Remember our demonstration of burning iron wool as an example of a composition reaction (Session III-1 in the lab manual)? Here’s a bit more information about what you saw.
First, when the iron in the wool reacted with the oxygen in the air it formed iron oxide, but what was the equation? Was it 2 Fe + O2 → 2 FeO ?
Because iron can be in different states – iron (II) and iron (III) – there are actually three potential products. FeO is one product, but other reactions might be:
4 Fe + 3 O2 → 2 Fe2O3
3 Fe + 2 O2 → Fe3O4
We can not tell which products we obtained by merely looking at the results.
By the way, the second equation above is for rust. We were creating rust more quickly because we added heat to get the reaction started. You can also speed up rust with salt water (see a simple experiment at Growing With Science blog.)
FYI: If you ever are in a survival situation and just happen to have steel wool and a 9-volt battery, you can use your chemistry knowledge to start a fire!
Want to see the reaction again? Check this NurdRage video:
Please let me know if you have any questions.