Liquid Oxygen

    The students love to see liquid oxygen. And why not, it is truly fascinating to see this life-sustaining chemical when it is cold, wet, blue (!!!), and more combustion-supporting than ever. To make condensed oxygen, I use three main components. First, I have a cylinder of compressed, industrial-grade oxygen (197 cubic feet). Second, I have a 10 L Dewar flask for liquid nitrogen. And last, I have a homemade condensor.
          The condensor was made by finding an appropriately sized can. An 8 ounce can with a tall, slender build will work. Mine is a Texas Pete hotsauce can. You need to purchase some copper tubing from the hardware store with a diameter of about 0.5 cm. This can be coiled by carefully wrapping it around a graduated cylinder or a broom handle. Leave a straight section at the top and bottom. A hole must be punched in the bottom of the can that allows the the tubing to pass through and extend several inches below the can. The can has to hold liquid nitrogen so you must seal this opening. I originally soldered the tubing to the can, but after contracting and expanding due to extreme temperature changes, the solder came loose. My second attempt at sealing the can involved pouring an epoxy resin mixture into the bottom. It has worked beautifully, but I am sorry to say that I can't recall the exact type or brand. I have the can in a ring clamp with a large stryofoam cup surrounding it to reduce heat transfer. Any space in the cup can be filled with styrofoam "peanuts".
         To make the LOX, the oxygen supply is connected to the top of the copper coil using rubber tubing. Liquid nitrogen is added to the can to begin cooling down the coils. This nitrogen will boil quickly until the heat is removed from inside the can. Now top off the can with nitrogen and it will last surprisingly long. To catch the LOX, I use half of a styrofoam packing container for concentrated acid and place a large styrofoam cup inside with "peanuts" filling up the gaps. Make sure the students notice that the peanuts are packed to the top of the container (they soon won't be - a nice demonstration of gas behavior). A large test tube is placed in the cup and placed so the bottom of the copper tubing is about 1/3 of the way down the tube. The cup is filled with liquid nitrogen to allow us to maintain the LOX as it is produced.
          The copper coils are now sufficiently devoid of kinetic energy (boiling point of liquid nitrogen = -196 *C). The oxygen is now allowed to enter the coils at a low rate. With a boiling point of -183 *C, the oxygen gas molecules experience "sticky" collisions on their journey through the coils. The whole set-up will be quite foggy now due to condensing water vapor so you will not be able to see the oxygen dripping from the bottom of the condensor. However, you will start to notice a blue tint in the bottom of your collection cup. It won't be long until the test tube is full. Keep your LOX in liquid nitrogen if you are not going to use it right away.
          There are a lot of interesting things that you can do with LOX, but many of them are life-threatening. Extreme combustion and explosions are a very real possibility if a combustible fuel is present. Keep in mind: liquid oxygen offers almost 4000 times the level of oxygen found in the air (also remember that the atmosphere around your boiling oxygen is becoming more concentrated). The key is to limit the quantity of combustible fuel. You can dip a wooden splint in the test tube of LOX, take it out, and light the end of it. The LOX does not soak into the wood well, but you will observe a much faster rate of combustion. Beware of using materials that will soak up a lot of LOX and offer a lot of potential energy. I've done personal experiments with charcoal sticks and the degree of fireworks is hard to predict. A demonstration that offers a great lesson in chemical reactions is to find two pans, one large and one small. Pour some liquid nitrogen in the large pan and put the small pan in the nitrogen. Allow the small pan to lose its heat and then add your LOX. Tell the students that you are going to throw a match into this concentrated oxygen. The students will retreat in hopes of avoiding the impending inferno. Most students believe that the oxygen will burn all by itself. Have them write a combustion equation and think about limiting reactants. They will soon realize that the match represents very little combustible fuel. However, the match burns spectacularly when compared to a match burning in normal oxygen concentrations.

Check out some liquid nitrogen pics (7 images, 67 K)

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