A substance is a general word that can be used for any kind of material including elements, compounds and mixtures. Perhaps the the word substance is used by mistake instead of compound. A third type of substance often referred in chemistry is an element.
An element is a substance that has only one type of atoms in its molecules. A compound is a substance formed by chemical combination of more than one elements. The molecules of compounds contain more than one type of atoms. However, the molecules of any one compound are all alike in composition.
A mixture is a combination of two or more substances that have not combined chemically. The constituent substances can be elements, compound, or a combination of the two. A mixture consists of combination of molecules of different types.
A SUBSTANCE has same type of molecules..But a MIXTURE has different molecules,,,,