Question

why the compound to be crystallized should be very soluble in the hot solveng and less soluble in the cold solvent ??

and why impure solid dissolves in the minimum volume of hot solvent ? why minimum ??

Answer 1

"The principle behind crystallization is that the solubility of a solid compound increases with increase in temperature and thus more sample can be dissolved in a less amount of solvent at elevated temperatures. when this sample later cools down, the solubility of a sample decreases and the sample becomes saturated at a lower concentration of solute. Due to this, the sample can no longer stay in the solution form and appears in the form of crystals from the solvent"

Crystallization only happens when the solution cools down and therefore if the sample dissolves in the cold solvent then there will be no formation of the crystal as the sample is soluble in cold solvent while if the sample is not soluble in cold solvent and soluble only in hot solvent then once the solvent will become cold then the sample will be crystallized out as the tendency of the sample is not to be soluble in the cold solvent.

Minimum solvent is required to maximize the crystal formation as more solvent will dissolve more amount of sample and the complete sample will not crystallize out from the solvent. secondly, at high temperature, minimum amount of solvent is required as the solubility of a compound greatly increases at high temperature thereby eliminating the need to use large amount of solvent.