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Name: Mendy
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 8/2/2004


Question:
Dear Scientist,

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?)


Replies:
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 chemical law

Vince Calder


Mendy,

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.

Regards,
ProfHoff 899


Hi Mendy!

The principle you mention is not only a rule, but it has an explanation. (By the way, in science there is 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!

Mabel
(Dr. Mabel Rodrigues)


Mendy,

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.

Bob Hartwell


Mendy,

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.

Sincerely,

Bob Trach



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