Why is iron sulphide a compound?

Iron sulphide is a compound because of the following reasons:

1. Iron sulphide is a compound formed when iron and sulphur combine chemically, on heating, in 7 : 4 ratio by mass.
2. Iron is a grey black metal and Sulphur is a yellow amorphous non-metallic solid whereas iron sulphide is a black solid.
3. Iron is attracted by a magnet and Sulphur is soluble in carbon disulphide but iron sulphide is neither attracted by a magnet nor it is soluble in carbon disulphide.

From the above, we can say that properties of iron sulphide are entirely different from its constituent elements i.e., iron and sulphur. Hence, iron sulphide is a compound.