Properties, states and phases of a pure substance

1. Properties, states and phases of a pure substance I am teaching Engineering Thermodynamics using the textbook by Cengel and Boles. Many figures in the slides are taken from that book, and most others are found online. Similar figures can be found in many places. I went through these slides in two lectures, each 90 minutes. Zhigang Suo

2. Thermodynamics relates heat and motion thermo = heat dynamics = motion

3. Pure Substance A substance: a collection of molecules or atoms A pure substance: A substance that has the same composition everywhere.

4. Liquid-gas mixture

5. Phases One species of molecules can aggregate into several forms, known as phases. ice water steam Solid liquid gas

6. actions and words • Parts of an experimental setup • A fixed number of H2O molecules • Cylinder • Frictionless, perfectly sealed piston • Weights • Fire • System • A fixed number of H2O molecules • The system interacts with the surroundings • Weights transfer energy to the system by work. • Fire transfers energy to the system by heat. • Thermodynamic variables (properties) of the system • Temperature, pressure, volume, energy, entropy… • Thermodynamic states of the system • The system approaches a thermodynamic state of equilibrium. • The states of the system has two independent variations.

7. A bit of high-school mathematicsFour ways to represent a function of two variables, f(x,y) • Contour plot (plane diagram) • Table • A surface in 3D • An equation

8. compressed liquid saturated liquid coexistent Liquid and vapor saturated vapor superheated vapor a • States • Specify states with two variables, T and V • Change of state • Continuous change of state • Phases • Two phases: liquid and gas • Change of phase • Discontinuous change of state • Co-existent phases: liquid-gas mixture • A state of coexistent phases

9. Represent states on TV • Specify states with two variables • A point on the TV plane represents a state • Pressure is a function, P (T ,V) • Curves of constant pressure

10. The dome of coexistent phases

11. Two paths to change from one state to another state

12. Heat causes giant motionwhen liquid turns to gas P = 100 kPa Tsat = 100 degC Vf = 10-3 m3/kg Vg = 1.7 m3/kg https://www.ohio.edu/mechanical/thermo/Intro/Chapt.1_6/Chapter2a.html

13. Represent states on PV

14. https://www.ohio.edu/mechanical/thermo/Intro/Chapt.1_6/Chapter2a.htmlhttps://www.ohio.edu/mechanical/thermo/Intro/Chapt.1_6/Chapter2a.html

15. Saturation Temperature and Saturation Pressure liquid gas

16. Two paths to change from one state to another state A path of discontinuous change of state A path of continuous change of state a a P P critical point critical point a a liquid liquid gas gas T T a a

18. Pressure cooker Invented by Denis Papin, France, 1679 Invention: increase pressure, increase temperature, reduce cooking time. Science: When water and steam coexist, temperature increases with time. Engineering: seal, strength, control pressure or temperature. P ~ 2 atm T ~ 120 dedC

19. Bottled gas by liquefaction Invention: store gas in small volume, at room temperature . Science: At room temperature and high pressure, some gases become liquids. Engineering: seal, strength. No need for thermal insulation. Ammonia, NH3 liquid gas

20. Fix temperature by using boiling point Invention: Fix temperature by using boiling points of various liquids. Science: When a liquid evaporates at the atmospheric pressure, the temperature is fixed. Engineering: seal, insulation.

21. Tables of properties outside the dome Compressed liquid or superheated vapor Specify a state by values of PT A partial listing of Table A–6.

22. Tables of properties in the dome coexistent liquid and vapor A partial list of Table A–4. • Table A–4: Saturation properties of water under temperature. • Table A–5: Saturation properties of water under pressure.

23. Specify a state of coexistent phases Specify a state by values of two variables: TV or PV, but not PT. 0 < x < 1: mixture of liquid and vapor x = 0: saturated liquid x = 1: saturated vapor Rule of mixture Two more ways to specify a state of coexistent phases: Tx or Px.

24. A bit of high-school scienceIdeal-gas law Equation of state: An equation that relates properties of a substance.

25. Is Water Vapor an Ideal Gas?

26. Principle of corresponding states • Use PT as independent variables. • Normalize them by critical vales. • Any property is a function of the two independent variables. • Pv/RT is a (dimensionless) property. • At low pressure, and all temperatures, all substances approach to ideal gas, Pv/RT ~ 1. • At high temperature, and all pressures, all substances approach to ideal gas, Pv/RT ~ 1. • Any property is a function of the two independent variables. • The function Pv/RT = f(P/Pcr, T/Tcr) is nearly the same for all substances.

27. van der Waals Equation of State Critical isotherm of a pure substance has an inflection point at the critical point.

28. Summary—states, properties, and phases • One puresubstance of a fixed number of molecules: H2O. • Two phases: liquid and gas. • Many (thermodynamic) states, specified by two independent thermodynamic variables (properties). • T,V as independent variables. Curves of constant P represent function P(T,V). A point on the left of the dome represents a state of liquid, a point on the right of the dome represents a state of gas, and a point under the dome represents a state of coexistent phases. • P,V as independent variables. Curves of constant T represent function T(P,V). • P,T as independent variables. Many states of coexistent phases fall on the same point on the phase boundary. • A change of phase: a discontinuous change of state. • A single state is represented by three points on three planes. • The states of coexistent phases are represented by the regions under the domes on the T-V plane and P-V plane, and by the phase boundary on the P-T plane. • P and T are intensive properties. V is an extensive property. liquid gas

29. Three phases Triple point liquid sublimation/condensation evaporation/condensation melting/freezing

30. https://en.wikipedia.org/wiki/Water_(data_page)

31. Liquid water is denser than ice The crystalline structure of ice is very open. Liquid water packs tighter. Ice floats on top of water http://chemistry.elmhurst.edu/vchembook/122Adensityice.html

32. http://www.wardteam.com/Blog/Preventing-Frozen-Pipes

33. https://commons.wikimedia.org/wiki/File:Phase_diagram_of_water.svghttps://commons.wikimedia.org/wiki/File:Phase_diagram_of_water.svg https://commons.wikimedia.org/wiki/File:Phase_diagram_of_water.svg

34. Phase diagram unlike that of water

36. The function P(T,V)

37. Project a surface in 3D to planes Borgnakke and Sonntag, Fundamentals of Thermodynamics

38. Project a surface in 3D to planes Borgnakke and Sonntag, Fundamentals of Thermodynamics

39. Aggression in dogs Zeeman, Catastrophe theory. Scientific of American, 65 (1976)

40. Phase diagram on P-V plane

41. Questions that motivate later lectures • What is temperature? • What is a thermodynamic state? • Why does a system isolated for a long time reach equilibrium? • What is equilibrium? • Once in equilibrium, the isolated system will never get out of equilibrium. Why? • The phase diagrams of many pure substances look similar (i.e., co-existent phases, triple point, critical point). Why? • Beside TVP, what are other thermodynamic properties? • How do we use diagrams and tables of properties to design engines? • How do we invent new devices? • How about impure substances, such as air and saltwater?