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This site is here to provide information about the Science Bag called "Urban Air - What's in There?" presented during January 2000 at UW-Milwaukee by Prof. Thomas Holme of the UWM Chemistry Department. If you want to order a video of this science back from Blue Sky Associates you can click here to get to the right web page. |
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Liquid Oxygen: My first demonstration uses liquid oxygen to show how much more rapid combustion reactions would be if there was more oxygen in air. In the case of liquid oxygen the percentage of oxygen is MUCH greater than what is found in air. We make our liquid oxygen by having an apparatus called a "cold finger" bathed in liquid nitrogen. Nitrogen liquefies at a colder temperature than does oxygen, so when we slowly pass oxygen (from a tank of oxygen) through the cold finger some of the oxygen liquefies. It takes us about 45 minutes to get the amount of liquid oxygen we have during the show. Remember...I have practiced doing these demonstrations many times. Even with that practice I have set a few small fires during the science bag. You should not try these demonstrations unless you take extreme precautions to prevent fires.
Dry Ice: My second demonstration is quite simple. We simply drop dry ice into a large graduated cylinder containing a solution with universal indicator. We make the solution basic (purple) by adding some 0.1 M NaOH. You can get universal indicator from science supply stores or from pool supply stores as well. We make our own at UWM because with the large containers we have in our demonstrations we need a LOT of indicator or the color doesn't show up well enough for people in the audience to see it.
Disappearing Ink: I have a whole page on disappearing ink already. Click here to get to that page.
Sulfur Oxides: Sulfur oxides can be dangerous to demonstrate, so we built a system that makes it safe to do in a classroom. Another problem with sulfur oxides is they are colorless gases - and thus invisible. We show their generation by taking advantage of the fact that they react with water to form acidic solutions. Bob Ponton, the glassblower at UWM, made a device that has an outer glass compartment with another glass vessel inside it. There is a tube (smokestack like) that goes up about 2/3 of the way to the top of the glass containment compartment. This inner vessel is also connected to the outside of the device by a tube that has a glass stopper. This tube is needed to provide a way to inject sulfuric acid to start the demonstration. So, once you have a containment system, you can generate SO2 by taking about 2.0 g of sodium sulfite and adding about 2 mL of 6 M sulfuric acid. Make sure you have all fittings tightly clamped to prevent this noxious gas from escaping the apparatus. We fill the bottom of the containment vessel with a neutral solution that has universal indicator (basically distilled water with indicator in it) and when the gas is generated in the head space above the liquid, it slowly dissolves in and causes a color change as the solution becomes acidic.
Nitrogen Oxides: Nitrogen oxides are also noxious gases, so the demonstration has to be safely constructed. Again we turned to our glassblower, Bob Ponton, to build a device. This device requires three chambers. The left and right chambers need valves above and below them. The central chamber has a valve at the bottom, and has tubes running from the two side chambers into it near the top. We evacuate the entire device with a rotary style vacuum pump and fill the right chamber with NO gas. (The tank we have been using is a little on the old side, so there is some NO2 in it because of the disproportionation reaction of NO with itself to form NO2). If we have clean, pure NO the gas would be colorless. But, having a little NO2 in there actually makes it easier to see what happens. We charge the left chamber with O2. To run the demo it is important to release the NO into the central chamber first, or there isn't enough vacuum to pull it over. Once the NO is in both the central chamber and the right chamber we release the O2 into the central chamber and the brown NO2 formation reaction is quite vivid. We can then look at the reaction of NO2 with water by placing a beaker with universal indicator under the central valve. The entire apparatus is still at vacuum relative to atmospheric pressure, so the liquid is sucked up into the device and the color change showing acid formation is another vivid demonstration - this time of the chemical nature of NO2 gas.
Acetone Vapor Pressure: This demonstration is designed to show the concept of volatility. We have once again constructed a special apparatus, but this demo wouldn't need one. A 500 mL Florence flask would work too. Take a one hole rubber stopper that fits the top of the Florence flask. You will need to insert a glass tube - the tube should be pulled to a slight taper at the stop and should go into the flask so that it is about 20-30 mm above the bottom of the flask. Then take a small test tube and attach it to the glass tube with rubber bands so it would be about 2/3 of the way up, but inside of the Florence flask. Fill the Florence flask about 60% full with water (I added blue food color to make it easier to see) and heat the water to about 85-90 oC. Put a few boiling chips in the small test tube and fill it with acetone. When you place the stopper - glass tube - test tube part into the Florence flask the small test tube should be 70% covered with the warm water. Make sure the seal is tight. Place you finger on the top of the drawn part of the glass tube for a little while so pressure from the acetone vaporization can build up. When you release your finger you should get a fountain of warm water coming out - like happened in the Science Bag show.
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