[rchok2]

why does crystals benzoic acid dissolve in hot water and when it cools down the crystals reformed again? could some pls give an scientific explanation? THANK YOU VERY MUCH^^

[Mitch]

Because solubility is a temperature dependent property of a substance.

[rchok2]

mind to explain a little bit in detail? what actually happens when it is hot? but why does the crystal reform again when it cools down?

[Mitch]

Well I was vague in answering, because I don't beleive in giving answers.

what actually happens when it is hot?

Great question, hard to answer though. What makes anything want to dissolve in the first place? I will say that when things are hot, then the kinetic energy of the solvent is higher. What do you think will happen when fast colliding molecules hit on a crystalline structure?

[rchok2]

i get your idea Mitch....but why does the crystals reforms when it is cold? u mean this is thetemperature can reverse the process?@_@

[Nucleophile]

How about a thermodynamic explanation?
think of it as an equilibrium that exists between the dissolved and the solid states. this equilibrium is dependant on temperature due to the old DG=DH-TDS. when the temperature is higher, the preffered reaction is the dissolution of the crystal. in lower ones, it's the other one. Now, think as to why this makes sense vis a vis the definition of H, G, and S. well, and T, too.

[Donaldson Tan]

i am not very sure how kinetics work for the reverse process, but i think it has to do hydrophobic interaction. at high temperature, the water molecules move at high velocity to prevent the benzene ring 'body' of benzoic acid molecules from grouping together. at low temperature, the water molecules are moving at slower velocity not sufficient to prevent intermolecular bonding among the benzoic acid molecules.

[rchok2]

i think i somehow got d answer, it's something to do with benzoic acid dimer o something like that...at high temperatures, the hydrogen bond of the benzoic acid dimers break, and then it begans to form hydrogen bonding with water...got this answer from a book but why must the hydrogen bind between molecules brak@_@ to form with water? sigh...

[ericandrews]

I'm going out on a limb here, I once heard hot water has the intermolecular distances a bit bigger hence less hydrogen bonding, ie so, your hildebrand solubility changes (hildeband solubility being calculated from dipsersion forces, hydrogen bonding, and polarity), remember, way out on a limb here