Question

Why does sulphur dissolve when carbon
disulphide is added to a mixture of iron and
sulphur but not when it is added to iron
sulphide ?​

Answer 1

Sulphur dissolve when carbon disulphide is added to a mixture of iron and Sulphur because they form a mixture. And as we know that mixtures retain their individual properties, the above step is possible.

Now, Sulphur does not dissolve when it is added to iron sulphide because they loose their individual properties. The atoms/molecules do not exist freely/separately in this situation.