A pure substance is made up of only one kind of particles. All the constituent particles of a pure substance are the same in their chemical nature. These particles may be atoms or molecules. For example, sulphur is made up of only one kind of particles called sulphur atoms, therefore, sulphur is a pure substance. Similarly, water is made up of only one kind of particles called water molecules. All the elements and compounds are pure substances because they contain only one kind of particles. A pure substance cannot be separated into other kinds of matter by any physical process. A pure substance has a fixed composition as well as a fixed melting point and boiling point.