Phase Diagrams and Curves

• The green line in a phase diagram divides the solid and liquid phases, representing melting (solid to liquid) and freezing (liquid to solid) points
• Phase Diagram


  • Graphical representation of the physical states of a substance under different conditions of temperature and pressure
  • Gives the possible combinations of pressure and temperature at which certain physical state or states a substance would be observed
  • Plots pressure versus temperature, divided into solid, liquid, and gaseous states
  • Boundary between liquid and gaseous regions stops at point C, the critical temperature for the substance
• The blue line in a phase diagram divides the liquid and gas phases, representing vaporization (liquid to gas) and condensation (gas to liquid) points
• Phase diagrams are plots of pressure versus temperature, usually in atmospheres and degrees Celsius or Kelvin
• Three Areas in Phase Diagram

  • Marked solid, liquid, and vapor
  • Under a set of conditions in the diagram, a substance can exist in a solid, liquid, or vapor (gas) phase
  • Labels on the graph represent the stable states of a system in equilibrium
  • Green line divides solid and liquid phases, represents melting (solid to liquid) and freezing (liquid to solid) points
  • Blue line divides liquid and gas phases, represents vaporization (liquid to gas) and condensation (gas to liquid) points
  • Red line divides solid and gas phases, represents sublimation (solid to gas) and deposition (gas to solid) points
  • Triple Point: Combination of pressure and temperature at which all three phases of matter are at equilibrium
  • Critical Point: Terminates the liquid/gas phase line, set of temperature and pressure where the liquid and gaseous phases of a substance merge together
• The red line in a phase diagram divides the solid and gas phases, representing sublimation (solid to gas) and deposition (gas to solid) points
• The curve on the phase diagram of water is sloped backwards, indicating that liquid water is more dense than solid and the melting point gets higher at lower temperatures
• Heating Curve

  Graphical representation of the correlation between heat input and the temperature of a substance. It can be used to determine the melting point and the boiling point of a substance. A plot of the temperature vs time is the heating curve
• Triple point

  The point on a phase diagram at which the three states of matter coexist
• The position of the triple point on the phase diagram of carbon dioxide is well above atmospheric pressure. The freezing-melting curve is also sloped forward
• At 1 atm pressure, carbon dioxide will sublime at a temperature of 197.5 K (-75.5 °C), which is why solid carbon dioxide is often known as "dry ice." There is no liquid carbon dioxide under normal conditions - only the solid or the vapor
• Critical Point

  • Terminates the liquid/gas phase line
  • Set of temperature and pressure where the liquid and gaseous phases merge into a single phase
  • Beyond the temperature of the critical point, the merged single phase is known as a supercritical fluid
• The triple point for water occurs at a very low pressure of 0.006 atm and at a temperature of 273.2 K. It is impossible to convert water from a gas to a liquid by compressing it above this temperature
• Cooling Curve


  Line graph representing the change of phase of matter, typically from a gas to a solid or a liquid to a solid. The cooling curve is not an identical mirror image to the heating curve
• Supercritical fluid


  A temperature above a substance that will ALWAYS be a gas regardless of pressure