The chemical change when a substance loses its luster is called tarnishing. Tarnishing occurs when a metal reacts with oxygen, moisture, or other chemicals in the environment to form a layer of corrosion on the surface of the metal. This layer of corrosion can cause the metal to lose its shine and appear dull or discolored.

The most common example of tarnishing is the formation of rust on iron and steel. Rust is formed when iron or steel is exposed to oxygen and moisture in the air. The oxygen atoms react with the iron atoms to form iron oxide, which is a reddish-brown compound. Iron oxide is a porous material, which means that it allows oxygen and moisture to continue to penetrate the metal, causing the rust layer to grow thicker and the metal to deteriorate further.

Other metals that can tarnish include copper, silver, and brass. Copper tarnishes to form a green layer of copper carbonate, silver tarnishes to form a black layer of silver sulfide, and brass tarnishes to form a brown layer of copper and zinc oxides.

Tarnishing can be prevented by protecting the metal from exposure to oxygen and moisture. This can be done by applying a protective coating to the metal, such as a lacquer or sealant. Alternatively, the metal can be kept in a dry, oxygen-free environment.