Plastic vs. Glass


Concepts Shown:

heterogeneous nucleation


The materials necessary to perform this experiment are few and inexpensive. All that is necessary is a plastic cup, a glass, and some soda. The plastic cup and glass could certainly be stored and used as often as they are desired. They both should be a decent size and clear so that the class can observe the demonstration. I have purchased two plastic cups at Meijer's for only $.95 each, and a set of four glasses for only $5.49. This way if anything gets lost or broken there is back up supply for the demonstration. the best part about these items is that the plastic cup and the glass are both relatively the same size and look fairly similar. It won't be easy for the class to determine which one is the glass prior to the demonstration. The soda used should be of a dark color so that the class can easily observe the nucleated bubbles. The soda will certainly have to be resupplied, which is no problem because it can be purchased at nearly any corner store.


During the lecture about nucleation, the demonstrator will place the glass and the plastic cup next to each other where the class can see. He/She will then ask the class to guess which one is the glass based on what happens when soda is poured onto them. At first they both may have many bubbles nucleated on the side, but after a minute or two the plastic cup will noticeably have more bubbles nucleated, and more foam on its surface. The instructor will then proceed to explain the reasoning behind it.


Heterogeneous nucleation is clearly demonstrated here. The scratches that exist in plastic cups are perfect sites for heterogeneous nucleation of carbon dioxide. Since glasses have fewer scratches than plastic cups, there will exist a detectable amount less of bubbles forming in the glass. Also, this demonstration will show that heterogeneous nucleation is a lot quicker than homogeneous nucleation. Soda is a supersaturated solution of carbon dioxide in water, which is why the gas bubbles out of it. The nucleation of carbon dioxide is comparable to the precipitation of supersaturated carbon in steel, which can be mentioned to the class also. Suggestions: The glass should be washed and thoroughly rinsed prior to the demonstration to get rid of any dirt that would act as undesired nucleation sites for carbon dioxide.



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