How does being both polar and nonpolar allow the soap to both dissolve in the so

One end of the soap molecule is very non-polar, like grease; the other end is is very polar and interacts with water. The "oily" ends of the soap molecules stick to the grease particles, so the outside of the particles are covered with the exposed polar ends, which allow the grease particles to be solublized in water.

In as plain English as possible:

The non-polar end adsorbs the oil or other hydrophobic dirt. The ionic end is highly soluble in water. This allows for an emulsion to be formed. The alkali metal (sodium or potassium ion) does not play a role in the action of the soap.

Soap use is not a chemical reaction, but a physical one. Under normal conditions, the soap does not react with the dirt chemically. If "hard water" minerals are present (magnesium or calcium) these can chemically react with the soap and lessen its effectiveness by removing the soap from solution.

The structure of the emulsion is such that the oil or oily dirt is surrounded by soap molecules with the ionic part of the molecules toward the outside where water will react with the ionic end (by hydrogen bonding) and keep the oil in "solution." Hot water helps in the formation and suspension of the emulsion.

This interface of oil and water is based upon the the old adage that "like dissolves like." The long hydrocarbon part of the soap adsorbs the oil, the ionic end is dissolved in the water.

Soaps are sodium or potassium fatty acids salts, produced from the hydrolysis of fats in a chemical reaction called saponification. Each soap molecule has a long hydrocarbon chain, sometimes called its 'tail', with a carboxylate 'head'. In water, the sodium or potassium ions float free, leaving a negatively-charged head.

Soap is an excellent cleanser because of its ability to act as an emulsifying agent. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed.

The organic part of a natural soap is a negatively-charged, polar molecule. Its hydrophilic (water-loving) carboxylate group (-CO2) interacts with water molecules via ion-dipole interactions and hydrogen bonding. The hydrophobic (water-fearing) part of a soap molecule, its long, nonpolar hydrocarbon chain, does not interact with water molecules. The hydrocarbon chains are attracted to each other by dispersion forces and cluster together, forming structures called micelles. In these micelles, the carboxylate groups form a negatively-charged spherical surface, with the hydrocarbon chains inside the sphere. Because they are negatively charged, soap micelles repel each other and remain dispersed in water.

Grease and oil are nonpolar and insoluble in water. When soap and soiling oils are mixed, the nonpolar hydrocarbon portion of the micelles break up the nonpolar oil molecules. A different type of micelle then forms, with nonpolar soiling molecules in the center. Thus, grease and oil and the 'dirt' attached to them are caught inside the micelle and can be rinsed away.

Although soaps are excellent cleansers, they do have disadvantages. As salts of weak acids, they are converted by mineral acids into free fatty acids:


CH3(CH2)16CO2-Na+ + HCl ----> CH3(CH2)16CO2H + Na+ + Cl-
These fatty acids are less soluble than the sodium or potassium salts and form a precipitate or soap scum. Because of this, soaps are ineffective in acidic water. Also, soaps form insoluble salts in hard water, such as water containing magnesium, calcium, or iron.


2 CH3(CH2)16CO2-Na+ + Mg2+ ----> [CH3(CH2)16CO2-]2Mg2+ + 2 Na+
The insoluble salts form bathtub rings, leave films that reduce hair luster, and gray/roughen textiles after repeated washings. Synthetic detergents, however, may be soluble in both acidic and alkaline solutions and don't form insoluble precipitates in hard water. But that is a different story...

Long chain organic (fatty) acids are an example of such compounds. With a long chain of nonpolar -CH2- groups separating a terminating nonpolar -CH3 group at one end and a terminating polar -COOH group at the other end, the two ends are far enough apart to allow one end to be nonpolar while the other end is polar.

A simple, but not wholly accurate, description of how soap (usually the sodium or potassium salt of a fatty acid) works illustrates this: The nonpolar end of the molecule can dissolve (or dissolve in) grease, while the ionic end of the acid salt can dissolve in water. This is how soap pulls grease off of dishes and allows the water to wash it away. See the link below for a more detailed description.