Question

Iron does not burn on heating but iron filings burn vigorously when sprinkled in the flame of the burner why

Answer 1

Iron does not burn on heating because the oxygen can't get inside, instead it forms a layer on the surface that keeps the oxygen from getting to the rest of the iron. Whereas if you grind up the iron and turn it into the iron fillings it would burn easily.

Answer 2

Iron wont burn in a flame as it would become glowing red hot, it forms an oxide coating, but it won't burn as it conducts heat quickly away toward the part that's not in the flame. Unlike this, in case of Iron Filings, there is even the possibility of vaporizing the metal long enough for it to support a flame. This is similar to the case when a log won't burn with a match or lighter held under it, but sawdust and wood shavings will burn easily.