Burning-Hot Demonstration of Surface Area
Most students would agree that metals aren’t expected to burn. However, this activity of burning steel wool demonstrates the importance of large surface area, rapid oxidation of metals, and conservation of mass.

Materials List: 1 g very fine steel wool 250 mL beaker 50 mL acetone, lab grade3 Triple beam balance Bunsen burner Crucible tongs Disposable aluminum pie plate Ring stand with clamp

Chemical Concepts:

• Metals are combustible if their surface areas are very high. Finely divided or powdered metals are very flammable.
• Combustion is an exothermic oxidation process that occurs with most organic compounds and some elements.
• The combustion of metals produces metallic oxides, which are usually solids. Therefore, as the combustion occurs, the mass will increase.

Advanced Preparation:
Place the steel wool in the 250 mL beaker and cover with acetone. Allow the steel wool to soak in the acetone for 20 to 30 minutes. Remove the steel wool from the acetone and allow it to dry overnight under a fume hood. This removes any oily coating placed on the steel wool by the manufacturer to prevent rusting.

Procedure:

1. Carefully weigh out about one gram of steel wool using a triple beam (or equivalent) balance. Use a disposable aluminum pie pan as a weighing dish. This will assist in collecting the entire residue that will form later in the demonstration.
2. Using the tongs, unfold the steel wool over the pie pan to collect any particles that may fall. Pull the strands as far apart as possible to get maximum surface exposure. Hang the steel wool from a clamp on a ring stand. Place the disposable pie pan containing any tiny particles under the hanging steel wool.
3. With students at a safe distance, ignite the steel wool with the Bunsen burner. It will burn rapidly surprising most students. Some steel wool strands will break off from the bulk. However, with the combustion done over the pie pan, most of the residue can be captured.
4. After about 2 minutes, the steel wool is cool enough to handle. Again, use the pie pan as a weighing dish and determine its mass. Ask the students to predict if it will be greater or less. They will usually say less, thinking about the ash from combustion of organic materials. Someone might predict no change due to conservation of mass. The mass actually increases due to the addition of oxygen.

Discussion:
Most students don’t recognize combustion as an oxidation process and don’t realize that many elements can burn. They may have burned sulfur or carbon but would be reluctant to accept the idea that many metals can burn violently. Finely divided metals, especially iron, magnesium, aluminum, and zinc dusts are extremely flammable. Under the right conditions, they can even form an explosive mix with air. The key to burning metals is the surface area of the metal and the amount of oxygen available. Airborne metallic dusts or fine strands of metals have large surface areas where rapid oxidation can occur. For this demonstration, the equation is:
   4Fe(s) + 3O₂(g) -> 2Fe₂O₃(s)

Ferric oxide (Fe₂O₃) has a bluish-gray color, rather than the typical rust color associated with the ferric ion in water solution or iron rusting in the presence of water.

If you were fortunate enough to achieve complete combustion, the increase in mass should be about 43% (molar mass Fe₂O₃ / molar mass 2Fe). However, due to incomplete combustion and losses in the collection process, an increase of about 25% is more likely.

Safety Precautions:
Acetone is extremely flammable and a dangerous fire risk. It is slightly toxic both by inhalation and ingestion. Use only with proper ventilation and keep away from sparks and any open flame. Remove all flammable materials from the demonstration area. Wear chemical splash goggles, chemical-resistant and insulating gloves, and a chemical-resistant apron.

Disposal:
The steel wool and combustion residue can be discarded in the trash. The excess acetone can be evaporated in the fume hood or, even better, recycled and used for the same demonstration next term.

References:
Bilash, B. B., Gross, G. R., Koob, J. K. A Demo A Day; Flinn Scientific: Batavia, IL 1995; p 201.

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