Re: Why do iron filings give off sparks in fire but not iron rods?

Iron filings give off sparks when they burn in air, converting from iron to 
iron oxide.  The simple answer to your question is because of surface to 
volume ratio.

In order for something to burn, the surface in contact with the air must be 
heated to its ignition temperature.  The surface then reacts with oxygen in 
the air, giving off heat and byproducts of the reaction.  As you attempt to 
heat a surface to reaction temperature, heat is conducted away from the 
surface, partially foiling your efforts.  The mass of a small particle is 
such that the entire particle can be heated to ignition temperature.

The rate of the reaction between iron and air is largely proportional to 
surface area of the iron exposed to air.  Once heated to ignition 
temperature, the small particle has a relatively large surface area for 
reaction; perhaps 1 to 10 square meters per gram.  A large rod of iron may 
have a surface area of 0.01 meters per gram.  So even if the rod of iron 
were heated to ignition temperature, there wouldn't be enough combustion to 
keep the rod at temperature.

The same is true with most combustible solids.  A small wood match is easy 
to light and keep burning.  A large, single log is difficult to burn.

Hope that answers your question.