Date: February 2007
I teach a unit on mixtures and solutions to sixth
graders. I am a bit unsure how to explain the solubility of a
sugar solution. I demonstrated that the solubility increases when
the solution is heated, but I am not exactly why that occurred. I
have found very good explanations about ionic solutions, such as
salt, but not sugar.
Unfortunately, this is a rather complicated issue - especially since
the effect of temperature is not always to increase solubility. For
example, while KBr and NaNO3 increase in solubility with increasing
temperature, Na2SO4 and Ce(SO4)3 decrease in solubility with
increasing temperature. This is further complicated by the fact that
it is often difficult to separate the effect of solubility (the
maximum amount of substance that can be dissolved) from solvation
(the speed at which a substance goes into solution) - and the effect
of temperature on solvation is not always in the same trend. For
example, some plastics are actually soluble in particular solvents
but their solvation is so low (it takes so long to dissolve them)
that it seems that the plastics do not or will not dissolve at all,
and heating the system just makes the two substances phase separate.
A good explanation that covers both the positive and negative
effects of temperature, and limited to the Thermodynamics
(solubility only) and not considering the Kinetics (solvation) would
have to involve the concept of spontaneity and Gibbs Free Energy (dG
= dH - T[dS]). The idea here is that if dS is positive (the solution
is more disorganized than the separate solute and solvent) than an
increase in T would result in a more negative dG which would make
the solution formation highly spontaneous. However, if dS is
negative than increasing T is counter to spontaneous solution
formation (makes dG less negative). But I do not know if that would
work for 6th graders.
I hope other respondents can tailor a better explanation.
Greg (Roberto Gregorious)
In general, solubilities of solids increase with temperature for the
same reason that solids tend to melt when heated: the increased kinetic
energy of the molecules disrupts the attractions between the molecules
holding them in the solid phase.
There is the added complication in a dissolving process that you have
more intermolecular interactions to consider: the dissolving process
involves interactions between solute and solute (both in the solid and
solution phase), between the solvent and solvent, and between the solute
and solvent. By Le Chatelier's principle, you can expect that solutes
that release a lot of heat when dissolving will actually be more soluble
at low temperatures than at high temperatures.
Department of Physics and Astronomy
University of Wyoming
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Update: June 2012