Question

when iron rusts and forms iron oxide, the iron oxide has more mass than the iron. which statement correctly explains this difference in mass? (1 point)
the iron atoms in iron oxide have more mass than pure iron.
iron oxide has oxygen atoms, and pure iron does not.
there are more iron atoms in iron oxide than in pure iron.
the oxygen atoms in iron oxide have more mass than the iron atoms in pure iron.

Explanation:

• Iron is a pure element. When it rusts (reacts with oxygen), it forms iron oxide (\(Fe_2O_3\) for example). The mass of a substance is the sum of the masses of its constituent atoms.
• In pure iron, there are only iron atoms. In iron oxide, there are iron atoms and oxygen atoms.
• The mass of iron oxide is the mass of iron atoms plus the mass of oxygen atoms. So, the increase in mass comes from the oxygen atoms that are part of iron oxide but not of pure iron.
• The first option is wrong because an iron atom is the same in iron and iron oxide (same number of protons, neutrons, and electrons). The third option is wrong because in a given amount of iron that rusts, the number of iron atoms doesn't increase (conservation of atoms in a chemical reaction, atoms are just rearranged). The fourth option is wrong because oxygen atoms (\(O\) has an atomic mass of about \(16\ g/mol\)) have less mass than iron atoms (\(Fe\) has an atomic mass of about \(56\ g/mol\)).

Answer:

Iron oxide has oxygen atoms, and pure iron does not.