Iron is a solid, gray metal. Oxygen is a colorless gas. When they chemically combine, rust is made. Rust has a reddsh brown color. Why is rust different from the iron or oxygen that it is made of?

1 Answer
Dec 30, 2015

Because they react.


Iron(#Fe^(2 or 3)#) reacts with oxygen creating two oxides:
#2Fe^(+2) + O_2 -> 2FeO# If it is Iron(II), create Iron(II) oxide;
#2Fe^(+3) +3O_2 ->Fe_2O_3# If it is Iron(III), create Iron(III) oxide (or Ferric oxide).

The color before the reaction only matter if both reagents are liquid or aqueous and if they don't react.