The Decay Curve of Red Licorice


I saw this idea somewhere (Chem 13 News ?) and tried it out with excellent results. If you know the original source, please tell me. I owe the author at least a big bag of Licorice!


Radioactive decay
Societal issues about radioactivity and pollution


-a supply of Red Licorice "Twizzlers"
-plastic or kitchen knives
-paper towels
-note paper

PROCEDURE: ...................................... [See the Safety Note below!]

Give each student (or pair) ONE piece of Red Licorice on a clean paper towel.
Instruct the students to trace out the length of the licorice on a page set up like a bar graph.
Now they must find the midpoint of the piece and cut the piece in half.
They must trace the length of the new piece beside the old trace.
They may eat the "left-over" piece [optional].

The process of cutting in half and tracing the new length onto the graph continues until they can no longer cut the remaining piece in half (by cross-section) accurately.
Each student or group now has a bar graph showing a "decay curve".


[These suggestions are NOT intended to be a complete review of all the safety issues involved with this activity. Professional judgement and practices are essential. If you are unsure of the safety precautions that should be taken, seek experienced assistance.]

Although allowing students to eat the "decay products" of the reaction has advantages, they do NOT outweigh the problems and hazards of allowing students to eat in a laboratory setting.

If you cannot arrange to have the class in a non-laboratory room, the experiment can be done using dry spaghetti or another unappetizing substitute.
Point 3 (below) is also a serious issue. If they are going to treat the material as food then appropriate "clean techniques" must be used. Of course, if any students have the good sense to decline the chance to eat the 'decay products', respect and perhaps even applaud their good judgement!


The relation between this and the nuclear decay curve is obvious enough.

Here are some fun questions to guide further discussion:

1. "At what point in the decay process was the 'free to eat' piece too small to care about?"

-A similar judgement has to be made about radioactive decay. Although an isotope will never decay completely and will always be capable of releasing radiation, it does reach a point at which so little radiation is released that it is no longer hazardous. Short half-lives are characteristic of medical isotopes. "Why?"

2. "As the licorice decayed, did the mass of licorice disappear?"

-Many students fall victim to imprecise language and assume that the radioactive substance "disappears" as it decays. Here, the licorice is changed by the body of the eater into another substance, but the total mass does not change.
(What is the mass of a 10 g sample of pure uranium-235 after one half-life? Still 10 g! But only half of that mass is still the original U-235. The rest is the decay product of the reaction and any other nuclear reactions that occurred with the daughter product.)

3. "Why do most students seem to think that Red Licorice cannot be contaminated with bacteria?"

- Students will hit each other with it, drop it on their books and generally do things with it that they would never dream of doing with a carrot, or celery stick or other food material. Of course, if you smell the solvent vapours coming out of a bag, it is hard to believe that any living thing could survive inside!

So: "What makes these chemicals different from 'normal' food chemicals"?
......."If they kill bacteria, how can they be sold as candy to children?"
....................... [This is a great entry into the idea of dose/kg and the definition of LD-50 for poisons.]