Students in Martha's Physical Science Honors classes have been studying states of matter such as solids, liquids, and gases. (Can you name the other two states?) Among the things we've studied about these different phases is how to change one state to another state such as melting, freezing, and boiling. A change that students might have been unfamiliar with until introduced to it in class is that of sublimation.
Sublimation involves a solid changing directly to a gas without stopping off in the liquid state. Dry ice is famous for its tendency to sublime (change directly from a solid to a gas). Martha brought in some dry ice for the students to observe this phenomenon on their own. They watched the dry ice form a cushion of gas under itself on the counter top such that it could float around with light pushes like a hockey puck on an air hockey table. She put some in a beaker of water to see it produce copious 'smoke' filled bubbles which soon popped, releasing lots of cloudy vapors. When those vapors were poured over a lit candle, the flame was soon extinguished, confirming that dry ice is just carbon dioxide in the solid state. If you put something warm on a surface of the dry ice, a bugle like scream would emanate. This was due to the rapid release of the gas in a confined area. The students enjoyed the odd noises it would make. Some fruits had been stored in the container with dry ice for a while. When Martha 'accidentally' dropped the banana that had been in the container, it shattered like porcelain. Dry ice is so cold it made the fleshy fruits brittle.
Well, that was fun, but what else could we do with dry ice? Make dry ice root beer, of course. Martha found a recipe for it online and divided up the responsibility for bringing in each necessary component among the students in her first period Honors class. The goal became to make the root beer the following Monday. Second period came in and the discussion of making the root beer came up again. Since first period already had the items necessary to make the root beer covered, what could they do to join in on this project? Bring in the items necessary to make not just root beer but make root beer floats! They divided up responsibility for bringing in ice cream, spoons, and straws.
On Monday, with all the items on hand, the root beer was brewed up. We used an uncolored root beer extract so our concoction was nearly as colorless as water – but oh, the delightful fragrance that filled the room let us know that we had, indeed, made our own root beer. The vat containing our brew was topped with the misty, smoky clouds that we had seen in the regular dry ice demonstrations. We had added so much dry ice that we actually made dry ice root beer slushies. None the less, this was ladled into cups that contained vanilla ice cream and the students did a tentative taste test. It was delicious! By the time second period had their floats, not a drop of root beer or ice cream remained.Better eating through chemistry!