Liquid Oxygen as an Oxidizing Agent
The end of a cigarette is dipped in liquid oxygen and lit. The cigarette bursts into bright flame. This demonstrates the effect of concentration on reaction rate. A fun demonstration with a powerful anti-smoking message.
This demo is usually used to illustrate the effect that increasing concentration has on the rate of a reaction, though it can also be used to show the oxidizing power of oxygen. Allow about 10 minutes for this demo.
The rate of a reaction depends upon many factors, among which are concentration and temperature. In this demonstration these two factors compete with each other. One would expect the increase in concentration to increase the rate of reaction and one would expect the decrease in temperature to decrease the rate of reaction. We see in the demonstration that the increased concentration of oxygen molecules due to oxygen being in the condensed liquid state more than compensates for the decrease in temperature. The heat supplied by the butane flame is sufficient to initiate a vigorous reaction.
- large test tube with a string tied around it containing 1/4 to 1/3 full of liquid oxygen immersed in a small Dewar flask of liquid nitrogen to keep it cold
- tongs
- cigarette
- butane lighter
- Grasp the cigarette with the forceps.
- Immerse about 1 cm of the end of the cigarette in the liquid oxygen. Let it soak for a couple of seconds.
- Quickly take several steps away from the liquid oxygen!
- Quickly, holding the cigarette with the forceps at arm's length, light the end of the cigarette with the butane lighter. The end of the cigarette will burst into flame.
- Crush the cigarette in the Petri dish to extinguish it.
- Liquid oxygen and the liquid nitrogen in which it is immersed in the Dewar flask are extremely cold. There is a risk of frostbite if they come in contact with your skin. Handle the test tube only by its attached string.
- Liquid oxygen is a very powerful oxidizer. If flammable materials come in contact with it they are capable of quite vigorous combustion. The vapor that results from the vaporization of liquid oxygen is almost pure oxygen gas, which is also a very powerful oxidizer. Make sure that you are not near the test tube of liquid oxygen when you light the cigarette.
- The cigarette bursts into quite vigorous flames. Be sure to light it at arm's length. Wear goggles.
- Liquid oxygen for this demonstration is condensed from gaseous oxygen from the tank in the demo prep room. Fill the small wide-mouth Dewar flask with liquid nitrogen from Science Stores in the basement of Onyx Building. Place the test tube in the Dewar flask and run a rubber hose from the oxygen tank to the bottom of the test tube. Adjust the regulator so that a very slow flow of oxygen runs into the test tube. Liquid oxygen will begin to condense in the test tube. Turn off the oxygen flow when the test tube is 1/4 to 1/3 full.
- Liquid oxygen should be condensed not more than 15 minutes or so before the demonstration and test tube should be loosely capped with foil to prevent nitrogen from the air from condensing in the tube and contaminating the oxygen.
- Make sure that the test tube is not too long so that the cigarette can reach the bottom when held with tongs.
