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Q & A: The Color of Evaporation

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Most recent answer: 10/22/2007
Q:
Does the color of a cover over a container or glass (for instance - colored sarah wrap) effect the rate of water evaporating?
- Chris Harrison (age 13)
Trinity Catholic School, Tallahassee, FL USA
A:
Chris -
These is a great question, and I can hopefully give you some ideas.

First of all, if you have a container of water that is covered with something like saran wrap, the water is not going to evaporate very much regardless of the color of the plastic, since the plastic will keep the water vapor from escaping. But since I don't think that's really what you're asking, here's some other thoughts...

Water evaporates depending on its temperature. (i.e. Higher temperature -> faster evaporation.) If you took two dishes of water and put a colored filter over one and put them in a dark oven (assuming that the filter itself doesn't stop the water from escaping), the color of the filter shouldn't matter. Both dishes of water will be the same temperature, so they'll evaporate at the same rate. Even if you did this in a lighted oven, you probably wouldn't see any difference. It's not the /light/ that makes the water evaporate... it's the /heat/.

But let's say that you set two identical dishes of water in the sun in two identical windows, except that one window is clear and the other is tinted blue. I would expect that the one in the clear window would evaporate faster (try this at home to check!). Here's why: We know that water will evaporate faster under a sunny window than in a dark room, since the light from the sun is warming the water up. So the more light there is, the warmer the water will get.

White light (like from the sun) is actually a mixture of all different colors of light. It has some light waves that are blue and some that are green and some that are red, etc. They only look white because they're all mixed together. When you put a colored filter (like the blue tinted window) in the way of white light, it only lets one color of light get through. So only the blue light will get through the blue window and all the other colors will get lost. As such, there will actually be /less/ light coming through the colored window than the clear one. Less light means that the water will be warmed up less, so it won't evaporate as quickly.

The best way to settle this question is to do an experiment. Please try it and tell us what you discover.

-Tamara

(published on 10/22/2007)

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