Yes HCL is a pure substanse, although compound of Hydrogen and Chlorine.
HCl (hydrogen chloride) is a pure substance.
In chemistry, both elements and compounds are pure substances.
An aqueous solution of hydrogen chloride is called hydrochloric acid,
and it is also usually given the same formula (HCl) as that of pure hydrogen .
HCl is gas as well as acid .
The elements and compounds are considered as the pure form of matter which have their properties and that is different from constituent elements in case of a compound and since CaO is an oxide (compound) of calcium which is separated as Ca and O-atoms only by chemical reactions so Calcium oxide is calledpure.