Re: Why does soap keep mirrors from fogging up in steamy bathrooms?

Beth,

Have you ever seen insects walking on water, or carefully placed a pin on the surface of water and had it float? These things float due to the surface tension of the water, which is due to the cohesive forces holding the water molecules together. It is the surface tension of water that also forms the droplets on the mirror that we call fog.

Soap decreases the surface tension of water (you will sometimes hear this referred to as a surfactant, meaning a surface active agent) to the extent that the droplets do not form, that is when stem condenses on the soap covered surface, the water spreads out so far that you do not notice any fogging. If you do the experiment with the pin on the surface of water, and carefully add a little soap as to not to disturb the water, the pin will sink.

You should be able to read more about surface tension and surfactants in any introductory chemistry text, such as Chemistry The Central Science by Brown, LeMay, and Bursten.

Rob Baron