Multiple Choice
Which of the following best explains why adding salt to water causes it to freeze at a lower temperature?
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14. Solutions
Freezing Point Depression
Multiple Choice
At the freezing point, the liquid form of a substance is in equilibrium with the solid form of the substance. Which statement best describes the relationship between the liquid and solid forms at this temperature?
A
The liquid and solid forms have the same vapor pressure.
B
The solid form has a higher vapor pressure than the liquid form.
C
The liquid and solid forms have different chemical compositions.
D
The liquid form has a higher vapor pressure than the solid form.
Verified step by step guidance1
Understand that at the freezing point, the substance is at equilibrium between its liquid and solid phases, meaning the two phases coexist without net change.
Recall that equilibrium between phases implies that the chemical potential of the substance in the liquid phase equals that in the solid phase.
Recognize that vapor pressure is related to the tendency of molecules to escape from a phase into the gas phase; at equilibrium, the vapor pressures of the coexisting phases must be equal to maintain balance.
Note that if the vapor pressures were different, one phase would evaporate or condense more than the other, disrupting the equilibrium.
Conclude that at the freezing point, the liquid and solid forms have the same vapor pressure, which is why they can coexist stably.
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