Likes Dissolve Likes
I have learnt the broadly stated principle that "Like dissolvesLike".
Although we can carry out an experiment to demonstrate the principle, I
am wondering if there are any theoretical explanation about that. (Or
is it just a rule?)
Yes there is/are theoretical principle(s) underlying the chemical
"mantra": Likes Dissolve Likes. For a substance to dissolve the cohesive
energy of the bonds holding the solid or liquid solute together, and the
energy cost of disrupting the solvent-to-solvent bonds must be overcome by
the cohesive energy released by the formation of the solute-to-solvent
bonds. Thus there are two energy "costs" (one solute/solute and one
solvent/solvent) and two energy "gains" (two solute/solvent bonds). If these
energies are approximately equal, which occurs when the solvent and solute
molecules are structurally similar, then the substance will dissolve in the
solvent. Hence the saying: "Likes dissolve Likes.".
Having said that, do not take the guiding principle too literally. It is
only a guiding generalization to which one can find many exceptions, because
what are the "Likes" is not always easy to sort out. And what one means by
"dissolves" is very loose. For example, what about partial solubility versus
miscibility (total solubility over all concentrations).
"Likes dissolve Likes" is a guiding rule of thumb, not a physical
Underlying "like dissolves like" is a recognition that there must be some
sort of compatible interaction between the solute (that dissolved) and the
solvent (that which does the dissolving). Sometimes this "compatibility"
is a matter of structural similarity -- for example, most alcohols will
dissolve (and dissolve in) in other kinds of alcohols. Ethanol, a water
soluble alcohol, will dissolve and dissolve in butanol, a rather water
insoluble alcohol. These alcohols get along so well because they have a
polar -OH end and a non-polar hydrocarbon "tail." Thus, they are
structurally compatible. On the other hand, even though butanol and water
both have polar -OH functions, the hydrocarbon "tail" of butanol is
structurally dissimilar to the remaining -H atom in water H-OH. Thus, they
are insoluble one in the other.
To the extent that we can find structural similarities between solute and
solvent is rather nicely related to the extent to which we can predict
whether one substance will dissolve in the other.
The principle you mention is not only a rule, but
it has an explanation. (By the way, in science there
always an explanation for rules, or facts. It could be
that the scientists of today do not know all the facts
explanation, what does not mean that the scientists of
tomorrow will not find an explanation, or hopefully
the explanation, if it is correct.)
So, when one says "like dissolves like" it has to do
with the forces that hold the atoms together forming a
compound. It has to do with the chemical bonds.Broadly
speaking, the chemical bonds can be
ionic or covalent. So the crystalline solids are held
together by the attraction of 2 oppositely charged
ions (ionic bond). Another way of bonding, the
covalent bond occurs when the bond is formed by a
shared pair of electrons. There are other in between
cases where the electron sharing is not equal, and it
occurs some part that is slightly positive. In these
cases, where the electron sharing is not equal, the
bond is called a polar covalent bond, or simply a
polar bond and in the cases where electrons are
equally shared are called non polar covalent bonds.
Then coming back to the solubility rule, non polar
(or slightly polar) solutes dissolves best in polar
solvents. The rule works well in this case. So fats,
oils and greases (that are non polar or slightly
polar) dissolve well in non polar solvents as benzene.
That happens because the forces that hold non polar
molecules together are mostly weak. So, the amount of
energy needed to separate the atoms at the molecules
of the solute and to break the attractive forces
between molecules of the solvent are small.
So there are a dissolution or a solution is formed.
But the rule that likes dissolves likes does not works
so well for polar substances and, in particular, for
aqueous solutions. The water solubility has to do with
the ability of water to form hydrogen bonds to the
solute molecules. So, molecules containing a high
proportion of nitrogen or oxygen atoms will dissolve
in water because these elements are the ones that
form hydrogen bonds.
Hope all that helped you somehow.
And thanks for asking NEWTON!
(Dr. Mabel Rodrigues)
Like dissolves Like is a general rule describing the fact that polar
solvents will dissolve polar solutes and non polar solvents will
dissolve non polar solutes. There is theory behind this which has been
proven through much observation.
One of the benefits of understanding this theory has been the
development of surfactants. These are molecules which are polar on one
end and non-polar on the other. Surfactants in laundry detergent
surround oils and other non-polar "dirt" on our clothing with their
non-polar side toward the oil and polar side toward the water. Now the
oil will be "attracted" to the water and lifts from the clothing.
When we examine the kinds of substances that dissolve in different kinds
of solvents, we discover an important principle often quoted as the like
dissolves like rule. That is to say that polar and ionic compounds tend
to be soluble only in polar solvents, whereas nonpolar compounds tend to
be soluble only in nonpolar solvents. For example, polar sugar molecules
and ionic salt crystals are soluble in polar water, and the nonpolar
molecules in oil are soluble in gasoline, a solvent composed of nonpolar
molecules. I hope that this helps.
Click here to return to the Chemistry Archives
Update: June 2012