Demonstration (Jason Newman)

Title: Tracing a Chemical Contaminant Through Logic.

Principle(s) Investigated: Diffusion; pH indicatiors; logic and reasoning.

Standards Chemistry 4d: Students know how to use the pH scale to characterize acid and base solution.

Chemistry 6a: Students know the definitions of solvent and solute.

Materials: Several test tubes (in pairs) from any chemistry lab; sodium hydroxide (NaOH) from a chemistry lab; deionized/distilled water (H2O); Phenolphthalein (an acid-base indicator) from a chemistry lab..

Procedure: Each student should be given two test tubes labeled with a group number and a letter (either E(xperimental) or C(ontrol)) and a dropper. When told to begin, each student will select another student (it is important that there is an even number of groups) and exchange a small bit of fluid between their "E" tubes. They should note who they traded with for future reference. After two minutes, each will select a new student and repeats this until each has made a total of three swaps. The studentss swapped with should be listed on a table and posted for easy reference.

Once all swaps have been made, all "E" tubes will be tested for the presence of the contaminant (NaOH) with phenolphthalein. Any tube which changes color (reddish) is "contaminated". By analyzing which tubes have intereacted, the students should be able to determine the source of the contaminant.

Once the source has been identified, the "C" tube of the groups will be tested. Only one should be positive.

Student prior knowledge: Students should understand basic concepts about pH. They should also understand the basic ideas of mixtures.

Explanation: This activity is designed to allow students to flex their "logic" muscles and to illustrate the principles involved with chemical (and biological!) contamination.

The C, or Control, test tubes serve as our reference guide and are untouched and unmixed, so that we can objectively identify which group started the contamination. Phenolphthalein is used because the color change is easy to observe and dramatic, and NaOH is a simple, easy-to-use source of OH- (basic) ions.

Each time the students interact and exchange water, this can be likened to the interactions between different organisms or machines, such as food-making machines (often with warnings about peanuts, fish, or other allergens). Despite the fact that the final interaction may be far away from the origin of the contamination, the contamination continues to be passed on to those who may be unaware of it. The experiment also makes an excellent model for contagious diseases, especially STDs.

When the results are posted and the students can see which tubes are contaminated, they can begin the process of figuring out the contamination's source. This is a process of determining in which order the tubes became contaminated (through logic) and figuring out which tube must have started things. As an example:

The red numbers are groups found to be contaminated at the end.

From this table, you can see that 14 was contaminated at the end, but 1 was not. Therefore, 14 could not be the source, because if it was, 1 would be contaminated. Similarly, 10 is contaminated but 12 is not, so 10 could not be the source.

Continuing these trains of logic it is possible to determine the original source of contamination, tube 2.

Questions & Answers:

Q1: What affects how contagious a pathogen is?

A1: A pathogen's contagiousness is associated with many factors, including its vector (how it is transmitted), its ability to survive in an environment before being "caught," and its ability to evade a host's immune response.

Q2: Why do foods have those warnings, "Made on machines which also process (nuts, fish, wheat, etc)"?

A2: Although efforts are made to clean these machines, it is possible that some contamination may have remained on them as they process the new foods. If this is so, then people with allergies to these foods may suffer a reaction while eating something which is apparently innocuous, like suffering a peanut allergy while eating bread.

Q3: How much of a contaminant is dangerous? I mean, aren't we talking parts per million (ppm) in some of these circumstances?

A3: Yes, but in biological systems a toxic chemical can kill with a very low concentration. For instance, in the US, the "safe" concentration of mercury is 1 ppm; anything above that is considered dangerous. Many of these contaminants can act as catalysts in humans or other life forms, causing multiple harmful reactions without being consumed.

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Applications to Everyday Life:

Disease contagion patterns can be traced using this kind of logic, by determining where the organisms involved (humans or birds, for example) must have come into contact with the original source of the disease.

Forensic pathologists may have to trace the source of a poison or contaminant responsible for someone's death or injury as part of an investigation.

By examining the possible modes of transmission for a disease (as in, what were the people who caught it all exposed to? How could they have caught it?) researchers may be able to characterize the disease more effectively and recommend prevention mechanisms, similar to those in cold and flu season (masks, washing hands, etc).

Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.