Quality

Whenever a substance exists as portion of liquid and portion of vapor at saturation conditions, its quality (x) is stated as the ratio of the mass of the vapor to the total mass of both liquid and vapor. Therefore, when the mass of vapor is 0.2 lbm and mass of liquid is 0.8 lbm, the quality is 0.2 or 20%. The quality is an intensive property. The quality has meaning whenever the substance is in a saturated state merely, at saturation pressure and temperature. The region under the bell-shaped curve is shown in figure below illustrates the area in which quality is significant.

Figure: T-V Diagram illustrating the Saturation Area