[confusedstud]

My teacher told as a scenario when a group of students were trying to obtain crystals through nucleation and precipitation. After a while they obtained some moderately sized crystals. Next, they left it alone for a few days and when they returned the crystals disappeared.

How did that happen? I thought that when they left it under room conditions, more solvent will evaporate making the solution even more saturated allowing for more of this precipitation to happen.

My teacher explained that when some crystals formed, the solution become less saturated causing some salt to redissolve. And the amount of solvent evaporation is not enough to keep the solution saturated so the salt eventually dissolved. However this makes little sense to me because
1) when some of the salt redissolved it will cause the solution to become saturated again propagating the precipitation so I would think there is an equilibrium happening but the overall size should remain the same
2) combining the effects of the re-dissolution of the salt and the evaporation, the solution should become even more saturated which will further cause the salt crystals to grow in size.

 A friend thought that by leaving the solution alone the crystals would continue to dissolve and forms a supersaturated solution. But I think its unlikely as a high temperature is needed to form a supersaturated solution. So is there a reason why the crystals redissolved?

[Borek]

Teacher explanation doesn't make sense to me.

[confusedstud]

Teacher explanation doesn't make sense to me.

He said that the "loss of solvent is not enough to compensate for the formation of  the crystals which results in the solution becoming less concentrated" but the sequence of events seems to suggest that no matter what the solution becomes more saturated favouring the formation of crystal. Do you agree with this?

[Borek]

No idea what was the real reason behind the crystals disappearing.

In general, crystals don't form lowering the concentration of the salt. Crystals appear when the solution becomes (over)saturated, and they grow when the solvent evaporates (at the rate dictated by the solvent loss). Solution is saturated throughout the whole process. There is no need for the "loss of solvent to compensate" as the solution was never below saturation.

At least that's what would happen in an isothermal process. We rarely deal with such processes, as the temperature in the lab is never exactly constant. For some salts even small differences in the temperature (say 5 degs, quite possible in the lab room without AC and with a changing insolation during a day) is enough to produce quite large differences in solubility.