[Chemistry] Freezing Point Depression question

[miles01110]

I'm really bad at chemistry, but I was wondering about this the other day as I was filling up my ice tray.

Freezing Point Depression is a well-known colligative property of liquids when a solute is added- for example, salt. It depends on the molar concentration of the solution (and an unimportant constant, for this discussion).

If I dissolve salt in water, it freezes at a lower temperature depending on how much salt I can dissolve in a given volume. But in this case doesn't the molar concentration change as more water freezes? I seem to remember something about how the solute doesn't freeze along with the water, in which case the volume would decrease with a corresponding increase in concentration. What am I missing here? I'm sure it's simple, but as I said... chemistry isn't my thing. Any enlightening information would be greatly appreciated.

 

[mkrishnan]

If I understand your question correctly, I think the missing element is that, while there is a set, maximum concentration of salt in water that can be achieved at a given temperature, like 0C, if solute is added at a higher temperature and the water is cooled ... gently (I'm not sure if the cooling has to be quite adiabatic per se), then the solute doesn't precipitate as the liquid cools. Subsequently, you have a concentration at the lower temperature that's richer in solute than is possible when the solute is dissolved in at that temperature (although certain shocks to the system can make it precipitate).(That's the way rock candy is made, for instance).

 

[miles01110]

If I understand your question correctly, I think the missing element is that, while there is a set, maximum concentration of salt in water that can be achieved at a given temperature, like 0C, if solute is added at a higher temperature and the water is cooled ... gently (I'm not sure if the cooling has to be quite adiabatic per se), then the solute doesn't precipitate as the liquid cools.

Ok, I hadn't thought of that, but I still don't think that was exactly what I was asking.

Basically I am not understanding how a freezing point depression calculation is valid for freezing the entirety of a liquid due to the variable nature of the volume of the unfrozen solution (and in turn, the concentration).