Informal vs. Formal Assessments. In order to properly understand what informal assessments are, we should first differentiate them from formal assessments. Formal assessments are designed to tell how well a student has progressed as evaluated against other students.
Rusting of iron consists of the formation of hydrated oxide, Fe(OH) 3, FeO(OH), or even Fe 2 O 3 .H 2 O. It is an electrochemical process which requires the presence of water, oxygen and an electrolyte. In the absence of any one of these rusting does not occur to any significant extent.
Another example of oxidation-reduction reaction is the tarnishing of silver in the presence of atmospheric oxygen gas. Similar example is corrosion of iron in the presence of oxygen to form ferrous oxide and ferric oxide which are deposited on the metal surface and called as rust.