When copper and sulfur combine, they form copper sulfide (CuS). This is a chemical reaction that results in the formation of a new compound with different properties than the original elements.

Here's a breakdown of the reaction:

* Copper (Cu) is a reddish-brown metal.

* Sulfur (S) is a yellow non-metal.

When heated together, copper and sulfur react to form copper sulfide (CuS), a black solid. This reaction is an example of a synthesis reaction where two or more reactants combine to form a single product.

The balanced chemical equation for the reaction is:

Cu + S → CuS

This means that one atom of copper reacts with one atom of sulfur to produce one molecule of copper sulfide.