Re: How and why do solids dissolve into a liquid?

After a solid is dissolved, it is in the liquid phase, and is every bit   
as much a part of that phase as the solvent.  Its molecules are now 
swimming around in the liquid along with the solvent molecules, rather 
than being confined in the solid structure.

It is common to look at the dissolution of a solid in terms of two 
effects: energy and entropy.

In general, systems tend to go to states of low energy (like a ball tends 
to roll down a hill).  Crystalline solids are in general low-energy 
states.  The change in energy when something dissolves depends on the 
interactions between the solid molecules and the solvent molecules.  For 
the most part, if the solid and the solvent are molecularly different 
(like paraffin was in water), the solid molecules would be 
energetically "uncomfortable" in the liquid solvent and would therefore 
tend to stay in the solid phase.

Entropy can be thought of as related to how "restricted" a system is -- 
the less restricted the more the entropy.  And systems (more properly, 
isolated systems) tend to go toward higher entropy according to the second 
law of thermodynamics.  A solid dissolving would generally represent an 
increase in entropy, because its molecules are now free to move around 
rather than being trapped in one place in the solid.  So entropy tends to 
push twoard dissolution.

Whether or not something dissolves is then a balance between the entropy 
and energy effects.  The exact combination of the two that determines 
things is called the "free energy", but that is about a college sophomore 
level concept so I won't get into that.  The one thing I will mention is 
that the entropy term becomes more important the higher the temperature 
gets, which is a main reason why things tend to melt or dissolve more at 
higher temperatures.

Things get more complicated if you have a solid that dissolves by breaking 
up into ions (like salts).  Breaking up into ions is a big increase in 
entropy, but also having charged particles produces strong energetic 
effects and also bigger changes in the entropy of the solvent.  All other 
things being equal, salts dissolve more in solvents with a high dielectric 
constant (like water).  The dielectric constant is a measure of how much a 
solvent acts to counterbalance an electric field imposed on it. A high 
dielectric constant reduces the energetic penalty of separating a salt 
into charged ions, making dissolution more thermodynamically favorable.

Allan Harvey, aharvey@boulder.nist.gov
"Don't blame the government for what I say, or vice-versa."