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Q & A: Melting Colored Ice

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Q:
Does the color of ice make it melt faster?
- Alissa
Jack D. Gordon Elem., Miami, FL
A:
Alissa -

The answer to your question is sometimes yes and sometimes no. If the ice is melting in the dark, then no. But if the ice is melting in the light, then the color will matter. Let me explain. White light (this is the type of light you're used to seeing) is actually made up of all sorts of different colors of light. You can see this if you shine a flashlight through a prism. The prism bends different colors of light differently, so you can see them all separately.

When light hits something, some of the colors of light are absorbed by the object, and some of them bounce off and come back to your eyes. This is why things look like different colors. For example, if you have a red shirt, that means that the shirt is absorbing all of the colors of light except for red, and the red is bouncing off so that you can see it. If something is white, that means that all of the light bounces off. And if something is black, then it's absorbing all of the light and nothing bounces off.

The reason that this matters is that the light that's absorbed heats up the object. You know this if you've ever been outside on a sunny day in a black shirt. Since black absorbs all of the light, you get pretty hot. But if you're wearing a white shirt, none of the light is absorbed, so you don't get as hot. Another example of this is if you've ever touched a blacktopped driveway on a sunny summer day. It's a lot hotter than a white cement sidewalk because it absorbs more light.

So this means that a dark colored ice cube will absorb more light and melt faster than a light colored one. (Of course this doesn't matter if the ice cubes are in the dark.)

The other thing that may matter is what is making the ice be colored. If water is colored, that means that it probably has something dissolved in it. This won't change how fast the ice will melt very much, but it could change what temperature it starts to melt at. Normal food coloring probably won't change this melting temperature much, but other types of dye might.

-Tamara

(republished on 07/25/06)

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