why is water called a pure substance ?

A pure substance is a substance which is a pure form of matter, i.e., all it's constituent particles are the same and have the same chemical composition. A pure substance is either an element or a compound. Water is a compound, one molecule of which is made of two atoms of hydrogen and one atom of oxygen. As it is a compound, it can be formed only when it's constituents hydrogen and oxygen combine together in a fixed proportion, i.e., 2:1.