CBSE Class 9 Answered

pure substances are those which are same throughout. If its element then it has 1 type of atoms, ( here the same atoms combine to form a molecule  eg; H₂)if its a compound its 1 type of molecules i.e different atoms combined in a fixed ratio eg; H₂O. In water you find 2 H and 1 O forming a molecule and its throughout the same H₂O molecules. suppose in this some other atoms or molecules added not in any particular ratio then its mixture or impure.