What Is a Molecule?
The word molecule means “smallest mass,” which indicates the very smallest division that can be made of any substance without destroying its identity. Every substance is made up of molecules, and in many cases the molecules of one substance will mix with those of another substance, while in other cases they will not. When you dissolve sugar in water or melt lead or change water into steam, the physical body of the substance is changed, but the molecules remain as they were. They are only changed in so far as their relations to each other and to those of another substance are concerned.