In lab the other day we did an acid/base extraction to separate mixtures. The top layer of the mixture is the organic layer, which consists of diethyl ether and fluorine. The bottom layer is considered to be the aqueous layer, which consists of water and benzoate anion.

Question

The different compounds have different polarities, my questions is: what is the reason we can separate compounds of different solubility, is because the solutions are insoluble in each other?

And then when choosing specific solvents for an extraction, what is the reason for choosing them? Is it again because of the fact that different molecules have different densities and mix or don't mix well with other solvents?

Thanks for the help!