Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: When iron and sulfur are mixed together in a certain ratio and heated, a chemical reaction occurs. This results in the formation of a unique new compound, called iron sulfide (FeS). A magnet cannot be used to mechanically separate the iron from the iron sulfide because metallic iron does not exist in the compound. Instead, another chemical reaction is required to separate the iron and sulfur. Answer: iron sulfide
When iron and sulfur are mixed together in a certain ratio and heated, what do they become?