Phases

Summary

• solid
  • definite volume and shape
  • high atomic order
• liquid
  • definite volume, indefinite shape
  • moderate and transitory atomic order
• gas
  • indefinite volume and shape
  • no atomic order
• glass
• plasma
  • indefinite shape and volume
  • a gas of electrons and ions
• metal
  • definite shape and volume
  • a gas of electrons, a solid of ions
• polymorphs and allotropes
  • There are often several ways to arrange the particles of a substance.
  • These variations are called polymorphs or allotropes.
• phase changes
  • melting, freezing, fusion
  • boiling, evaporation, vaporization, condensation
• Equilibrium can be used to describe two very different situations.
  • Static equilibrium occurs whenever the components of forces and torques acting in one direction are balanced by the components of forces and torques acting in the opposite direction.
    • A system in static equilibrium will have a constant translational and angular velocity.
  • Dynamic equilibrium occurs whenever a change in the statistical behavior of a large group of particles is balanced by an opposite change in the statistical behavior of a similarly large group of different particles.
    • A system in dynamic equilibrium will have a constant mass, pressure, temperature, and volume.
    • Dynamic equilibrium is a state where no macroscopic change is observed.
  • Phase changes occur whenever a large group of particles is out of dynamic equilibrium.
• The dynamic equilibrium phase plotted on a pressure-temperature graph is called a phase diagram.
  • Each substance has its own characteristic phase diagram.
  • The lines separating phases on a phase diagram are known as phase boundaries.
    • liquid-gas
      • The liquid-gas phase boundary is known as the vaporization curve or vapor pressure curve.
      • The value of the liquid-gas phase boundary at a given pressure is a boiling point.
      • The value of the liquid-gas phase boundary at atmospheric pressure is the normal boiling point
      • The liquid-gas phase boundary terminates at a critical point with a critical pressure and critical temperature.
        • A gas cannot be liquefied by compression if it is hotter than its critical temperature. It will remain a gas.
    • solid-liquid
      • The solid-liquid phase boundary is known as the fusion curve or melting curve.
      • The value of the solid-liquid phase boundary at a given pressure is a melting point (or freezing point).
      • The value of the solid-liquid phase boundary at atmospheric pressure is the normal melting point (or normal freezing point).
    • solid-gas
      • The solid-gas phase boundary is known as the sublimation curve.
      • The value of the solid-gas phase boundary at a given pressure is a sublimation point.
      • The value of the solid-gas phase boundary at atmospheric pressure is the normal sublimation point.
  • The point where three phase boundaries meet is a triple point.
    • All three phases exist in dynamic equilibrium when a substance is at its triple point.
    • A gas cannot be liquefied by cooling if the pressure is less than the triple point pressure. It will go directly to the solid phase.