What happens if sodium sulfide is added to iron(II) sulfide? [Fe^2+] increases and [S^2] decreases because sulfide is a common ion.

[Fe^2+] decreases and [S^2] increases because sodium is a common ion.

[Fe^2+] and [S^2] remain the same because sodium is not a common ion.

[Fe^2+] decreases and [S^2] increases because sulfide is a common ion.