Colligative properties have valuable applications, such as the salting of roads in cold-weather climates. By using salt to an icy road, the melting point of the ice is decreased, and the ice will certainly melt more quickly, making driving safer. Sodium chloride (left( ceNaCl ight)), and also either calcium chloride (left( ceCaCl_2 ight)) or magnesium chloride (left( ceMgCl_2 ight)) are supplied the majority of generally, either alone or in a mixture. Sodium chloride is the least expensive choice, but is much less reliable bereason it only dissociates into two ions, instead of three.

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Freezing Point Depression

The figure below shows the phase diagram for a pure solvent and exactly how it transforms when a solute is included to it. The solute lowers the vapor push of the solvent, leading to a lowering of the freezing point of the solution compared to the solvent. The freezing point depression is the distinction in temperature between the freezing point of the pure solvent and also that of the solution. On the graph, the freezing allude depression is represented by (Delta T_f).

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Figure (PageIndex1): The vapor pressure of a solution (blue) is lower than the vapor pressure of a pure solvent (pink). As a result, the freezing suggest of a solvent is reduced when any type of solute is liquified right into it.

When a pure solvent freezes, its pwrite-ups become more ordered as the intermolecular pressures that operate in between the molecules end up being irreversible. In the case of water, the hydrogen bonds make the hexagonally-shaped netoccupational of molecules that characterizes the framework of ice. By disaddressing a solute right into the liquid solvent, this ordering procedure is disrupted. As a result, even more power need to be removed from the solution in order to freeze it, and the freezing suggest of the solution is reduced than that of the pure solvent.

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The magnitude of the freezing point depression is straight proportional to the molality of the solution. The equation is:

The proportionality constant, (K_f), is called the molal freezing-allude depression constant. It is a continuous that is equal to the readjust in the freezing allude for a 1-molal solution of a nonvolatile molecular solute. For water, the worth of (K_f) is (-1.86^ exto extC/ extitm). So, the freezing temperature of a 1-molal aqueous solution of any nonvolatile molecular solute is (-1.86^ exto extC). Eextremely solvent has a unique molal freezing-allude depression continuous. These are presented in the table below, together with a related value for the boiling allude called (K_b).

Table (PageIndex1): Molal Freezing-Point and also Boiling-Point Constants
Solvent Regular Freezing Point (left( ^ exto extC ight)) Molal Freezing-Point Depression Constant, (K_f) (left( ^ exto extC/ extitm ight)) Typical Boiling Point (left( ^ exto extC ight)) Molal Boiling-Point Elevation Constant, (K_b) (left( ^ exto extC/ extitm ight))
Acetic acid 16.6 -3.90 117.9 3.07
Camphor 178.8 -39.7 207.4 5.61
Naphthalene 80.2 -6.94 217.7 5.80
Phenol 40.9 -7.40 181.8 3.60
Water 0.00 -1.86 100.00 0.512