Does rust make this heavier or lighter?

Question

"You place a nail with a mass of 13.8g in a moist place for two weeks. When you retrieve the nail, it is covered with a crusty brown coating of rust. You then place the nail on a balance to measure its mass again, Will the nail have a mass that is greater less than or equal to 13.8. Explain."

I have no clue. At first I thought it'd be heavier because the rust would be built up on it, but then i thought maybe the rust eats away at the nail? Idk

Help :]!?

Answer 1

Rust has a tendency to flake off, but as long as all of the rust is still on the nail, the nail will be heavier.  The reason is that you are adding oxygen and hydrogen.

It takes both oxygen and water to make rust:

4Fe(s) + 3O2(g) + 2H2O(l) --> 4FeO(OH)(s)

Rust can typically exist in three forms:

Fe(OH)3 - in water

FeO(OH) - in humid air (common form of rust)

Fe2O3 - in arid conditions, very dry