MadSci Network: Chemistry

Re: A homogenous aqueous solution is frozen. Does it all melt 'evenly'?

Date: Mon Jul 31 22:03:11 2000
Posted By: Kai Bester, Post-doc/Fellow, Environmental Science, Institute for Reference Materials and Measurements
Area of science: Chemistry
ID: 961440125.Ch

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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