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Solid-Liquid Equilibrium

Consider a system containing solid water (ice) and liquid water. The equilibrium set up between the two phases may be represented as
H2O(l) H2O(s)

The following characteristics of the above equilibrium may be established.
  1. At 1 atmospheric pressure, the two phases, namely, solid water (ice) and liquid water, exist together in equilibrium only when the temperature of the system has a constant value of 0°C.
  2. If the temperature of the system is more than 0°C, we find that all the ice has melted and the system contains only liquid water.
  3. If the temperature of the system is less than 0°C, we find that all the liquid water has solidified and the system contains only solid water.
  4. For a thermally isolated system at 0°C, the masses of solid water and liquid water remain constant with time. However, as stated earlier, the system is at dynamic equilibrium where water molecules are being continuously exchanged between the two phases at equal rates, i.e. the rate of transfer of water molecules from liquid water to solid water is the same as that from solid water to liquid water . 

  Schematic diagram displaying dynamic equilibrium
H2O(l) H2O(s)

Normal Melting (or Freezing) Point of a Pure Substance. 
We have seen above that the solid and liquid phases of a pure substance exist at equilibrium only at one particular temperature for a given pressure. When the pressure is 1 atm this temperature is referred to as normal melting (or freezing) point of the substance.

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