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Properties of Pure Substances: Classification, Phases, and Phase Change Processes

Explore the classification, phases, and phase change processes of pure substances. Learn about homogeneous and heterogeneous mixtures and the variations in properties during phase changes.

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Properties of Pure Substances: Classification, Phases, and Phase Change Processes

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  1. CHAPTER 3: PROPERTIES OF PURE SUBSTANCES EMT 230 THERMODYNAMICS SITI SALWA MAT ISA

  2. Classification of matter • Chemists can classify matter as solid, liquid, or gas. • But there are other ways to classify matter, as well as pure substances and mixtures. • Classification is one of the basic processes in science. All matter can be classified as either a pure substance or a mixture.

  3. What is PURE SUBSTANCE? PURE SUBSTANCE : A pure substance or chemical substance is a material that has constant composition (is homogeneous) and has consistent properties throughout the sample. A pure substance does not have to be of a single chemical element or compound. However a mixture of various chemical elements or compounds also qualifies as a pure substance as long as the mixture is homogeneous

  4. Examples • A mixture of liquid and water vapor is a pure substance, but a mixture of liquid and gaseous air is not pure substance. • EXAMPLES: • Water (solid, liquid and vapor phases) • Mixture of liquid water and water vapor • Carbon dioxide • Nitrogen • Mixtures of gases, such as air, as long as there is no change of phase

  5. Pure SubstanceExamples continue.. • Tin, sulfur and diamond are examples of pure substances which are chemical elements. All elements are pure substances. • Water, sugar, salt and baking soda are pure substances which are chemical compounds. Chemical compounds also are pure substances. • Depending on who you talk to, homogenous mixtures may be considered examples of pure substances. Examples of homogenous mixtures include vegetable oil, honey and air. • Inhomogeneous mixtures are not pure substances. Examples of materials which are not pure substances include gravel, your computer and a tree.

  6. Mixture • Mixtures are physical combinations of pure substances that have no definite or constant composition — the composition of a mixture varies according to who prepares the mixture. • Mixtures can be either homogeneous or heterogeneous • A homogeneous mixture, sometimes called a solution, is relatively uniform in composition; every portion of the mixture is like every other portion. • For example, if you dissolve sugar in water and mix it really well, your mixture is basically the same no matter where you sample it.

  7. Mixture • A heterogeneous mixture is a mixture whose composition varies from position to position within the sample. • For example, if you put some sugar in a jar, add some sand, and then give the jar a couple of shakes, your mixture doesn’t have the same composition throughout the jar. Because the sand is heavier, there’s probably more sand at the bottom of the jar and more sugar at the top.

  8. Phases of A PURE SUBSTANCE The substances exist in different phases. E.g. at room temperature and pressure, Copper is solid but Mercury is liquid It can exist in different phases under variations of conditions. There a 3 phases: Solid Liquid Gas Each with different molecular structures.

  9. Phases-change Processes of PURE SUBSTANCE The arrangement of atoms: Solid: strong intermolecular bond, molecules are at relatively fixed positions Liquid: Intermediate intermolecular bonds, groups of molecules move around each other Gas: Weak intermolecular bond , molecules move around at random

  10. PHASE CHANGE PROCESS OF PURE SUBSTANCE • COMPRESSES LIQUID • SATURATED LIQUID • SATURATED LIQUID- VAPOR MIXTURE • SATURATED VAPOR • SUPERHEATED VAPOR

  11. At 1 atm and 20˚C, water exists in the liquid phase • COMPRESSED LIQUID: A substance that is not about to vaporize • 1 atm = standard atmospheric pressure • = 101.4 kPa At 1 atm and 100˚C, water exists as a liquid that is ready to vaporize SATURATED LIQUID: A liquid that is about to vaporize

  12. As more heat is transferred (but T maintained at 100 C), part of the saturated liquid vaporizes SATURATED LIQUID-VAPOR MIXTURE: The state at which the liquid and vaporphases coexist in equilibrium At 1 atm pressure, the temperature remains constant at 100˚C until the last drop of liquid is vaporized SATURATED VAPOR: A vapor that is about to condense

