Answer the following question given this paragraph: 

A: Another example of matter changing color is a penny changing from reddish brown to greenish brown as it becomes tarnished. The color change indicates that a new chemical substance has been produced. Copper on the surface of the penny has combined with oxygen in the air to produce a different substance called copper oxide.


Q: What substance is created when a copper turns from reddish brown to greenish brown?


A:
copper oxide