03.11.10

IMFs, Phase Transition and Frozen Tequila Shots

Posted in Art, Science at 2:12 am by Administrator

Several years ago, on a hot humid summer day in Portland OR, me and a friend were at a grocery store, and the dry ice freezer caught my eye. Kind of an impulse buy, I have to admit, I asked the cashier for a pound. They wrapped it up in paper and sent me on my way. Once we got home we unwrapped it, and for a while we were not quite sure what we were going to do with it. As the dry ice sublimated I noticed that it would sink, and kind of hover along the table top. This got me wondering.

I had just taken the first Gen Chem class and we were taught, among other gross oversimplifications of reality, that gas will expand evenly to fill a container in a manner which is independent of the gas. If you have a mixture of gas, they will mix evenly in all directions, as the gas phase is independent of the identity of the gas. The relationship is described in the equation pV=nRT.  This means that if you have a volume of gas at a certain temperature and pressure, you can say how many molecules of gas are contained therein, no matter what the gas is composed of. You can think of it like all gas behaves like food coloring diffusing into warm water. They do not separate like liquids of differing densities such as oil and water.

So the question this raised was, is the fog coming off of dry ice a gas? Most people would say yes. Dry ice is composed of CO2 which hates being in its solid state so badly, that it jumps straight into the gas phase via sublimation. But why can I see it? Why does it disappear? Why does it sink? These are questions that aren’t easy to answer with my best friend google. In fact I even get different answers from different professors. Apparently Physicists and Chemists agree to disagree on this point.

Perhaps what I am seeing is in fact the carbon dioxide, en route to its more excited gas phase, but still retaining the cool temperature causing it to sink below the room temperature air. But but carbon dioxide gas is not a visible gas. When I look inside a bottle of carbonated water it is not cloudy. When I open a warm beer(I get impatient sometimes), I do not encounter the smokey CO2 pouring out of it like with a cold beer.

All of this comes down to a few core questions. Is a gas visible? Why don’t heavier gases sink and lighter gases rise? What exactly am I looking at when I see steam? clouds? smoke? dry ice fog at a dance party? Well intermolecular forces are the culprits for all the bizarre phenomenon we see at the macro level. At a very small level we have these molecules of differing sizes running into each-other, attaching to each-other, repelling each other, and sometimes just hanging out and not doing a whole lot. These interactions between molecules are called intermolecular forces (IMFs) and rely on a few key points. Electro-negativity, Polarity and Surface area are chief among these. Basically, a happy, non-reactive element, which has no reason to interact with anything else will be a gas. As the interactions with other molecules increase, it takes more energy to make this substance into a gas, in order to combat the intermolecular forces. Once it is a gas, however, and it has gained enough energy to overcome it’s intermolecular forces, it will zoom around, and bump into other gases, and fill its container just like all good gases tend to do. But if you cool it down it will start to shift phase as the intermolecular forces are allowed to take over, and things like polarity begin to pull the molecules into an incompressible density like that of water. This less exited phase makes things immiscible (had to add that one to the dictionary) and densities separate due to weights as the molecules shift around each-other.

When we see gas, it is usually adulterants in the gas that allows us to see its behavior. As the gas settles, we see that this substance in its gas phase is in fact not visible, and mixes with its surroundings, and PV is still equal to nRT.

So besides keeping you up at night with questions about physical chemistry, we discovered that dry ice is capable of freezing tequila to the point of being a kind of slurry around -25 degrees ferrenheit, and does a great job of cooling you off on a hot summer day.

Above is a photograph by Alberto Soveso of which I am very jealous demonstrating the immiscible nature of enamel as it is suspended in water. The colors are gorgeous.

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