Give reasons: Aluminium utensils should not be kept in water overnight.

(N/A) The oxygen present in water reacts with aluminium to form a thin layer of aluminium oxide $(Al_2O_3)$.
This layer acts as a protective coating,but when water is stored in an aluminium vessel for a long time,some amount of aluminium oxide may dissolve in the water.
Since aluminium ions are toxic and harmful to health,water should not be stored in aluminium vessels overnight.
The reaction is: $2Al(s) + 3H_2O(l) \rightarrow Al_2O_3(s) + 3H_2(g)$.