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Name: Michelle
Status: student
Grade: 6-8
Location: FL
Country: N/A
Date: November 2006

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?



Replies:
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)


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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