Ask A Scientist Chemistry Archive

name         Justin D.
status       student
age          17

Question -   What is it that determines if a compound can be
super-saturated. I know compounds such as salt will not allow themselves
to be super saturated, but others like KCL will allow themselves to.
And if it can be super-saturated in one solvent, will it be possible for
other solvents as well?
------------------------------------------------------
Supersaturation is a thermodynamically unstable state. There are many
contributing factors that play a role: 1. The viscosity of the solution or
liquid is a factor. Glycerin (glycerol) has a m.p. of 20 C, but very few
people have seen the solid. The same is true of common "glass" which is
super cooled SiO2, at least approximately, also honey. 2. The presence (or
absence) of "seeds" like dust is important. 3. The complexity of the crystal
structure, e.g. water, makes some substances more prone to super-saturation.
4. The rate of cooling, if that is happening can play a role.
There is no single answer; many factors play a role.

Vince Calder
=========================================================
This question introduces you to the field of crystal growth, which is very
important, much studied, and poorly understood.  In fact, the American
Chemical Society publishes an entire journal devoted to this topic, Crystal
Growth and Design.

Your question is really the same as asking why salt readily crystallizes
from a saturated solution, while compounds such as potassium chloride do not
crystallize as readily, even when it is thermodynamically favorable to do
so.  The general answer is that crystallization is a complicated process
that requires the molecules to arrange into just the right alignment, so
that they will induce others to join them and propagate the arrangement into
a large solid crystal.  A lot of things can happen to inhibit the
"nucleation" step and to disrupt the continued growth of a crystal once it
starts.

Solvent can play a big role in crystallization.  For instance, it is very
difficult to get most sugars to crystallize from supersaturated solutions in
water (such as honey, which can stay liquid for months).  If the sugars are
dissolved in pyridine, however, the sugars can usually form crystals more
readily.

As for the specific cases of salt (NaCl) and potassium chloride (KCl), some
information about the crystallization processes are probably known, but not
by me.  The crystallization of sodium chloride is of considerable economic
significance, and the many forms of salt that are sold - blocks, granulated,
fine and coarse powders, rock - differ primarily in crystal size.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
============================================================