phase is called the saturation pressure Boiling can only occur when the temperature reaches the saturation temperature, which is corresponding to the specified pressure, or when the pressure drops below the saturation pressure corresponding to the specified temperature. It is clear that T, increases with Ps. Thus, a substance higher pressures boils at higher temperatures ts=f(Ps) 6.2 Phase Change Process of Water In this section, we mainly focus on the phase change process of a substance from liquid to vapor. As a familiar substance, water is chosen to demonstrate the basic principles involved. Remember, however, that all pure substances exhibit the same general behavior 6.2. 1 Phase Change Process of water (1)Pre-heat stage: compressed liquid to saturated liquid Consider a piston-cylinder device containing liquid water at 20C and I atm pressure(state 1, Fig 6-1(a)). Under this condition, water exists in the liquid phase, and it is called a compressed liquid, or a sub-cooled liquid, meaning that it is not about to vaporize. Heat is now transferred to the water and thus its temperature rises to, say, 60C. As the temperature rises, the liquid water expands slightly, and thus its specific volume increases. To accommodate this expansion, the piston moves up slightly. The pressure in the cylinder remains constant at I atm during this process since it is determined by the outside barometric pressure and the weight of the piston, both of which are constant. Water is still compressed liquid at this state since it has not started to vaporize. As more heat is transferred, the temperature keeps rising until it reaches 100C (state 2, Fig. 6-1(b)). At this point, water is still a liquid, but any heat addition will cause some of the liquid to vaporize. That is, a phase-change process from liquid to vapor is about to take place. a liquid that is about to vaporize is called a saturated liquid. Therefore, state 2 is the saturated liquid state (2)Vaporization stage: saturated liquid to saturated vapor Once boiling starts, the temperature stops rising until the liquid has completely vaporized. That is, the emperature will remain constant during the entire phase-change process if the pressure is held constant This can easily be verified by placing a thermometer into pure boiling water on top of a stove. At sea level(p=l atm), the thermometer will always read 100C if the pan is uncovered or covered with a weightless lid. During a boiling process, the only change observed is a large increase in the volume and a steady decline in the liquid level as a result of more liquid turning to vapor. Midway about the vaporization line(state 3, Fig. 6-1(c), the cylinder contains equal amounts of liquid and vapor. As heat is continuously transferred in, the vaporization process continues until the last drop of liquid is vaporized (state 4, Fig 6-1(d). At this point the entire cylinder is filled with vapor that is on the orderline of the liquid phase. Any heat loss from this vapor will cause some of the vapor to condense A vapor that is about to condense is called a saturated vapor. Therefore, state 4 is a saturated vapor state. A substance at states between 2 and 4 is referred to as a saturated liquid-vapor mixture since the liquid and vapor phases coexist in equilibrium at these states. The amount of energy absorbed during vaporization is called the latent heat of vaporization and is equivalent to the energy released during condensation. Once the phase-change process completes, the water turns back to a single phase egion again, this time vapor99 phase is called the saturation pressure Boiling can only occur when the temperature reaches the saturation temperature, which is corresponding to the specified pressure, or when the pressure drops below the saturation pressure corresponding to the specified temperature. It is clear that Ts increases with s p . Thus, a substance at higher pressures boils at higher temperatures. ( ) s ps t = f （6-1） 6.2 Phase Change Process of Water In this section, we mainly focus on the phase change process of a substance from liquid to vapor. As a familiar substance, water is chosen to demonstrate the basic principles involved. Remember, however, that all pure substances exhibit the same general behavior. 6.2.1 Phase Change Process of Water (1) Pre-heat stage: compressed liquid to saturated liquid Consider a piston–cylinder device containing liquid water at 20°C and 1 atm pressure (state 1, Fig. 6–1(a)). Under this condition, water exists in the liquid phase, and it is called a compressed liquid, or a sub-cooled liquid, meaning that it is not about to vaporize. Heat is now transferred to the water and thus its temperature rises to, say, 60°C. As the temperature rises, the liquid water expands slightly, and thus its specific volume increases. To accommodate this expansion, the piston moves up slightly. The pressure in the cylinder remains constant at 1 atm during this process since it is determined by the outside barometric pressure and the weight of the piston, both of which are constant. Water is still a compressed liquid at this state since it has not started to vaporize. As more heat is transferred, the temperature keeps rising until it reaches 100°C (state 2, Fig. 6–1(b)). At this point, water is still a liquid, but any heat addition will cause some of the liquid to vaporize. That is, a phase-change process from liquid to vapor is about to take place. A liquid that is about to vaporize is called a saturated liquid. Therefore, state 2 is the saturated liquid state. (2) Vaporization stage: saturated liquid to saturated vapor Once boiling starts, the temperature stops rising until the liquid has completely vaporized. That is, the temperature will remain constant during the entire phase-change process if the pressure is held constant. This can easily be verified by placing a thermometer into pure boiling water on top of a stove. At sea level ( p = 1 atm), the thermometer will always read 100°C if the pan is uncovered or covered with a weightless lid. During a boiling process, the only change observed is a large increase in the volume and a steady decline in the liquid level as a result of more liquid turning to vapor. Midway about the vaporization line (state 3, Fig. 6-1(c)), the cylinder contains equal amounts of liquid and vapor. As heat is continuously transferred in, the vaporization process continues until the last drop of liquid is vaporized (state 4, Fig. 6–1(d)). At this point, the entire cylinder is filled with vapor that is on the borderline of the liquid phase. Any heat loss from this vapor will cause some of the vapor to condense. A vapor that is about to condense is called a saturated vapor. Therefore, state 4 is a saturated vapor state. A substance at states between 2 and 4 is referred to as a saturated liquid–vapor mixture since the liquid and vapor phases coexist in equilibrium at these states. The amount of energy absorbed during vaporization is called the latent heat of vaporization and is equivalent to the energy released during condensation. Once the phase-change process completes, the water turns back to a single phase region again, this time vapor