The Formation of Ice by Crystallization

A simple experiment can be carried out to note the ice point of water and understand the concept of latent heat of fusion. Take some pure ice in a beaker and place a thermometer in it as shown in the figure given below. At normal atmospheric pressure, as the ice starts to melt, the temperature is noted to be 0°C and it remains constant at that temperature until all the ice melts into liquid water. After this, the temperature of water starts to rise above 0°C.

The latent heat of fusion arises from the work required to overcome the forces that hold the molecules of ice together. As the ice melts, the temperature remains constant at 0°C. The heat given by the surrounding air is used to break the crystal structure of ice. A crystal structure is maintained by forces of attraction that hold the individual oscillating atoms in their average positions. As heat is supplied to the system, the oscillations gradually become violent and at the melting point the forces of attraction are no longer sufficient to maintain the crystal structure. Thus, the latent heat of fusion is used to complete the transition to the liquid state.

Effect of Pressure and Impurities
The ice point of water is 0°C or 32°F at normal atmospheric pressure. However, if there are impurities present in the solution or if the atmospheric pressure is not 1, then the value of the ice point changes.

· Pressure: If the pressure is increased on the system, the molecules are forced to come closer. As water takes up the lesser volume as compared to ice, an increase in pressure leads to faster melting of ice into water. Thus the ice point is lowered. For example, if the pressure is maintained at 2,000 atm, water can supercool to as low as -70°C before turning into ice.

· Impurities: The addition of impurities on the ice below 0°C leads to a depression in the melting point. Thus, the temperature drops further and the ice melts faster into the water in the presence of impurities. This is why salt is added as an impurity on frozen streets, which melts the ice, even at temperatures below 0°C.