  13. As more heat is transferred, the temperature of the vapor starts to rise (e.g. 300 C) SUPERHEATED VAPOR: A vapor that is not about to condense (i.e. not saturated vapor)

  14. If the entire process between state 1 and 5 described in the figure is reversed by cooling the water while maintaining the pressure at the same value, the water will go back to state 1, retracing the same path, and in so doing, the amount of heat released will exactly match the amount of heat added during the heating process T-v diagram for the heating process of water at constant pressure

  15. PROPERTY DIAGRAMS FOR PHASE-CHANGE PROCESSES The variations of properties during phase-change processes are best studied and understood with the help of property diagrams such as the T-v, P-v, and P-T diagrams for pure substances. T-v diagram of constant-pressure phase-change processes of a pure substance at various pressures (numerical values are for water).

  16. Alternatively, if we join the saturated liquid and saturated vapor line, the diagram will resulted as below with three different region; Compressed Liquid region, Saturated liquid-vapor region and Superheated region • saturated liquid line • saturated vapor line • compressed liquid region • saturated liquid–vapor mixture region (wet region) • superheated vapor region Critical point : The point at which the saturated liquid and saturated vapor states are identical e.g. : T-v diagram of a pure substance

  17. At supercritical pressure (P>Pcr), there is no distinct phase-change (boiling) process * Pcr = pressure line @ Critical Point

  18. Point to ponder • Water boils at 100 C  incorrect!!! • Water boils at 100 C and at constant pressure of 1 atm ( 101.325 kPa)  Correct  • If the pressure is raised to 500 kPa, boiling point of water will INCREASE to 151.8 C

  19. Saturation Temperature and Saturation Pressure • The temperature at which water starts boiling depends on the pressure; therefore, if the pressure is fixed, so is the boiling temperature (refer below graph) • Water boils at 100C at 1 atm pressure but at 151.8 C at 500 kPa. • Saturation temperature Tsat: At a given pressure, the temperature at which a pure substance changes phase is called Tsat. • Saturation pressure Psat: At a given temperature, the pressure at which a pure substance changes phase is called Psat. The liquid–vapor saturation curve of a pure substance (numerical values are for water).

  20. ExampleAltitude vs Boiling point • Higher elevation  lower pressure • Less energy for the water to boils  lower boiling point • Water boils at lower temperature with longer cooking time at the top of Mount Kinabalu as compare to Lahad Datu town • Effect of low pressure: • Dry atmosphere • Windy

  21. Point to ponder gain “ I understand that as the elevation increases, the atmospheric pressure, and therefore vapor pressure, decreases, which means the boiling point is lower on top of a mountain. Wouldn't that technically mean it would take a shorter amount of time to boil, let's say, water on top of a mountain because the boiling point is lower, rather than it actually taking longer (which it actually does)?”

  22. Answer “It takes less time to boil water, because the boiling point is lower. Thus, faster time to boil water. However, it takes longer to cook food in boiling water because less heat is supplied by the water.”

  23. Example continue Pressure Cooker • Pressure developed in the container because the expansion of steam is blocked by the cooker body. • Higher pressure  higher water boiling point. • Higher boiling point  shorter cooking time + save energy • High energy steam is used to cook the food inside the container • Normal pan : beef stewed in 1 to 2 hours • Pressure cooker : beef stewed in 20 minutes

  24. Phase change

  25. Energy for phase-change process • Latent heat  The amount of energy absorbed or released during a phase-change process. • Latent heat of fusion: The amount of energy absorbed during melting. It is equivalent to the amount of energy released during freezing. • Latent heat of vaporization: The amount of energy absorbed during vaporization and it is equivalent to the energy released during condensation. • The magnitudes of the latent heats depend on the temperature or pressure at which the phase change occurs. • E.g. At 1 atm pressure, the latent heat of fusion of water is 333.7 kJ/kg and the latent heat of vaporization is 2256.5 kJ/kg.

