Phase diagram

1. Phase
    A homogenous part of the system in contact with the other parts of the system but is separated from them by a distinct boundary. 

2. Types of phase

• Solid
• Liquid
• Gaseous/vapour

3. Phase diagram
    (a) Phase diagram is a graphical plot of pressure versus temperature that shows the condition at which a substance exists as solid, liquid or gas.
    (b) It describes the most stable phase under particular conditions of temperature and pressure.
   

4. Triple point

    Triple point is the point at which the three phases (solid-liquid-vapour phase) can be in equilibrium with one another at the same temperature and pressure.

5. Critical point

    (a) Critical point is the highest temperature at which a gas can be liquefied. (the point at which there is no boundary between the liquid and gas phase)

    (b) Above this point, it is impossible to liquefy a gas, no matter how great the applied pressure is.

6. Phase diagram of water
   

    (a) Triple point (T): 0.006 atm, 0.01^oC
         Critical point (C): 218.3 atm, 374^oC
         Normal melting point: 1.00 atm, 0^oC
         Normal boiling point: 1.00 atm, 100^oC

    (b) Anomalous behavour of water:

• The phase diagram of water is not typical.
• Line of solid-liquid equilibrium slopes to the left (or negative slope). It shows that as the pressure increases, the melting temperature decreases.
• It is because ice is less (occupy larger volume) dense than liquid water.
• At high pressure, it favours the formation of liquid water. Less heat is required to melt the ice. Thus, ice melts at lower temperature at high pressure.

7. Phase diagram of carbon dioxide
   
    (a) Under room temperature and pressure (298.15 K and 1 atm), CO2 exists most stable at gas (or vapour) phase.
    (b) Triple point: 5.11 atm, 216.8 K (-56.35^oC). Triple point is above the atmospheric pressure (1 atm). Thus, solid CO2 does not melt but sublime when heated at atmospheric pressure (1 atm).
    (c) Liquid CO2 is only stable at high pressure (above 5.11 atm).
   
    (d) Description of phase diagram of CO2:

• The phase diagram of CO2 is typical.
• Line of solid-liquid equilibrium slopes to the right (or positive slope). It shows that as the pressure increases, the melting temperature increases.
• It is because solid CO2 is more (occupy smaller volume) dense than liquid CO2.
• At high pressure, it favours the formation of solid CO2. More heat is required to melt the solid CO2. Thus, solid CO2 melts at higher temperature at high pressure.