|MadSci Network: Chemistry|
Indeed it matters a lot and the solutions do not freeze evenly. Thus they essentially do not melt evenly. Why is that? In general, the solubility of "foreign" molecules in crystals is low. Structure and bonding possibilities between "Hostmatrix" such as water in frozen and liquid state is completely different. Solubility in the crystal will change also in dependence of the kind of crystals: snow may be different from glacier ice..... This effect is used in organic and inorganic chemistry for cleaning compounds and is called fractionated crystallization. This crystallization happens during the freezing process while crystals and solution both are present.... AT the start, the later "inner" parts of the crystals as the concentration of the solute molecule is low. The more water crystallizes (without solutes included) the higher the concentration in the remaining solution gets. In the end, even these parts will freeze and will include the remainig solute molecules. The result is: the crystals are low in concentration in the inner parts and "dirty" or high in concentration at the tips and outer parts. NB: in some cases the solute is better soluble in the crystal than in the solution, i.e., the concentration is highest in the inner parts of the crystal. NB2: I do not know of any cases were a homogeneous distributions may be assumed. NB3: If You have to melt a part of a very expensive frozen solution (such as enzymes), it may be feasible to make a homogeneous (ice) powder from the crystals, take away an aliquot of the resulting powder and melt this aliquot totally. The better way is definitely to freeze small portions of the solution and thaw as needed. I hope this will answer the question.
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