SMOKE AND MIRRORS

Mirror, mirror on the wall, who's the fairest of them all? The trouble is, if the mirror is on a bathroom wall, then, after a shower, we cannot tell at all. Vanity aside, all that condensation on the mirror certainly makes life difficult when you're trying to shave. Wiping it with the hand only buys a few seconds of viewing time and using a towel compounds the problem by covering the mirror with lint. There may, however, be a simple solution to the dilemma. But before looking at it, let's reflect on why the mirror fogs up in the first place. 

Everyone knows that water can turn to a vapour and then change back into a liquid. After all, that is what rain is all about. Water evaporates from the oceans, the vapour rises and then condenses to form tiny water droplets. We call these aggregations of water droplets "clouds". Eventually the water droplets coalesce and fall to the ground as rain. 

A similar scenario takes place in the bathroom during a shower. Hot water vaporizes and the vapour then condenses on any cool surface. A mirror is ideal; in a jiffy it is covered with little droplets. And then our struggle to peer through the fog begins. The obvious solution is to have a heated mirror! If the surface is as warm as the water vapour, there will be no condensation. Classy hotels actually have rear-heated mirrors but this is a luxury rarely encountered in a home. But commercial self-heating mirrors are available! These are attached to the main shower pipe so that hot water circulates through the mirror, heating it. The only problem is that these mirrors are usually too small for most purposes. 
There is, however, an even more ingenious solution to the problem. I was alerted to this as I cruised through a trade show one day and was stopped in my tracks by a salesman pitching a product that claimed to solve the problem of the fogged mirror. After alerting the crowd to the gravity of the situation and how fogged mirrors may eventually spell the end of western civilization, he proceeded to rub a pink fluorescent crayon-like substance over half a mirror. He then attempted to steam the mirror with a kettle; the treated half did indeed remain clear. Seemed like a bargain at ten dollars a crack! 
Then I started to think about what was going on here. What was this miracle product all about? I had a suspicion. So, I went home and tried the same experiment with a bar of soap. I made a few marks on a mirror and then rubbed it in with a cloth. The protective effect was the same! 

Now let's look at the science involved here. Water forms droplets because molecules on its surface attract each other. This surface tension, as it is called, is the reason we can fill a glass so that it overbrims with water. But add a little soap or detergent, and the attraction between the water molecules is eliminated. The water cascades out of the glass. Similarly, if a little "surfactant", as soaps or detergents are technically called, is added to water, instead of forming beads on a surface, the water spreads into a thin layer. This is just what happens when we rub the mirror with soap. Instead of beads that create a fog, we get a thin transparent film of water. So there is really no need to shell out ten bucks for a little piece of soap. 

The same bit of chemistry will work on eyeglasses. You can rub the lens with a bit of soap, or if that sounds unappealing, with a little rubbing alcohol. This can also reduce the surface tension of water; in fact it is the active ingredient in commercial anti-fogging products. And if you want to be really economical, you can always reduce surface tension the way divers do. The components of saliva have detergent action! Not recommended for the bathroom mirror though.