Curriculum Notes 

This is a very versatile demonstration. It is an example of a formation reaction, a redox reaction, and a combustion reaction. A lot of instructors like to use it when they are talking about the factors that affect the rates of reactions. This demo takes about three minutes to perform.

One day of lead time is required for this project.

Discussion 

Iron combusts in oxygen to form various iron oxides, mainly iron(III) oxide:
$\ce{4Fe(s) + 3O2(g) -> 2Fe2O3(s)}$

Iron in its usual bulk solid form will only burn when in pure oxygen with when a great deal of heat is supplied. This is what a cutting torch does. As we can see in this demo, however, when iron is subdivided finely it burns readily enough in air. We also see that when the concentration of oxygen is increased, the rate of reaction also increases greatly. Air is about 20 percent oxygen, so when we burn the steel wool in pure oxygen, we have increased the concentration by a factor of about five. 

Materials 

• a small ball of steel wool about 1 cm in diameter
• a large, wide mouth, stoppered Erlenmeyer flask with sand in the bottom filled with pure oxygen
• large forceps
• propane burner with striker

Procedure 

• loosen but do not remove the stopper
• light and adjust the propane burner
• holding the ball of steel wool in the forceps, ignite it in the flame of the propane torch
• quickly remove the stopper and insert the smoldering steel wool into the flask
• after it has burned for a couple of seconds, drop the steel wool, otherwise it will weld the ends of your forceps together
• turn of the propane torch and replace the stopper in the flask

Safety Precautions 

Pure oxygen is a strong oxidizer. It will greatly accelerate combustion. Keep flammable materials away. Keep a fire extinguisher on hand.

Prep. Notes 

Make sure that there is plenty of sand in the bottom of the Erlenmeyer flask. To fill the flask with oxygen, run a rubber hose from the regulator into the bottom of the flask with the stopper resting lightly on top. Adjust the flow rate to about 1 or 2 liters per minute and let it run for a couple of minutes. Remove the hose and stopper the flask tightly. Make sure that there is no spark, flame, or flammable materials around while you are filling the flask.