Dissolving Solids

name         Michelle
status       student
grade        6-8
location     FL

Question -   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?
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Michelle,

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)
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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.

Don Yee
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