Question

Give reason for the following:
The shining surface of some metal becomes dull when exposed to air for a long time?

Answer 1

Hint: The shining surface of some metals become dull when exposed to air because the metal reacts with air to form a metal oxide layer on the metal surface.

Complete step by step answer:
Surface of some metals acquire dull appearance due to the process of corrosion. Corrosion is the process of slowly eating up of the metals due to attack of atmospheric gases such as oxygen, carbon dioxide, hydrogen sulphide, water vapour, etc. on the surface of the metals so to as to convert the metal into oxide ‘carbonate’ sulphide is known as corrosion. For example aluminium objects lose their lustre and become dull after sometime. This is due to the corrosion of aluminium. When aluminium is exposed to moist air its surface is covered with a thin impervious layer of aluminium oxide${Al_2}{O_3}$.

Additional Information: Methods to prevent corrosion:
(a) By painting the iron article such as window grille, iron gates, steel furniture, railway coaches, bodies of care buses etc.
(b) By greasing and oiling of the iron article such as mechanical tools, machine parts,
(c) By galvanisation i.e. coating the surface of iron objects with a thin layer of Zinc.

Note:
Corrosion of all metals is not always troublesome, sometimes corrosion of metals is an advantage because it prevents the metal underneath from further damage. More reactive the metal the more easily it corrodes while noble metals such as gold and platinum which lie at the bottom of the activity series do not corrode readily.