Expert: Daniel Mazur - 5/9/2007

Question
QUESTION: I am doing a science project on the effects of color on evaporation. I have food coloring in 3 cups (red, green and blue) and 1 cup clear water. i also have red bulbs above each cup. The red water is absorbing fastest, then the clear, then green, then blue. I am wondering why the clear water is 2nd fastest? Is it because of the red bulb? THANK YOU SO MUCH for your help!!!
Madison
ANSWER: Hi Madison,

I need to ask *you* a few questions first about your experiment to give you a sensible answer. All of them are highly relevant, I try to justify each one below.

1) What color were the cups you kept your samples (water) in during experiment?
* Black cup will heat up itself a lot, white cup will too, somewhat, mirror (shiny metal) cup will reflect the heat back to the water inside, clear cup will let heat pass through (simplified view, but may work).

2) Did you measure water temperature (and cup temperature) during the experiments?
* This would tell us, where exactly the heat gets mostly absorbed.

3) Did you do all colors at the same time or subsequently? In either case, did you make any arrangements to have the same ambient temperature, pressure, humidity and light (apart from the red bulb)?
* Rate of water evaporation will *seriously* depend on ambient temperature, pressure, humidity and light. If these were not controlled, it's hard to make conclusions from the experiment.

4) What dyes *exactly* did you use?
* Some dyes are in fact mixtures of other dyes, e.g. green may be a mixture of yellow and blue. When it comes to red, it beats *me* that it ranked first and not last - the color match should mean the *least* heat absorption...!

5) What were the concentrations of the dye in your water solutions?
* Ideally, we should have the same number of dye particles per unit volume. To evaluate it seriously one needs to know exactly the chemical composition (and other properties) of the dye... But it is necessary to know at least if the amount (mass) of dye was the same for all colors.

6) What was the air flow around your water cups during the experiment? Did you control it?
* Air flow increases evaporation. Above water level there is always increased vapour concentration, which in turn slows the continuing evaporation. If dry air blows in, the inhibition is removed and evaporation goes on at maximum speed.

7) Did you stir?
* Probably not, but that would make it possible for some dyes to accumulate on the surface and block water evaporation.

I cannot write more now (lack of space), so I'll wait for your answers about the experiment details. I'll try to help more then.
Cheers, Daniel

---------- FOLLOW-UP ----------

QUESTION: Thank you for helping me. Should I just start over, and if so, what is the best way for me to measure and test my experiment?
*I used clear plastic dixie type cups with white caulk on the bottom in order to hold them in place on a board.
*I only measured the temperature above the cup which was 83 degrees.
*I did all the cups at the same time. 4 cups exactly 5 inches apart with C7 Christmas lights 6 inches above the cups.
*I used food coloring. I put 20cc of water with 2 droppers of food coloring in the colored and 20cc and 2 droppers of plain water in the clear one.
*I have been checking the amount every 48 hours with a syringe.
*the air flow has been the same. I have been keeping my experiment on the floor in the closet.

Thank you so much!!!
Madison  

Answer
Hi again,

I would definitely suggest repeating the experiment - reproducibility is a *mandatory* feat of any experiment in natural sciences. How long did the experiment last, anyway? How many repetitions can you manage? One, four, ten? When you redo the experiment, I would suggest to swap/rotate the positions of the different colors in your cabinet. Although there should have been no air flow, it could happen that one corner is just dryer or warmer, which would distort your result.

I thought about your project a bit and I realized that you are in fact measuring a slightly different effect than what you are *trying* to measure. A bulb (colored or clear) produces both visible and invisible radiation. By using the red bulbs, you have made sure that no blue or green light (higher frequency) is radiated. However, you have not cut away the infrared radiation, which in fact delivers more power than the visible red light. So, the majority effect measured will be the absorption of differently colored water  in the *infrared* region. Because our eyes cannot see in the IR region, we have *no idea* regarding what dye should be more absorbing and which one is less. The good news is that if you re-formulate your project topic, any result will be a good result, as long as it is reproducible. No need to worry about the rankings of different colors, because there is no way of telling, what your result *should* be.

If you continuously (!) measure the temperatures of your solution, you might find that the temperatures are different during the experiment. The higher the temperature, the higher evaporation rate. Now, if repeated experiments exactly confirm the respective rates of evaporation you have from the first run, you conclude that the red dye *enhances* the total absorption of the IR radiation, while the green and blue perhaps have very high heat capacity (amount of energy stored per degree), so their temperature doesn't rise as high as for red and clear water, which explains the slower evaporation rate.

This is from me for now, I hope it helps a bit. I will be happy to discuss this project some more, if you wish.

Good luck and cheers,
Daniel