module 3

• The state exhibited by a given sample of matter depends on the identity, temperature, and pressure of the sample

  
• Phase diagram


  A graphic summary of the physical state of a substance as a function of temperature and pressure in a closed system

  
• A typical phase diagram consists of discrete regions that represent the different phases exhibited by a substance

  
• Each region corresponds to the range of combinations of temperature and pressure over which that phase is stable

  
• Solid phase


  Favored at high pressure and low temperature
• Gas phase


  Favored at high temperature and low pressure
• Supercritical fluid


  The combination of high temperature and high pressure
• Heating and cooling curves can be used to determine a substance's melting (or freezing) point

  
• A phase diagram combines plots of pressure versus temperature for the liquid-gas, solid-liquid, and solid-gas phase-transition equilibria of a substance

  
• Phase diagrams indicate the physical states that exist under specific conditions of pressure and temperature, and also provide the pressure dependence of the phase-transition temperatures

  
• Liquid-vapor curve

  Separates the liquid and gaseous regions of the phase diagram and provides the boiling point for water at any pressure

  
• The liquid-vapor curve terminates at a temperature of 374 °C and a pressure of 218 atm, indicating that water cannot exist as a liquid above this temperature, regardless of the pressure

  
• Supercritical fluid


  A unique state of matter with properties intermediate between those of its liquid and gaseous phases
• Solid-vapor curve

  Indicates the temperatures and pressures at which ice and water vapor are in equilibrium, corresponding to the sublimation, or deposition, points for water

  
• Solid-liquid curve

  Shows the temperatures and pressures at which ice and liquid water are in equilibrium, representing the melting/freezing points for water

  
• The solid-liquid curve for water exhibits a slight negative slope, indicating that the melting point for water decreases slightly as pressure increases
• The solid-liquid curve for carbon dioxide exhibits a positive slope, indicating that the melting point for CO2 increases with pressure as it does for most substances
• The triple point for carbon dioxide is well above 1 atm, indicating that carbon dioxide cannot exist as a liquid under ambient pressure conditions
• Cooling gaseous carbon dioxide at 1 atm results in its deposition into the solid state, and solid carbon dioxide does not melt at 1 atm pressure but instead sublimes to yield gaseous CO2
• The critical point for carbon dioxide is observed at a relatively modest temperature and pressure in comparison to water
• Heating curve

  A graph showing the temperature of a substance plotted against the amount of energy it has absorbed

  
• Melting


  The phase change as a substance changes from a solid to a liquid
• Boiling


  The phase change as a substance changes from a liquid to a gas
• Freezing


  The phase change as a substance changes from a liquid to a solid
• Condensation


  The phase change as a substance changes from a gas to a liquid
• Vaporization


  The phase change as a substance changes from a liquid to a gas
• Cooling curves are the opposite of heating curves, showing how the temperature changes as a substance is cooled down

  
• During freezing, energy is removed and during melting, energy is absorbed
• During the diagonal line segments on a heating or cooling curve, a single state of matter exists and the sample is either getting hotter or cooler

  
• During the horizontal flat parts (phase changes) of a heating or cooling curve, both phases exist in various ratios

  
• State changes from gas to liquid, or from liquid to solid

  1. Heating curve

  2. Cooling curve

  
• Lauric acid has a melting point of about 45°C and is easily melted in a test tube placed in a beaker of hot water
• The temperature can be followed using a thermometer or temperature probe connected to a data logger
• The liquid may be cooled by putting the boiling tube in a beaker of cold water or just leaving it in the air
• The melting and freezing occur at the same temperature
• Temperature


  A measure of "Average Kinetic Energy"

  
• During the horizontal line segments, there is no change in temperature, so kinetic energy remains constant

  
• All the energy that is absorbed or released is related to changes in potential energy
• 3 Ps

  Plateau, Phase change and Potential Energy Change
• The temperatures at which phase transitions occur are determined by the relative strengths of intermolecular attractions and are, therefore, dependent on the chemical identity of the substance