[mikegoldi (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 could  simply be that the oil is so viscous that the dissolution is very slow.
Also 1 gram in 5ml is quite a lot.

You could try heating the mixture until the acid melts, then letting it cool.
If it's really a solubility limit then the acid will crystalise out again on cooling.