Ingredients: water, methanol, ethanol, glycerol

Procedure: A complete recipe follows.

1. Acquire small quantities of water, methanol, ethanol and glycerol.

2. Observe the systems using sight and smell and touch.

Understanding: The compounds known as alcohols exist in a variety of forms. All alcohols have in common the -OH functional group. The electronegative oxygen bonded to the electropositive hydrogen creates a substantial separation of partial charge - the oxygen being negatively charged relative to the positively charged hydrogen. The difference of electronegativity creates a dipole on the -OH functional group.

Since the polar hydrogen of one alcohol can hydrogen bond with the oxygen of another molecule, there is a good deal of intermolecular hydrogen bonding in alcohols. That hydrogen bonding attraction is strong relative to the energy of motion associated with temperature. The ability to hydrogen bond makes the molecules "sticky," lowering the vapor pressure of the alcohols relative to the corresponding hydrocarbon - CH₃OH/CH₄, C₂H₅OH/C₂H₆, C₃H₅(OH)₃/C₃H₈.

The more alcohol groups that a molecule has, the more hydrogen bonds it might form, and the stickier it will be. So we expect a molecule like glycerol, with three -OH groups, to be "stickier" than water. That is just what is observed - glycerol is far more viscous than water or simple alcohols such as methanol or ethanol, whose viscosity is much like water. Why is that so? Glycerol has more hydrogen bonds formed per molecule!