Here's why:

* No chemical reaction: The iron and sulfur particles are physically mixed, but they haven't chemically reacted to form a new compound.

* Individual properties: You can still see and separate the iron and sulfur particles. They retain their original properties.

* Variable proportions: The ratio of iron to sulfur can vary in the mixture.

Upon heating, the iron and sulfur react to form iron sulfide (FeS), which is a compound.