[GilbertJobs (OP)]

Hi!

This question was from a lab a few days ago. Stearic acid (octadecanoic acid) was observed to not dissolve in water (makes sense because water is polar and stearic acid is not really) but the acid also didn't dissolve in vegetable oil, which doesn't make too much sense because they are both non-polar. Everything was done in small quantities (1g stearic acid in 5 mL water or oil) and at room temperature. Any idea why? I'm thinking it might be because of that carboxyl group at the end of the stearic acid that makes is slightly polar. Thanks in advance!

[Bored chemist]

It will dissolve but possibly rather slowly, also 1 gram in 5 ml is quite a lot.
If you heat the mixture until the acid melts (about 70 C) they will mix and you can then let it cool down.
The acid may well crystallize out of the cool mixture.

[Hyperbole]

The tail is basically an alkane, and so readily dissolves in fat, oil and grease, but not in water. Thus the tail is said to be hydrophobic (water-hating). The head-group, however, is polar, and so easily dissolves in water (hydrophilic - water-loving) and will not dissolve in oil or grease.

[Evageasse]

I congratulate, your idea simply excellent

[mileys1990]

Some of the most insoluble things are compounds that are both polar and nonpolar at the same time; solvents that interact strongly with one part do poorly with the other. It’s often hard to find a solvent that can thread the needle between them!