Which of the following are not compounds?
$(a)$ Chlorine gas
$(b)$ Potassium chloride
$(c)$ Iron
$(d)$ Iron sulphide
$(e)$ Aluminium
$(f)$ Iodine
$(g)$ Carbon
$(h)$ Carbon monoxide
$(i)$ Sulphur powder

(A, C, E, F, G, I) compound is a substance formed when two or more chemical elements are chemically bonded together. Elements are pure substances consisting of only one type of atom.
$1$. Chlorine gas $(Cl_2)$ is an element.
$2$. Potassium chloride $(KCl)$ is a compound.
$3$. Iron $(Fe)$ is an element.
$4$. Iron sulphide $(FeS)$ is a compound.
$5$. Aluminium $(Al)$ is an element.
$6$. Iodine $(I_2)$ is an element.
$7$. Carbon $(C)$ is an element.
$8$. Carbon monoxide $(CO)$ is a compound.
$9$. Sulphur powder $(S)$ is an element.
Therefore,the substances that are not compounds are: Chlorine gas,Iron,Aluminium,Iodine,Carbon,and Sulphur powder.