Knowing the different effects that determine solubility between compounds is of great importance in organic chemistry since solvent choice can directly determine recrystallization and reaction yields. More fundamentally, if reagents are insoluble in the solvent, no reaction will even occur! Therefore, it is important to be able to determine whether two compounds will be soluble in each other just from looking at their structures.

            One easy way of determining solubilities is to remember that "like dissolves like." In other words, compounds with similar properties are generally soluble in each other, and compounds with different properties are generally insoluble in each other. Polar compounds are generally soluble in other polar compounds; non-polar compounds are generally soluble in other non-polar compounds. Molecules that can form hydrogen bonds are generally soluble with other molecules that can form hydrogen bonds (see picture below), and so forth.

            Oil and water don't mix. This is because alkyl chains (which make up the bulk of oil structures) can be thought of as hydrophobic (water hating). Interestingly enough, hydrocarbons and water do in fact have weak attractive interactions through London Forces; however, these forces are very weak compared to hydrogen bonding that occurs between the water molecules. By dissolving a hydrocarbon, such as propane, a compound that has no hydrogen bond forming capabilities, water must give up some of its hydrogen bonds that it has to other water molecules to form a cage-like structure around the propane. Doing so would be thermodynamically unfavorable because the energy of the system due to the broken hydrogen bonds would be increased (remember, nature is lazy and will always choose the lowest energy state possible). Therefore, to minimize the energy, oil and water separate, with oil generally remaining on the surface because of its lower density.

Ethanol is completely miscible in water because its -OH group can be involved in hydrogen bonding with water		Propane is insoluble in water because it is not polar, nor capable of forming hydrogen bonds