The surface of some metals acquires a dull appearance when exposed to air for a long time due to the formation of a thin layer of oxide, carbonate or sulphide on their surface by the slow action of the various gases present in air.

It is because they react with various gases present in atmosphere like oxygen and carbon dioxide to form Metal oxide and metal compound .The surface of some metals acquires dull appearances when exposed to air because they corrode.

Introduction:
When metals are exposed to air for a long time, some of them develop a dull appearance on their surface. This phenomenon is commonly referred to as metal tarnishing or oxidation. The process occurs due to the reaction between the metal and the oxygen in the air, resulting in the formation of a layer of metal oxide on the surface.

Explanation:
Metal tarnishing can be explained by the following steps:

1. Formation of Metal Oxide:
When metals are exposed to air, they react with the oxygen present in the atmosphere. This reaction leads to the formation of metal oxide on the surface of the metal. For example, iron reacts with oxygen to form iron oxide (rust), while copper forms copper oxide.

2. Protective Layer Formation:
In some cases, the metal oxide layer that forms on the surface acts as a protective layer, preventing further oxidation of the metal. This layer can be seen on metals like aluminum and stainless steel, which acquire a dull appearance but do not corrode extensively.

3. Lack of Luster:
The dull appearance of metals after oxidation is due to the change in their surface properties. The metal oxide layer is often porous and rough, which scatters light and reduces the reflection of light from the metal surface. As a result, the metal loses its luster and appears dull.

4. Chemical Reactions:
Apart from the metal-air reaction, other chemical reactions can also contribute to the dull appearance of metals. For example, sulfur compounds present in the air can react with metals like silver and lead, forming a dark layer of metal sulfide on the surface.

Prevention:
To prevent metal tarnishing, various methods can be employed:

1. Protective Coatings: Applying a protective coating, such as paint or varnish, can act as a barrier between the metal surface and the air, preventing direct contact and oxidation.

2. Galvanization: Coating metals with a layer of zinc through a process called galvanization can provide protection against oxidation. Zinc acts as a sacrificial anode, corroding in place of the underlying metal.

3. Alloying: Creating alloys by combining metals can enhance their resistance to tarnishing. For example, stainless steel, which contains chromium, is more resistant to oxidation than pure iron.

4. Storage in Controlled Environments: Storing metals in environments with controlled humidity and reduced oxygen levels can minimize metal tarnishing. Vacuum-sealed bags or nitrogen-filled containers are commonly used for this purpose.

Conclusion:
The dull appearance of metals when exposed to air for a long time is primarily due to the formation of metal oxide on the surface. This oxidation process alters the surface properties, resulting in reduced luster and a dull appearance. Taking preventive measures and employing protective coatings or storage methods can help maintain the original appearance and properties of metals.