Chemical Change
Oxidation
Oxidation is the reaction that occurs when an inflammable substance is exposed to oxygen and this results in a change in the chemical properties of substances. When iron is exposed to moisture and oxygen, for example, the iron element interacts with oxygen. Oxygen and iron combines forming a compound called iron oxide. Iron oxide is a reddish substance that is commonly called rust.
Tarnishing
When a bright metal is exposed to a nonmetal, usually oxygen, sulfur dioxide, or hydrogen sulfide, it produces a dark color. This chemical reaction is called tarnishing. Unlike rusting, tarnishing affects only the top layer of an object. When silver tarnishes because of the reaction between oxygen and hydrogen sulfide, it forms a black film over the shiny surface of the silver object.
