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Name: Debi M.
Status: student
Age: 40s
Location: N/A
Country: N/A
Date: 2001 - 2002


Question:
How and why does the least nonpolar substance dissolve first in a nonpolar solution?


Replies:
There is an old chemical mantra: "Likes dissolves Likes" but that does not tell you much about why. The answer is complex because the solution process is a complex process, but here is a simplified view:

For a solid to dissolve in a solvent, the atoms/molecules must be separated from the solid. This process requires an input of energy of some sort. An analogy is the boiling of a liquid, where there must be sufficient thermal energy (high enough temperature) so that some of the molecules have enough energy to escape the liquid.

In the case of solution of a solid by a solvent, the solvation of the atoms/molecules by the solvent must be sufficiently strong to overcome the forces holding the solid together and also to keep the atoms/molecules in solution.

Nonpolar solvents, where only van der Waals force operate (approximately), these forces are not strong enough to overcome the polar and/or ionic forces that hold those types of solid solutes together. Non-polar solid solutes, e.g. paraffin, are held together by van der Waals forces (approximately), so the solvent has a competitive chance to dissolve such solutes.

This is very simplified, because there is no detailed theory that can predict solubility reliably. As I said above, the process of the dissolution of solids is a complex business.

Vince Calder



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