Solubilities of Alcohols in Water

This is an early lab in the semester, done before much new chemistry has been learned. Its purpose is to explore a simple association between the polarity/nonpolarity of molecules and their solubility in water. In an introduction, the relative polarities of O-H and C-H bonds are discussed, with consequences for water solubility.  In the lab, students are given dropper bottles of methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, and 1-octanol. They are also given examples of molecular structures; they have been told about the significance of the prefixes meth-, eth-, etc., but that's about all the nomenclature I care about at this level.

Students are instructed to put 2 mL of water in seven small test tubes. To one tube, they are to add one drop of 1-butanol and mix well to see if it dissolves.  If it does (which it does), they are to add a drop at a time until no more dissolves, at least until they reach a maximum of 25 drops (they should find that 1-butanol becomes insoluble after several drops). They must be careful in their observation not to confuse air bubbles with undissolved droplets; apparently this can be a little tricky. They then repeat the procedure with the remaining six alcohols.

In a table, they are told to draw the molecular structures of the alcohols they used, based on the few examples given them, and to record the number of drops of each alcohol they added to their tubes. The exercise wraps up with questions asking them (1) what is the significance of the "1-" in front of some alcohol names, and why methanol and ethanol don't need that prefix, (2) why solubilities decreased as carbon chain length increased, and (3) given its molecular structure, would they predict that cholesterol, a secondary alcohol, would be soluble in water, explaining their choice.

Revised 9/6/06