PIMSO 2023 - Science - Secondary 4 - National Round Question 7

Question 7

After months of exposure in air, 1.9 g of nail made of steel rusted partially. The rusting process, forming a compound of iron and oxygen (which comes from air) stopped only once the whole surface of the nail was covered in rust, effectively protecting the iron under the surface from rusting. What can you say about the mass of this nail upon rusting?

Editorial assumption: all metal and corrosion products remain attached; evaluate the mass claim separately from the stated protective-layer mechanism.

The insides were protected so the mass should still be 1.9 g because the rusting stops
The mass should be less than 1.9 g because the insides were protected
The mass should still be 1.9 g because of the law of conservation of mass
The mass should be more than 1.9 g because of oxygen incorporates in the material
Nothing meaningful could be said based on the information given