Stanford Science Bus
Myth Buster Lessons (Autumn 2008)
December 2 and 3, 2008
Alex Sugarbaker (Tuesday lesson by Andrei Garcia)
Bacteria Lesson (finish from last time – stolen from Vivian's lesson) – 7 minutes
Goals:
Were our hypotheses correct? Which areas of the classroom and school had the most bacteria?
Materials:
-LB agar plates from last time
-latex gloves for the head tutor
Lesson:
Recap what we did with the bacteria. We swabbed the area to test using a q-tip and then swiped the q-tip in a zig-zag on the plate. The plates were placed at room temperature and then moved to 4C once growth was visible. The head tutor should then call the kids to the front and have them help to organize the plates by number of colonies. Note that the whitish-green fuzzy stuff (looks like bread mold) is mold, not bacterial colonies. Did the most bacteria grow where you expected them to?
Dissolving Teeth and Bones (finish from last time – stolen from Vivian's lesson) – 7 minutes
Goals:
Were our hypotheses correct? Did the teeth and teeth-like objects dissolve? Is the myth that soda can dissolve your teeth busted or upheld?
Materials:
-jars with teeth, eggs, and bones in vinegar, water, and coke from last time
-latex gloves for the head tutor
-dry teeth (in the box with the plates)
Lesson:
Vinegar has a pH of 2.4-3.4. Coke has a pH of 2.5. According to wikianswers coke does not dissolve a tooth overnight. But according to the Southern Illinois University School of Medicine pieces of enamel left in a variety of sodas for 48hrs experienced up to 5% weight loss. We left them in for two weeks to see what happened. We also tried it with an egg and chicken bones. If you cannot test the exact material in the hypothesis, you can try using other materials that have similar properties. At the front of the room the head tutor should help the students compare the control teeth to the teeth we left in liquids for two weeks. Was there any weight loss? Was the myth busted? Keep in mind that soda doesn't actually remain on your teeth undiluted for two weeks!
Soda Cans - 25 minutes
Goals:
To understand that the bubbles in soda pop result from dissolved carbon dioxide. To understand, at a basic level, the concept of pressure. To reiterate the importance of experimental controls. To determine experimentally whether tapping a soda can that has been shaken reduces the risk that it will explode when opened. To have fun spraying soda everywhere (outside).
Materials:
-soda in cans (preferably seltzer water, 6 per group of 4 => about 36 each day)
-ice chest and ice
-worksheets for data (about 30 each day)
Lesson:
Start this section off with a mini lecture on bubbles in liquids. Ask, why does soda have bubbles? What are the bubbles in soda made of? Carbon dioxide is the source of the bubbles. Henry's law says that the amount of gas dissolved in a liquid is proportional to the pressure of the gas in equilibrium with the liquid. Soda is bottled at 2 or 3 times atmospheric pressure to force a lot of carbon dioxide into solution. Ask the students what pressure is, and then help them understand that carbon dioxide is forced into the water by the high pressure.
The growth of bubbles of a gas in a liquid is governed by two competing effects. (1) The gas will tend to flow from the liquid into the bubble, causing it to grow. (2) The attractions of the water molecules around the bubble will tend to pull it back together. As a result, there is a critical bubble size. Anything smaller will quickly disappear, anything larger will expand until it floats to the surface and joins the high-pressure gas above the soda.
When you shake a can of soda, you create tons of sub-critical bubbles in the soda via agitations at the interface between the liquid and gas. If you open the can before they disappear, these bubbles will rapidly expand as the external pressure drops, propelling some of the liquid out of the can.
Ask, can you do anything to prevent this? We will now do a little experiment in groups of 4 to test if tapping the can, waiting, or cooling have any effect. Use the worksheets provided to rate the eruptions on a scale of 1-5. Tutors should help the students record the time between shaking and opening for each trial. The last three rows are intentionally left blank so the students can retry or mix and match conditions. At the end, ask, what would you do to avoid getting splashed by soda next time?
Please see the references if you want a more detailed understanding of bubbles in soda.
Mentos® and Diet Coke® – 5 minutes
Goals: Introduce nucleation. Have fun with even bigger explosions.
Materials:
-index cards or dropping apparatus
-3 or 4 two liter bottles of Diet coke
-mint mentos
Lesson:
When everyone is done with their worksheets, you can give another brief lecture on nucleation. In order to start growing beyond the critical size, the bubbles need nucleation sites. These provide the cracks and crevices needed to reduce the shrinking effects described above. For instance, it has been discovered (remarkably recently) that the bubbles in a glass of soda typically nucleate on tiny cellulose fibers from the cloth or towel you last used to wipe the glass. The mint mentos we use here provide excellent nucleation sites for a dramatic result. The artificially sweetened coke is also essential because there is actually a little chemistry beyond the scope of this lesson that accelerates the bubble development.
To do the experiment, just open the 2 liter bottle, stack 4 or 5 mentos in a card tube on top of a card. Then just slide the bottom card out and run. (Or use our specially designed dropping apparatus.)
Crushing a Soda Can – 5 minutes (filler if there is still time)
Goals:
To learn that it is much easier to fold sheet metal than it is to truly compress it.
Materials:
-the dozens of empty cans we now have
-pencils
Lesson:
It is much easier to bend sheet metal, like the side of an aluminum can, than it is to compress it. In bending it, you only need to alter the interactions between the few aluminum atoms that lie along the crease. To compress it, you would either have to force adjacent aluminum atoms to begin to overlap (very difficult!) or rearrange the the atoms more dramatically. When you stand on top of the can, it often doesn't crumple right away because there is an unstable equilibrium. The forces you apply are mainly trying to compress the straight walls of the can. However, if you apply even a slight force against the side of the can with a pencil, you can upset the equilibrium and the can will be crushed.
References
source of the below)
Cragin, J. H., “Soft drink bubbles,” Journal of Chemical Education, 60, No. 1, 71 (January 1983)
Deamer, D. W., and B. K. Sellinger, “Will that pop bottle really go pop? An equilibrium question,” Journal of Chemical Education, 65, 518 (1988)
Wilt, M., “Why do bubbles form in a soft drink?” in “Questions Students Ask,” Physics Teacher, 23, No. 6, 366-367 (September 1985)
Pascuzzi, E., “Carbonation speculation,” Physics Teacher, 30, No. 3, 173 (March 1992)
Edmiston, M. D., and C. Parsons, (letters) “Agitation solution,” Physics Teacher, 30, No. 6, 325 (September 1992)
Weiss, P., “The physics of fizz. Toasting a burst of discovery about bubbles in champagne and beer,” Science News, 157, 300-302 (6 May 2000)
Liger-Belair, G., “The science of bubbles,” Scientific American, 288, 80-85 (January 2003)