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. 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.

Pure substances are: Ice, Iron, Hydrochloric Acid, Calcium oxide and Mercury. All these substances are pure because these are made of same kind of atoms or molecules. They have fixed melting or boiling points.

On the other hand, Brick, Air, Milk and wood are not pure substances because all of these do not have a fixed composition. Air is a mixture of gases in which the composition of gases is not fixed. Milk is a colloidal solution.