Why do all of the resources I find say that any point along the saturated liquid line is 2 phases?

How can statement 1 and 2 both be correct?
  1. Steam Quality @ p,v,t on sat liquid line= 0 =>(100% liquid)
  2. number of phases present on saturated liquid line = 2

It has to be 1 phase the liquid phase. I say that because if I calculate the steam quality on the saturated liquid line it's 100% liquid.


I was reviewing materials studying for PE...I hit this, and now feel like everyone else is wrong and I am right. Since I know that is not reasonable, someone please help me understand what I am missing here. By intuition I solved it correctly but when i looked at the answer I felt like the gibbs phase just doesn't make sense, because I can't get over the fact that a saturated liquid line means 0% vapor.

The problem is as follows: enter image description here

  • $\begingroup$ Water has a triple point where it can be any of 3 phases.... $\endgroup$
    – Solar Mike
    Commented May 1, 2021 at 17:53
  • $\begingroup$ while true it does not answer the question. Especially how can statements 1 and 2 both be correct but not conflicting. $\endgroup$
    – LeeRuns
    Commented May 1, 2021 at 22:43
  • $\begingroup$ So what happens if you have 100% steam, 0% liquid and decrease energy until you are on the line? $\endgroup$ Commented May 2, 2021 at 0:11
  • $\begingroup$ @LeeRuns comments are comments and don’t need to answer the question. Answers are different and I posted a comment. I wasgiving you a pointer so you could expand your research & understanding to figure out the answer for yourself. $\endgroup$
    – Solar Mike
    Commented May 2, 2021 at 5:34
  • $\begingroup$ @SolarMike sorry if I came across as rude was looking at your comment and thought maybe he we answer if I point out I'm still missing an answer. Now that I have one, I see that its not what I thought it was. Meaning sat liquid line is not the last stop on the bus before getting into vle town. Instead it is really the first stop in vle town. That jives with the context you provided on triple point. $\endgroup$
    – LeeRuns
    Commented May 3, 2021 at 12:18

1 Answer 1


The saturation line is a two phase line.

Saturated liquid is primarily liquid with an infinitesimal amount of vapor. Because we cannot express infinitesimals with a finite number of significant digits, we say that the saturated liquid has 100% liquid. We need only to cool that liquid by an infinitesimal change in temperature at constant pressure or increase the pressure by an infinitesimal amount at constant temperature to truly have 100% liquid.

Comparable arguments apply for saturated vapor.


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