When Can a Mixture be Treated as a Pure Substance ?

Ch 2, Lesson A, Page 2 - When Can a Mixture be Treated as a Pure Substance ?

• Consider the Heterogeneous Mixture shown here.  The composition near the top of the vessel is very different from the composition near the bottom of the vessel.
• Therefore, this system cannot be considered a pure substance.
• What about the homogeneous mixture shown on the left ?  It is not a pure substance because it contains different types of molecules.  But, can it ever be treated as a pure substance anyway ?
• Yes, sometimes it can.
• Because the mixture is homogeneous or uniform, the composition is the same throughout.
• As long as the composition remains constant and uniform throughout, this mixture can be treated as a pure substance.  We call this a pseudo-component.
• Air is made up of approximately 79 % nitrogen molecules and 21 % oxygen molecules and yet we often consider it to be a pure substance or pseudo-component.
• If, for example, a chemical reaction involving only one of the species in a mixture occurs, then the composition will change.  Therefore, it would not be useful or accurate to treat the system as a pure substance.