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Name: Phil
Status: educator
Grade: 6-8
Location: ME
Country: N/A
Date: February 2007

Question:
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.



Replies:
Phil,

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.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming



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