Phase Changes:

Every phase change is accompanied by a change in the energy of the system.

• As the temperature of a solid increases, the units of the solid vibrate with increasingly energetic motion. When the solid melts, the units that made up the solid are freed to move with respect to one another.

  This melting process is called fusion (different from nuclear fusion). The increased freedom of motion of the molecules or ions comes at a price, measured by the heat of fusion, or enthalpy of fusion, denoted ΔH_fus.
   

• As the temperature of a liquid increases, the molecules move with increasing energy. With increasing temperature, the concentration of gas-phase molecules just above the liquid increases. These gas-phase molecules exert a pressure on the liquid called vapor pressure.

  Vapor pressure increases with temperature until it equals atmospheric pressure. At this point the liquid boils (boiling point). The energy required to cause this transition is called heat of vaporization or enthalpy of vaporization, denoted ΔH_vap.
   

• The molecules of a solid can be transformed directly into the gaseous state. The enthalpy change required for this transition is called the heat of sublimation, denoted ΔH_sub.