hello so my exam is this wednesday but i am a little confuse

hello, so my exam is this wednesday, but i am a little confused about what my professor posted as hints on the exam this is what she said

\"Exam #3

Posted on: Wednesday, November 23, 2016 5:06:00 PM CST

Dear All,

I have just posted the review notes for chapter 6.

Also, the exam will include two big problems (in addition to the small questions). For the two big problems, there will be ideal gas as well as substance that exhibits liquid-vapor phase changing behaviors....\"

i understand the ideal gas problem, im simply confused about \"a substance that exhibits liquid-vapor phase changing behaviors\".

if you could clarify this for me or give me and example of how such a problem would look like, it would be greatly appreciated

thank you

Solution

A subbsbtance (for example water) can change phase from liquid to vapor. But during the process, there comes a stage when some portion of it is liquid and some portion as vapor. This regime is called 2-phase regime or phase-changing regime as mentioned by your professor. During the phase change process, the quality x of the steam is given by

x = mass of vapor / (mass of liquid + mass of vapor)

During phase changing process, both pressure and temperature remain constant.

So when you start to heat water (at constant pressure) from say, room temperature of 25 C, and 1 atm pressure, it\'ll first reach a point where it\'ll be at 100 deg C but it\'ll still be 100% liquid. Then starts the phase change process. So some portion of it will be liquid water at 100 deg C and some will be steam at 100 deg C. On further heating, entire water will be in vapor phase at 100 deg C.

In terms of liq-vapor saturation curve, the left side curve denotes saturated liquid and right side curve denotes vapor. Whatever is in between the two curves, denotes the two-phase portion.

Hope this helps.