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Chapter 3 - Chapter 3 Properties of a pure substance Read...

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Chapter 3 Properties of a pure substance Read BS, Chapter 3 3.1 The pure substance We define a Pure substance : a material with homogeneous and invariable composition. To elaborate, Pure substances can have multiple phases: an ice-water mixture is still a pure sub- stance. An air-steam mixture is not a pure substance. Air, being composed of a mixture of N 2 , O 2 , and other gases, is formally not a pure substance. However, experience shows that we can often treat air as a pure substance with little error. 3.2 Vapor-liquid-solid phase equilibrium Often we find that different phases of pure substances can exist in equilibrium with one another. Let us consider an important gedankenexperiment (Latin-German for “thought experiment”) in which we boil water. Ordinary water boiling is shown in Fig. 3.1. However, this ordinary experiment has constraints which are too loose. Most importantly, the mass of water leaks into the atmosphere; thus, the water vapor and the air become a mixture and no longer a pure substance. Let us instead consider a more controlled piston-cylinder arrangement. Inside the cylin- der, we begin with pure liquid water at T = 20 C . The piston is free to move in the cylinder, but it is tightly sealed, so no water can escape. On the other side of the piston is a constant pressure atmosphere, which we take to be at P = 100 kPa = 0 . 1 MPa = 10 5 Pa = 1 bar . 41
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42 CHAPTER 3. PROPERTIES OF A PURE SUBSTANCE Figure 3.1: Water boiling isobarically in an open environment. We slowly add heat to the cylinder, and observe a variety of interesting phenomena. A sketch of what we observe is given in Fig. 3.2. We notice the following behavior: P = 100 kPa liquid water T = 20 ˚C P = 100 kPa liquid water T > 20 ˚C P = 100 kPa saturated liquid water T = 99.62 ˚C P = 100 kPa liquid,T = 99.62 ˚C P = 100 kPa saturated water vapor T = 99.62 ˚C Q P = 100 kPa water vapor T > 99.62 ˚C Q Q Q Q vapor, T=99.62 ˚C Figure 3.2: Sketch of experiment in which heat is added isobarically to water in a closed piston-cylinder arrangement. The pressure remains at a constant value of 100 kPa . This is an isobaric process. The total volume increases slightly as heat is added to the liquid. The temperature of the liquid increases significantly as heat is added to the liquid. At a very special value of temperature, observed to be T = 99 . 62 C , we have all liquid, but cannot add any more heat and retain all liquid. We will call this state the saturated liquid state. We call T = 99 . 62 C the saturation temperature at P = 100 kPa . As we continue to add heat, CC BY-NC-ND. 2011, J. M. Powers.
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3.2. VAPOR-LIQUID-SOLID PHASE EQUILIBRIUM 43 The temperature remains constant (this is isothermal now as well as isobaric). The total volume continues to increase. We notice two phases present: liquid and vapor, with a distinct phase boundary.
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