Q.7:- Which of the following materials fall in the category of a “pure substance”?
(d) hydrochloric acid,
(e) calcium oxide,
Ice, iron, calcium oxide and mercury are pure substances since they contain particles of only one kind of matter. In contrast, milk, hydrochloric acid (hydrogen chloride gas dissolved in water), brick and air cannot be called pure substances because they consists of particles of more than one kind of matter.