  26. SATURATION SATURATION is defined as a condition in which a mixture of vapor and liquid can exist together at a given temperature and pressure SATURATION PRESSURE, Psatis the pressure which at which a pure substance changes phase at a given temperature SATURATION TEMPERATURE, Tsatis the temperature at which a pure substance changes phase at a given pressure. For a pure substance, there is a definite relationship between saturation pressure and saturation temperature. The higher the pressure, the higher the saturation temperature

  27. SATURATED LIQUID : If a substance exists as a liquid at the saturation temperature and pressure SUBCOOLED LIQUID/COMPRESSED LIQUID : If the temperature of the liquid is lower than the saturation temperature for the existing pressure SATURATED VAPOR : If a substance exists entirely as vapor at saturation temperature SUPERHEATED VAPOR : When the vapor is at a temperature greater than the saturation temperature * The pressure and temperature of superheated vapor are independent properties, since the temperature may increase while the pressure remains constant EMT 230 THERMODYNAMICS : PROPERTIES OF PURE SUBSTANCE SITI SALWA MAT ISA

  28. Some Consequences of Tsat and Psat Dependence The variation of the temperature of fruits and vegetables with pressure during vacuum cooling from 25°C to 0°C. In 1775, ice was made by evacuating the air space in a water tank.

  29. homework • Give one example of phase change from solid  vapor • Explain how it happened • Group work? • Individual work!!! • Send it after semester break.

  30. QUALITY When a substance exist as part liquid and part vapor at saturation conditions, its quality (x) is defined as the ratio of the mass of the vapor to the total mass of both vapor and liquid. The quality is zero for the saturated liquid and one for the saturated vapor Example: If the mass of vapor is 0.2g and the mass of the liquid is 0.8g, then the quality is 0.2 or 20%

  31. MOISTURE CONTENT The moisture content of a substance is the opposite of its quality Moisture: As the ratio of the mass of the liquid to the total mass of both liquid and vapor Recall the definition of quality Then,

  32. Take specific volume as an example. The specific volume of the saturated mixture becomes The form that is most often used Let Y be any extensive property and let y be the corresponding intensive property, Y/m then

  33. PROPERTY TABLES For example if the pressure and specific volume are specified, three questions are asked : For the given pressure,

  34. Before we know how to use property table, we need to define a new property called ENTHALPY ENTROPY, s: is a property associated with second law of thermodynamics. ENTHALPY, h: is a combination properties u + Pv h = u + Pv (kJ/kg) or H = U + PV (kJ/kg)

  35. Example 1

  36. Example 2

  37. Example 3 Refer table A5 (saturated water-pressure table)

  38. Example 4 A11 Refer table A11 (saturated refrigerant-134a table)

  39. Example 5

  40. Refer table A6 (superheated water)

  41. Important Definition • Critical point - the temperature and pressure above which there is no distinction between the liquid and vapor phases • Triple point – the temperature and pressure at which all three phases can exist in equilibrium • Sublimation – change of phase from solid to vapor • Vaporization – change of phase from liquid to vapor • Condensation – change of phase from vapor to liquid • Fusion or melting – change of phase from solid to liquid

  42. IDEAL GAS LAW Robert Boyle formulates a well known law that states the pressure of gas expanding at constant temperature varies inversely to the volume, or As the result of experiment, Charles concluded that the pressure of gas varies directly with the temperature when the volume is held constant, and the volume varies directly with temperature when the pressure is held constant, or

  43. By combining the results of Charles and Boyles experiments, the following relationship can be obtained The constant in the above equation is called the ideal gas constant and is designed by R, thus the ideal gas equation becomes In order to make the equation applicable to all ideal gas, a universal gas constant Ruis introduced, R is gas constant and M is molar mass/molecular weight

  44. For example the ideal gas constant for air, Rair The amount of energy needed to raise the temperature of a unit of mass of a substance by one degree is called the specific heat constant volume Cvfor a constant-volume process and the specific heat at constant pressure Cp for a constant pressure process. They are defined as

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