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Why the aluminium becomes dull after sometime.

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Some metals often lose their shine and appears dull, because of the action of air on them. Some metals as a result of oxidation with the atmospheric oxygen form oxides and by which their appearance becomes dull. Similarly aluminium reacts with oxygen in the atmosphere to for aluminium oxide.

4Al +3O2 ---------> 2Al2O3

At this time no further reaction can takes place, hence this oxide layer acts as protection from corrosion. Due to this oxidation reaction and oxide layer formation, aluminium becomes dull after sometime.

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When Aluminum is exposed to air for some time then the it forms an Aluminum Oxide layer by reacting with Oxygen present in air. Due to which it looks dull after a time period. The reaction is such as-

2Al + 3O2 → Al2O3

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almunium look dull after some time because it reacts with oxygen present in air and forms almunium oxide

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Aluminium is a self protecting metal.

When exposed to air it forms its own oxide. This is a thin layer of aluminium oxide formed over the aluminium metal to prevent further oxidization.

4Al + 3O2-----2Al2O3

It is due to this reaction aluminium looks dull after some time.

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