Science Questions

Why does shaving foam stop the mirror from steaming up?

Sat, 24th Jul 2010

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Brett Kuyper asked:

Hello Guys, 


Very good job! I whole heartedly approve of making science approachable for non-scientsists. You guys manage to do this very well.


When I take a shower the mirror in the bathroom mists up. This is clearly a process of condensation of warm moisture from the shower, condensing on the cool surface of the glass of the mirror. If one wipes the mirror, with say a towel, then the condensation returns relatively rapidly. However, I have noticed that if I rub a small amount of shaving foam onto the mirror surface, then the condensation is removed and no further condensation is formed on the mirror. I would like to know what is happening to cause this effect? Is it something along the lines of a wet t-shirt becoming translucent when wet? Why does the foam get rid of the moisture and how does it prevent further condensation from occurring? 


Kind regards




Dave -   In some senses, it doesn’t actually stop it steaming up.  Steam is lots of little droplets of water.  When the light hits it, the light gets bent and so you get a very distorted image which, when you move away from it, just looks like a kind of mist.  What the shaving foam does: there are lots of detergents in it and those detergents reduce the surface tension of the film of water so it doesn’t form lots of little droplets.  It just forms a big flat sheet that you can see through much better, so you can see through it even though the water is still condensing.

Ben -   So there’s literally the same amount of water there; it’s just a change in the structure of the water.  So why do the droplets make it so hard to see things?

Dave -   Because water has quite a high refractive index - when light hits it, it bends.  If it hits this curved surface of a droplet, each one basically acts as a little tiny lens, light is bounced off in all sorts of different directions and makes it looks essentially white.  This breaks up the image and makes it look misty.

Ben -   Fantastic!  Well very interesting.  Not perhaps what you're supposed to use shaving foam for but if it works, it works.


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something to do with glycerin, maybe? CZARCAR, Mon, 12th Jul 2010

There are products in shops that are used to avoid the inside of car's glasses to mist. When water condense on greasy chemicals, it doesn't stick and so it falls down immediately, even tiny drops. Probably shaving foam contains similar chemicals. lightarrow, Mon, 12th Jul 2010

The condensation that is causing your consternation consists of a lot of little water droplets that form on the cold glass surface. The water droplets retain their shape because of surface tension. The droplets scatter the light in all directions, so the mirror is not effective.

The soap in your shaving cream destroys the surface tension and allows the droplets to coalesce into a continuous sheet of water that coats the surface of the glass. Light is able to travel through that coating without scattering.

At least, I think that could be what's going on. A small amount of dish washing detergent might have a similar effect. Geezer, Mon, 12th Jul 2010

hmmm, very interesting, I didn't think to it. lightarrow, Mon, 12th Jul 2010

I rub soap on my mirror and it does do the same thing. I think the chemical in your shaving foam is a soap or a detergant. Detergants have a hydrophilic end and a hydrophobic end. The hydrophilic end will bind to water, thus stopping it from bonding to itself. Just as Geezer says, it breaks down the surface area of water making water, wetter. The other end of the detergant, the hydrophobic side will not bind to water. Instead it binds to oils and grease. This means it will clean the dirt of plates but it would work equally well in removing grease from your chin.  Make it Lady, Mon, 12th Jul 2010

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