a) Corrosion: Corrosion is defined as a process where a substance, generally metal deteriorates and forms a layer of oxide on its surface. Metals get converted into their hydrated oxides or sulphides. Ex; Iron, Cu, Ag

\[
4Fe + 3O_2 + nH_2O \rightarrow
\underset{\text{Hydrated iron oxide}}{2Fe_2O_3 \cdot nH_2O}
\]

\[2Ag+H_2S \rightarrow Ag_2S+H_2\]

b) Rancidity: It is a process in which food items like fats and oils are oxidised. This results
in the change in taste and smell of the food item and is called rancidity. For example, when
any fried food is exposed to air for a long time then it has a change in smell and taste and it
becomes rancid.

Rancidity can be avoided by:

Adding antioxidants like BHA (Butylated tri hydroxy anisole)
Refrigerate the food items
keep food items in airtight containers
Adding nitrogen to food packets to prevent oxidation