Video Demo: Burning Iron
Sunday, June 14, 2009 at 7:31AM 
Many of the demos that I use in teaching chemical reaction are taken from a terrific book called A Demo A Day–Chemical Demonstration Book. The book contains a year’s worth of chemistry demonstrations that are easy to follow and to use. This demonstration shows that when iron has a large surface area it can react with oxygen and burn easily. I first use the 9 volt battery and touch the ends to various steel surfaces. The students see that nothing happens. I then touch the battery to the steel wool (held by the clamp on the ring stand) and the battery provides enough energy to make the iron burn. The steel wool has a larger surface area than pieces of steel like the ring stand. The steel wool is exposed to more oxygen molecules. You can also use this as a conservation of mass demo. Simply place the steel wool in a plastic bag and measure the mass. After burning the wool (of course it shouldn’t be in the bag at this point) return the leftover material back in the bag and mass it again. The mass should be the same (although it might be greater afterwards, since oxygen has now bonded with the iron.).
Preparations: You may have to soak the steel wool in acetone for 15-20 minutes to remove the outer coating, so that the steel is exposed. Take the wool out of the acetone and left it air dry.
Reader Comments (2)
I suspect this has less to do with "surface area" and more to do with the quantity of iron. The current available from a small 9 V battery is not enough to heat the quantity of iron in a ring stand to a high enough heat to get it to react with the oxygen in the air quickly. However, the tiny filaments of iron, with their very high resistance, should get quite hot when short-circuiting the battery.
To test the "surface area" idea, take a single 3 inch strand of steel wool versus the same length of ring stand. In this case, the ring stand would have more surface area, but the wool would still ignite.
Very interesting idea. I should try that.