Functional Groups - Water Solubility
The size of the molecule and functional groups present on the molecule also determine whether or not it will be soluble in a particular solvent. Many of the same functional groups that raise the boiling point of a compound (carbonyl and hydroxyl groups) also increase its solubility in water. As a general rule monofunctional compounds with three or fewer carbon atoms will be soluble in water. Those with six or more carbon atoms will be insoluble. Those compounds with four or five carbons are borderline.
Using this rule we should be able to predict that methanol (an alcohol with one carbon) will be soluble in water, octanol (an alcohol with eight carbons) will be insoluble and that butanol (an alcohol with four carbons) may or may not be soluble.
If water and a soluble compound are added to a container, the two will mix (below right). If water and an insoluble compound are added to a container, the two do not mix and we observe two layers (below left).