Dear Dr. Oblivious

NB: I am completely snowed in with preparing two camera-ready papers, reading and grading final project reports, and staying out of the rain, so today I offer a brief piece of advice I wrote a while ago, and saved for an occasion such as this.


Dear Dr. Oblivious,

I have been going through my deceased mother’s kitchen and have become increasingly annoyed by her habit of not labeling her spice jars. She has two large urns, one containing white sugar and the other salt. But I have no idea which is which; and I don’t want to just throw them out. What should I do?

Sweet or savory, or savory or sweet — I just don’t know.


Dear “Sweet or savory, or savory or sweet — I just don’t know,”

I am sorry to hear about your mother’s penchant for not labeling her spices. Telling the difference between these two chemicals is extremely difficult and she has left you with what could be a considerable task. Table salt, or sodium chloride (NaCl), looks exactly like sugar; and sugar, or sucrose, looks exactly like sodium chloride (table salt). See Fig. 1.

sugar-salt-l.jpg

Figure 1: Which one is salt and which one is sugar? It is extremely difficult to tell just by looking, even through a magnifying glass!


But do not worry! Here is a quick way that you can tell the difference. You will need: 1 bunsen burner with ethanol fuel, 1 inert wire loop, 2 scoopulas, 1 rubber policeman, 5 ml 10% Hydrochloric acid (HCl) solution, 10 ml deionized water, 2 10 ml graduated cylinders. Since one cannot emphasize safety in the laboratory enough, you will also need the following: 1 pair of safety goggles, 1 full length lab coat with long sleeves, 1 chemical hood, 1 pair of high temperature gloves, 1 pair of clean room shoe covers, and 1 face mask with vapor filters. If you have long hair, make sure it is pulled back.

After you have donned the safety gear, move your unknown chemicals and the rest of the equipment under the chemical hood.
Pour 1 ml of deionized water into each of the empty 10 ml graduated cylinders. With each scoopula, transfer a few pinches of one unknown (salt or the sugar) into one of the cylinders, and then a few pinches of the other unknown into the other. Mix each solution thoroughly.

Set up the bunsen burner and turn it on. Make sure the flame burns nearly colorlessly. Take the inert wire loop and submerge the end into the hydrochloric acid. Then rinse it off in the remaining deionized water. Place the metal loop into the flame and heat it up until the flame becomes colorless.

Once the wire loop is clean and has cooled, dip it into the first unknown solution to be tested. Place the loop into the hottest part of the flame and observe and record the color change to the flame in your properly dated lab book (Fig. 2).

flametest.jpg

Figure 2: A chemistry student demonstrates proper technique for “flame testing,” except her is missing one person to watch out for “air in the lines”, another person at the ready with Class C fire extinguished, and a third person to take notes.


If your unknown solution contains sodium ions, then the flame will burn a bright orange red. If, however, the solution does not contain sodium ions, the flame will not turn orange red. After testing the first unknown, dip the wire loop into the 10% HCl solution, rinse in the deionized water, and heat the metal loop in the flame until the flame becomes colorless.

If your unknown solution contains sodium ions, the flame will turn a bright orange red. If it is sugar, then it will not turn orange red.

Now submerge the wire loop in the other unknown and place in the flame. If this solution creates a orange red flame, then you know that solution contains sodium ions.

And that is it! Through the wonder and ease of modern chemistry, you have now determined which of your mother’s urns contains sodium chloride, and which doesn’t. HOWEVER, just because you have determined one urn contains a substance that when dissolved in H2O produces sodium ions, can you be absolutely certain that the other contains sugar? From our little experiment, and the wonderful problem of induction, you only know that the other urn does not contain salt, or at least did not when you tested it. There are any number of things that the non-salt urn contains, including strychnine, anthrax, and crack cocaine — all of which look exactly like sugar. In order to really make sure it is sugar, and the other urn contains sodium chloride you will need the following: 2 rubber policemen, 1 centrifuge, and 1 mass spectrometer. We will return to this problem at a later date.

Happy spicing!


Do you have an obvious question for Dr. Oblivious? Send yours in today for some overly complex and utterly unhelpful answers.

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