# Why does the temperature-volume diagram look the way it does?

Suppose we have a piston-cylinder system containing compressed water (water that is not about to vaporize). The pressure of water is equal to the sum of atmospheric pressure and the pressure exerted by the piston weight. As heat is transferred to the system, the liquid expands and exerts work to move the piston upward.

Net heat transferred to the system + Net work done by the system=Change in potential energy+ Change in K.E+Change in Internal energy

Can the change in potential energy be neglected because the center of mass of the system is raised only a small distance, since liquids do not expand as much as gases do, and because, after looking at the property tables of specific internal energy, the change in potential energy would be smaller in comparison? The change in kinetic energy can also be neglected because, yes, from the force balance, there would need to be an additional force to move the piston upward, but it must be negligible to keep the pressure of the gas constant.

Energy balance would reduce to

Net heat transferred to the system + Net work done by the system=Change in Internal energy

As more heat is transferred and the liquid reaches the saturated liquid state and is about to vaporize, the temperature and pressure are no longer independent. At the saturation temperature, does the temperature of the system remains constant because the net heat absorbed is used in the work done by the system to move the piston upward and to break the intermolecular bonds during the vaporization process?.

If we carry out the same experiment but add more weights on top of the system, compared to the system with the piston and no weights, we have done work on the system because the volume is initially compressed, so its specific volume is lower. Why is the specific volume of saturated liquid at a higher saturation pressure higher compared to a lower saturation pressure? Is it because outside the liquid-vapor mixture region, pressure and temperature are independent properties, and they are no longer independent. As the saturation pressure increases the saturation temperatures also increases, and the specific volume of liquids is a stronger function of temperature than pressure?

Why is the line connecting the saturated liquid and saturated vapor shorter as the pressure of the system increases? Is it because as more heat is transferred to the liquid-vapor mixture, the latent heat of vaporization required to break the intermolecular bonds in the liquid phase (which now occupies a larger volume for the same amount of liquid with larger distance between molecules) decreases at higher saturation pressure. Consequently, a larger fraction of the heat is used to exert more work to push the piston, weights, and atmosphere upward instead of doing work and breaking the molecular bonds.

• If there is compressed water in the cylinder, what/where is the gas? Commented Jul 9 at 15:23
• I am sorry for the confusing introduction, I have edited the post. Commented Jul 9 at 15:36
• OK, so the gas is atmospheric air. Commented Jul 9 at 16:24
• Yes, the gas above the piston is atmospheric air. Commented Jul 9 at 21:44