reaction of vinegar and baking soda. As the carbon dioxide gas bubbles rise to the top, they bring the small gummy bear pieces with them. The beaker with no baking soda produces no carbon dioxide gas, and the gummy bear pieces just sit at the bottom.

One day I cheerfully performed this demonstration in front of my class and noticed that the results were unremarkable. After class ended, due to the limited amount of time before the arrival of students for the next class, I left the setup on the bench top until the following morning. At the end of the day, the contents of both beakers looked identical: little pieces of colorful gummy bears in clear liquid.

The following morning, however, when I approached the beakers to discard the contents and clean them, I noticed that the beaker that had the baking soda added to it contained completely clear liquid and no solid material. The other beaker still contained pieces of colorful gummy bears. I was surprised! What had taken place? Where did the gummy bear pieces go? Where did the color go?

The reaction of the acetic acid in the vinegar with sodium bicarbonate (i.e., baking soda) in solution produces carbon dioxide along with sodium acetate and water (see Figure 1).

FIGURE 1
equation showing The reaction of the acetic acid in the vinegar with sodium bicarbonate (i.e., baking soda) in solution produces carbon dioxide along with sodium acetate and water

The reaction of the acetic acid in the vinegar with sodium bicarbonate (i.e., baking soda) in solution produces carbon dioxide along with sodium acetate and water.

The beaker with the added baking soda contained carbon dioxide produced during the reaction. The beaker without baking soda added produced no carbon dioxide. Could the presence of carbon dioxide cause the solid gummy bears to disappear? Could the presence of the carbon dioxide cause the color to disappear?

The traditional gummy bear is made from a mixture of cornstarch, beeswax, carnauba wax, citric acid, starch, fractionated coconut oil, sugar, dextrose, corn syrup, natural and artificial fruit flavors, glucose syrup, and artificial colors (yellow 5, red 40, and blue 1). The food dye molecules in the artificial colors have large electron clouds because of highly extended conjugated π bonding.

If carbon dioxide were an oxidizing agent, a reasonable scenario can be formulated. The generated carbon dioxide causes oxidative cleavage of the polyene of the food dye. The π-electron cloud of a double bond, located above and below the molecular plane, is polarizable. This can lead to the process in which the carbon–carbon bonds are broken and the initial alkene carbons are oxidized to form carbonyl compounds. This is analogous to oxidative cleavage of alkenes involving the permanganate anion, the periodate anion, and ozone.

Under this assumption, the generated carbon dioxide reacts with the food dye molecules via a redox reaction to break them into fragments. This fragmentation eliminates the color (see Figure 2)

FIGURE 2
equation showing Generated carbon dioxide reacts with the food dye molecules via a redox reaction to break them into fragments.

Generated carbon dioxide reacts with the food dye molecules via a redox reaction to break them into fragments.

Why are food dye molecules colored in the first place? They are colored because their conjugated systems of double bonds absorb certain wavelengths of light. If the conjugated system of double bonds is too small, the light absorbed is in the ultraviolet (non-visible) range and no color is seen. Each extension in this conjugated system, however, reduces the energy required for electrons to transition to higher energy states.

This allows the molecule to absorb light in the visible range, with each extension leading to the absorption of progressively longer wavelengths. Recall that in electromagnetic radiation, energy is inversely proportional to wavelength. This is why different food dyes have different color—their conjugated systems have different lengths.

When the carbon dioxide breaks the food dye molecules into fragments, each fragment is too short to absorb light in the visible range. Each fragment can only absorb light in the ultraviolet range, which the human eye cannot see.

For the more advanced student, a slightly more sophisticated explanation involving quantum mechanics can be offered. When an electron in the system absorbs a photon of light of the right wavelength, it can be promoted to a higher energy level. A simple model of the energy levels is provided by the quantum-mechanical problem of a one-dimensional particle in a box of length L. The length L represents the movement of a π electron along a long conjugated chain of carbon atoms.

In this model the lowest possible absorption energy corresponds to the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The energy difference (∆E) is inversely proportional to the square of L. The photon wavelength would be l = hc/∆E. Bigger L means smaller ∆E, which in turn means longer wavelength.

Recall that h and c are constants. Therefore, increasing the length of the conjugated chain of carbon atoms would bring the wavelength into the (relatively lower) range of visible light and, conversely, breaking the chain up into fragments would bring the wavelength into the (relatively higher) ultraviolet range, removing the visible color.

Carbon dioxide, however, is not known to be a strong oxidizing agent. Nonetheless, this reaction was reproduced by several groups, resulting in the same observations. How then can a chemical not known to be a strong oxidizing agent oxidize the conjugated systems of double bonds in the food dye molecules and cause them to fragment?

This is the conundrum. If the generated carbon dioxide did not fragmentize the food dyes in the gummy bears, where did the color go? What exactly happened in that beaker?

I challenge you, the readership, to answer these questions. Can you supply a suitable mechanism that explains the observations and supply corresponding balanced chemical equations?

Topics

Chemistry Labs

Levels

High School

Asset 2