Date: November 2006
How does dissolving work? I have read on your site
that "likes dissolve likes," but that still does not help me
understand exactly how it works. For example, when you put solid
sugar into water, it seems to become a liquid without
melting. What makes a solid dissolve when it touches a
liquid? What limits the amount of solid that can be dissolved?
You are correct in realizing that dissolving and melting are two
different processes. Melting requires that the solid
particles/molecules/atoms gain enough kinetic energy to break the
forces that are holding them in the solid phase and so that they
enter the liquid phase. Dissolving requires three distinct
processes: (1) the solute gains enough energy to break the forces
that are holding it together, (2) the solvent gains enough energy to
break some of the forces that are holding it together, and (3) the
system releases enough energy so that the solute and the solvent
particles/molecules/atoms interact with each other.
Thus, while melting only requires that we raise the internal energy
of a solid so that it breaks its own interparticle attraction,
dissolution means that not only do the solute and solvent
interparticle attraction have to be broken, but that solute-solvent
interaction has to be formed.
In melting we can supply the energy required to break the solid
interaction by heating the solid. In dissolution, the energy
released by the formation of the solute-solvent interaction has to
be sufficient to break the solute-solute and solvent-solvent
interaction. This means that the energies have to be matched, and
the solute-solvent interaction has to be readily formed (the solute
and the solvent have to have similar types of interparticle forces,
they have to "like" each other). This is why we say that "like dissolves like".
Greg (Roberto Gregorius)
Molecules of a substance dissolve when they mix into and are
surrounded by molecules of a solvent. This happens because there are
forces of attraction between the dissolving solid "solute" (e.g.
sugar) and the solvent (e.g. water), which allow the solvent
molecules to organize around the solute and overcome the forces of
attraction among the solute molecules (or atoms in the case of
salts) that would otherwise cause them to form solids. When there
is too much solute, their molecules end up close to each other in
the solution and are attracted to each other. When this attraction
between the solute molecules overcomes attraction for the solvent
molecules, the solid forms again, until enough solute drops out that
the remaining molecules can stay dissolved.
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Update: June 2012