# Equilibrium in a pure substance

1. Apr 24, 2017

### mech-eng

The book says when the water is heated, the pressure stays constant. But I do not understand pressure for a liquid-water system. What is the pressure for a liquid here? the pressure on the surface, botton or sides?

Source: Fundamentals of Thermodynamics by Sonntag/Borgnakke/Van Wylen.

Thank you.

2. Apr 24, 2017

### CWatters

The pressure at the top surface of the water is determined by the weights and the area. The pressure at other points depends on the surface pressure AND the density and depth at that point.

3. Apr 24, 2017

### CWatters

In this case the pressure everywhere stays constant because as the water expands the density reduces. I believe these two things cancel out.

The pressure is primarily determined by the weights and area.

4. Apr 24, 2017

### mech-eng

I especially ask about vapor-pressure curve. On my copy of book, I cannot find it, I can its picture later. In that curve perpendicular axis is pressure and horizontal axis is temperature and there is parabolic curve with positive slope called vapor-pressure curve again. I cannot understand what it represents. Can it be related to pic a, at the top, only compressed water case?

Thank you.

5. Apr 24, 2017

### mech-eng

This is the diagram.

It seems strange to me because for picture a at the top, compressed liquid case, there is no vapor so there should be no vapor pressure but there is a temperature value. Then should the graph start from somewhere on temperature axis, at which vapor pressure is zero, then going horizantal to saturation temperature then making the curve. But I am confused that I cannot surely determine if the pressure should only belong to vapor?

Thank you.

6. Apr 24, 2017

### Staff: Mentor

Vapor pressure refers to the pressure you would get in the gas phase in a closed system with room for the gas. But it may not be the pressure of the liquid. If you have a glass of water (between 0 and 100 °C), then the water is at 1 atm pressure, whatever the value of the vapor pressure.

The process in the OP is at constant pressure, so it corresponds to a horizontal line on the graph above. Point (a) is to the left of the vapor-pressure curve, point (b) is on the curve, and point (c) is to the right of the curve.

7. Apr 24, 2017

### mech-eng

I.e pressure is constant while temperature is always rising? And I do not understand why point c should be to the right of the curve instead of being on it?

Thank you.

8. Apr 24, 2017

### Staff: Mentor

If the external temperature is exactly equal to the phase transition temperature, then nothing will happen when the liquid reached that temperature. To be converted into a gas, additional energy is required, which is usually achieved by having an external temperature above the phase transition temperature. As a result, when the phase transition is completed, the vapor will most often be heated.

In theory, figs. 2.1 (a)-(c) could all be exactly at the transition temperature, but in practice this is never the case.

9. Apr 24, 2017

### mech-eng

I understood from your last post. It is the curve denoting the saturation temperatures corresponding to saturation pressures. I thought it was about vapor pressures, saturated or superheated, corresponding to temperatures. But it is not.I think the authors names it incorrectly. It is name is confusing.

Thank you.

10. Apr 24, 2017

### Staff: Mentor

I don't think there is anything named incorrectly. Look up the definitions for the different names and see if you can try and sort them out.

11. Apr 24, 2017

### mech-eng

This diagram from another source, Çengel/Boles, is quite different from the belonging to Sonntag/Borgnakke.

Thank you.

12. Apr 24, 2017

### Staff: Mentor

I don't see any significant difference (especially keeping in mind that the first one is schematic).