When sulfur and copper are heated together, they undergo a chemical reaction to form copper sulfide. The reaction can be represented by the following equation:

Cu + S → CuS

In this reaction, copper atoms (Cu) combine with sulfur atoms (S) to form copper sulfide (CuS). Copper sulfide is a black, crystalline compound that is insoluble in water. It is commonly used as a semiconductor material in electronic devices, such as solar cells and photodetectors.