When an aqueous solution containing a nonvolatile solute freezes,

Options

A. the vapor pressure of the solvent becomes abnormally high
B. it is the water that solidifies first
C. it is the solute that solidifies first
D. the solvent and the solute solidify at the same time

Answer

2

Explanation

It is well known that the freezing point of a solution of a nonvolatile solute is lower than the freezing point of the pure solvent. The salt water of the sea does not freeze as redily as fresh water lakes and rivers do.
Let's examine how a solution of `NaCl` in `H_(2)O` behaves as the temperature drops. A solution of `NaCl` in `H_(2)O` does not freeze when the temperature reaches `273K`, the normal freezing point of pure water. The solution freezes at a lower temperature that depends on the concentration of `NaCl` on the solution. When the appropriate temperature is reached, ice starts to from. This ice consists of pure water. The remaining solution becomes more concentrated, since it contains the same amount of NaCl in a smaller amount of liquid `H_(2)O`.
Pure `H_(2)O` maintains a constant temperature as it freezes but a solution behaves differently. The temperature of a solution drops steadily as ice freezes out. The freezind point of the solution decreases as the concentration of the solution increases. Eventually, the solution becomes saturated in `NaCl` and its temperature can go no lower. The solution becomes saturated at `252 K`, which is called the eutectic temperature of the solution.
If the solution is maintained at the eutetic temperature, a solid with the composition `NaCl, 2H_(2)O` crystallizes out of the solution. Eventually, the entire solution freezes, giving a solid called the eutectic solid. This consists of a mixture of ice and `NaCl. 2H_(2)O`.
Note that eutectic mixture is a solid solution of two or more substances having the lowest freexing point (eutectic point) of all the possible mixtures of the components.