What if More Than One Phase is Present ?

Ch 2, Lesson A, Page 3 - What if More Than One Phase is Present ?

• Consider the glass of ice water shown here.  If we consider only the water and not the air above the ice water, is this a pure substance ?
• Yes, because everywhere in the system, all of the molecules are water.  The phase does not matter.
• What if we included the gas above the ice water ?  Would the system contain a pure substance ?
• Well, that depends.  The gas above the ice water might be pure water vapor…no air.  In that case the matter would be a pure substance.
• However, if there were AIR in the container then the composition of the gas phase would not be pure water like the liquid and solid phases.  Thus the matter in the container could NOT be considered to be a pure substance.
• Now, let’s take a little closer look at the whole concept of a “phase.”