Correct Answer - 5
When a metal like iron is exposed to moist atmoshphere, a layer of rust, that is, a hydrated oxide `Fe_(2)O_(3)`. `xH_(2)O` is formed on its surface. This also contains some amount of `FeCO_(3)`. Apart from this, when impure metal, that is, iron having other metals as impurities comes in contact with water having `CO_(2),` a voltaic cell comes into existence. Iron passes into solution as ferrous ion.
Passivity on the other hand is the process wherein the metal stops reacting due to the formation of an impervious layer on its surface. `Fe,Al,Cr,` when dipped in conc. `HNO_(3)` have initially some reaction which ceases completely after some time. Thereafter, the metals stops reacting with any reactant.
