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What is the effect of impurities on freezing point?

The freezing point of a liquid is the temperature at which the solid and liquid forms of the substance are in equilibrium (co-exist). When water is cooled, at 0 degrees Celsius, water starts freezing. On further cooling, the temperature will remain unchanged until the freezing is complete. This temperature (0 C) at which ice and water co-exist is called the freezing point of water. 


When a non-volatile solute is dissolved in a liquid, the vapor...

The freezing point of a liquid is the temperature at which the solid and liquid forms of the substance are in equilibrium (co-exist). When water is cooled, at 0 degrees Celsius, water starts freezing. On further cooling, the temperature will remain unchanged until the freezing is complete. This temperature (0 C) at which ice and water co-exist is called the freezing point of water. 


When a non-volatile solute is dissolved in a liquid, the vapor pressure of the solvent is lowered. The vapor pressure of the solution thus formed will be always less than that of the pure solvent. This phenomenon is called lowering of vapor pressure. Lowering of vapor pressure is a colligative property. A colligative property depends on the number of solute particles in the solution, and is independent of the nature of the solute. The magnitude of a colligative property is proportional to the mole-fraction(concentration) of the solute in the solution.  


When a dilute aqueous solution of NaCl is cooled, at a particular temperature ice starts forming. This temperature is called the freezing point of the solution. The freezing point of the solution will always be below that of the pure solvent. This is a consequence of the lowering of the vapor pressure. This phenomenon is called freezing point depression, which also is a colligative property.


Thus, an aqueous NaCl solution will freeze at a temperature below 0 degrees C, which is the normal freezing point of water. In general, a solution with a solute (regarded as an impurity in the study of colligative properties of dilute solutions) has a freezing point below that of the pure solvent.


Measurements of colligative properties can be used to determine the molar mass of the solute.                                     

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