MadSci Network: Chemistry |
Ferdie, you are an accurate observer. The acetone does "boil" whenever it comes into contact with anything that is a bit warmer than -77 deg C. (The references all say that this is the temperature of the dry ice/acetone slush. The temperature you are observing may be several degrees different because of calibration errors in the low temp thermometer or impurities in the acetone). Why does it boil in this way? Well, the whole point of an acetone/dry ice slush bath is that carbon dioxide dissolves well in acetone. When you dissolve the solid carbon dioxide, the acetone solution cools down. This is mostly because the dry ice itself is even colder than -77, and partly because the solution takes up heat when the carbon dioxide turns from solid to liquid in becoming part of the solution. The solution will continue to dissolve carbon dioxide (dry ice) until it is saturated. The next bit becomes fairly heavy Phys Chem, but quite standard. You can find it in textbooks like PW Atkins "Physical Chemistry". I didn't manage to find the details as they relate to a slush bath in a quick web search. We have to think about the phase diagram of the CO2/acetone system. We are above the freezing points of both substances, so solid phases do not really come into play. There is a liquid solution phase (which we might prefer to think of as a gas dissolved in a liquid) and the vapour phase. The vapour pressure over the liquid phase will increase rapidly with increasing temperature. When this vapour pressure becomes greater than 1 atmosphere, the liquid solution will boil. According to Raoult's Law, each substance will contribute to the vapour pressure a pressure equal to its own vapour pressure times its mole fraction. At -77 deg C, the saturation vapour pressure of CO2 is rather more than one atmosphere, while that of acetone is very small indeed. So the solution will boil, and the vapour that comes off will be almost entirely CO2, with very little acetone. Another way of looking at it -- simpler but less rigorous -- is to think of the liquid slush phase as a gas dissolved in a liquid. We know from school chemistry books that gases are less soluble the warmer a liquid gets, so anything that warms up the solution makes some of the gas come out of solution (like soda water).
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