Why does polarity affect solubility

Polar species are soluble in water, while nonpolar species are soluble in oils and fats. Covalent solubility uses the like dissolves like rule. This means that substances with the same type of polarity will be soluble in one another. Moreover, compounds with differing polarities will be insoluble in one another.

In this class, we will not explore molecular geometries that are used to determine polarity. Instead, the polarity of a substance will be provided. In determining polarity, chemists look to the power of atom's' nucleus. The protons from an atom's nucleus are capable of attracting another atom's electrons.

Within a covalent bond, valence electrons are pulled toward's an atom that has a more powerful nucleus.

This pull is called electronegativity. If different atoms are connected in a bond, then one tends to be more electronegative than the other. Molecules that have an overall pull in one direction are labeled as being polar species.

Look at the structure of water that is shown below. This structure of this molecule shows the bonding electrons being pulled towards oxygen. Therefore, oxygen has a more powerful nucleus than hydrogen. Water's bent molecular shape does not cancel out the individual dipole pulls.

As a result, water has an overall pull and is classified as being polar. Carbon dioxide also has local dipoles arrows that pull in opposing directions. This molecule does not have an overall pull in one direction and is classified as being nonpolar. Some other molecules are shown in the figure below. The top three assymetrical molecules are all polar. They all have dipoles pulls that do not cancel. All of these molecules would be soluble in water. The bottom three molecules are nonpolar.

These are symmetrical molecules that have dipoles that cancel. Both of these molecules would be oil or fat soluble. Covalent solubility is important in the pharmaceutical industry. If a medication is not water soluble, then it will not dissolve in the bloodstream and react in the active site of the body in a timely and potent fashion. The polarity of vitamins can affect how long they remain in the body.

Hence, the enthalpy change to break these interactions step 1 is small. The nonpolar solute molecules do not form strong interactions with the polar solvent molecules; therefore, the negative enthalpy change for step 3 is small and cannot compensate for the large, positive enthalpy change of step 2. Therefore, the dissolution does not occur spontaneously. The nonpolar solvent molecules are also held together only by weak van der Waals interactions, so the enthalpy change for step 2 is also small.

The principles outlined in the green box above explain why the interactions between molecules favor solutions of polar vitamins in water and nonpolar vitamins in lipids.

The polar vitamins, as well as the polar water molecules, have strong intermolecular forces that must be overcome in order for a solution to be formed, requiring energy. When these polar molecules interact with each other i. Hence, the overall enthalpy change energetics is small. The small enthalpy change, coupled with a significant increase in randomness entropy change when the solution is formed, allow this solution to form spontaneously. Nonpolar vitamins and nonpolar solvents both have weak intermolecular interactions, so the overall enthalpy change energetics is again small.

Hence, in the case of nonpolar vitamins dissolving in nonpolar lipid solvents, the small enthalpy change, coupled with a significant increase in randomness entropy change when the solution is formed, allow this solution to form spontaneously as well. For a nonpolar vitamin to dissolve in water, or for a polar vitamin to dissolve in fat, the energy required to overcome the initial intermolecular forces i. Hence, in these cases, the enthalpy change energetics is unfavorable to dissolution, and the magnitude of this unfavorable enthalpy change is too large to be offset by the increase in randomness of the solution.

Therefore, these solutions will not form spontaneously. There are exceptions to the principle "like dissolves like," e. In general, it is possible to predict whether a vitamin is fat-soluble or water-soluble by examining its structure to determine whether polar groups or nonpolar groups predominate.

Will it dissolve in water? The molecules of ethanol are attracted to each other by H-bonding. And the molecules of water are attracted to each other by H-bonding. Water and ethanol are miscible in all proportions. What if you have a nonpolar substance such as hexane? The attractive forces among the water molecules are the relatively strong H-bonds.

The only attractive forces among the hexane and water molecules are London forces. Water and hexane are immiscible. They do not dissolve in each other.

There is little resistance to a molecule of one compound moving into the other layer. Thus, polarity affects solubility. If solute and solvent have approximately the same polarity, they will probably form a solution. Related questions How do I determine the molecular shape of a molecule? What is the lewis structure for co2? What is the lewis structure for hcn? How is vsepr used to classify molecules?

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