Phase Diagram 
 

Solid-Liquid Equilibrium Line


As the substance is subjected to the temperature and pressure pairs that follow this line the solid and liquid phases to coexist in equilibrium. The rate of change between the pressure and temperature to maintain this equilibrium is: 

The quantities (delta)Sfus (entropy of fusion) measures the change of disorder in the substance and (delta)V measures the change of volume for the same amount of substance as it melts. The volume change reflects the change in the substance's density between the two phases. 

 
 
Solid-Gas Equilibrium Line
As the substance is subjected to the temperature and pressure pairs that follow this line the solid and gas phases to coexist in equilibrium. The vapor pressure of the solid is related to its surrounding temperature by: 

The value of P0 is 1 Atm or 760 Torr and the Ts (in Kelvins) is the normal sublimation temperature. (delta)Hs and R are the enthalpy of sublimation and universal gas constant, respectively. 
 
 

 
Liquid-Gas Equilibrium Line
As the substance is subjected to the temperature and pressure pairs that follow this line the liquid and gas phases coexist in equilibrium. The vapor pressure of the solid is related to its surrounding temperature by: 

The value of P0 is 1 Atm or 760 Torr and the Tb (in Kelvins) is the normal boiling temperature. (delta)Hv and R are the enthalpy of vaporization and universal gas constant, respectively. 
 
 

 

Solid Region


Within the range of temperatures and pressures as specified by this area the substance exists as a solid. The liquid and gas phase of the substance does not exist under these conditions. 
 
 

 

 


 
 

 

Liquid Region


Within the range of temperatures and pressures as specified by this area the substance exists as a liquid. The solid and gas phase of the substance does not exist under these conditions. 
 

 

 


 
 
 

 

Vapor and Gas Region

Within the range of temperatures and pressures as specified by this area the substance exists as a vapor or gas. The Ideal Gas Law which can approximate the relationship between pressure(P), volume(V), amount of gas(n) and temperature(T) of the gas, is: PV = nRT

 The Van der Waals Equation, shown below, may be used to model the vapor phase of the substance.

The parameters, a and b, measure the amount attractive interaction between the gas molecules and effective volume occupied by the molecules, respectively. 


 

Normal Freezing Point 


At the normal melting temperature the solid substance liquefies at a pressure of 1 Atmosphere or 760 Torr, when heated. If the substance were cooled from its liquid phase to form the solid, this same temperature would be called the Normal Freezing Point. The solid form of the substance coexists with its liquid form, in equilibrium, at this temperature and pressure. 
 
 


 
   
 

 

Normal Boiling Point


At the normal boiling temperature the liquid substance vaporizes with a vapor pressure of 1 Atmosphere or 760 Torr, as the substance is heated. The vapor form of the substance coexists with its liquid form, in equilibrium, at this temperature and pressure. 
 
 
 
 
 
 


 

 

Triple Point


At the triple point temperature, the solid and liquid phase for the substance have the same vapor pressure. Only under this set of temperature and pressure do all three phases (i.e.: solid, liquid, and gas) coexist together. 
 
 
 
   

 


 

 

Critical Point

At this set of temperature and pressure, the liquid phase and gas phase of the substance becomes indistinguishable. The critical pressure is the pressure that is needed to condense the fluid to a liquid at the critical temperature. Above the critical temperature the gas cannot be condensed into a liquid regardless of the pressure applied to the substance. Below this temperature, the gas is a condensable vapor. 
 
 


 
 
 

The Phase Diagram of a Pure Substance