|MadSci Network: Chemistry|
Did you notice that soap is not particularly slippery if it is perfectly dry? Slipperiness is related to the reduced friction between two solid objects (e.g. a bar of soap and the bathroom floor). Friction is a force acting perpendicularly (at a right angle) to the direction of movement, thus making the movement less easy. Two dry, clean, flat objects are very difficult to move against each other i.e. they adhere strongly to each other. As soon as you put a liquid or liquid-like film between the objects, friction is much reduced (Skating takes advantage of this: a water film is created by melting ice under pressure of the skate). The structure of soap molecues is such that, in presence of water, they tend to build up a thin, liquid-like film at the surface of a solid, which allows objects to glide easily against each other. More detailed explanation of the phenomenon (keyword: lubrication) rapidly becomes very complex.
Try the links in the MadSci Library for more information on Chemistry.