Solution to Chemical Mystery #10: Out of the Blue!

Color-changing straw

In Chemical Mystery #10, plastic straws are observed to “magically” change color when waved in the air. Check it out in the video below:

This trick makes use of straws that contain thermochromic dyes,1 which are dyes that display different colors at different temperatures.2,3 The dyes in the straws happen to be colored (either red or blue) at cold temperatures but colorless at warm temperatures. It is obvious then, that the straws are colored when cool but colorless when warm.

But how is it that the straws only display color when they encounter one of two liquids and are subsequently waved in the air? Given the thermochromic nature of the dyes, one might guess that the color change somehow results because one of the liquids is cool, while the other is warm. However, both liquids are at room temperature! The difference results because one of the liquids is acetone, while the other is water. If you would like to see how I set up this experiment, check out the video below.

Room temperature is high enough to cause the thermochromic dyes in the straws to be colorless. Thus, straws placed in room temperature acetone or water will have no color. However, when a straw is removed from a liquid and waved in the air, residual liquid clinging to the straw evaporates off the surface of the straw. The energy to drive the evaporation (remember, evaporation is an endothermic process) come from the straw. Therefore, the straw loses energy – and drops in temperature – as the liquid evaporates. If the temperature drops enough, the thermochromic dye in the straw displays its color.

The color change only occurs in straws that have come into contact with acetone. Liquid acetone evaporates very quickly, due to the relatively weak intermolecular forces that exist between acetone molecules. Because of this, a straw dipped in acetone and waved in air will drop in temperature very quickly as acetone rapidly evaporates off its surface. The temperature drop is rapid enough to allow a color change to be observed in the dyes in the straw. Water molecules, on the other hand, have very strong intermolecular forces. As a result, a straw dipped in water and waved in the air will drop in temperature slowly: so slowly that the temperature of the straw does not drop quickly enough to register a color change.

This experiment allows for a discussion of the difference between thermodynamic and kinetic explanations. This experiment makes no sense if one compares the enthalpies of vaporization of acetone (31 kJ mol-1 ) and water (44 kJ mol-1).4 It actually takes MORE energy for water molecules to evaporate off of the surface of the straw than it does for molecules of acetone. Since this is the case, the greater temperature drop observed in the straws dipped in acetone must result from much more rapid evaporation of acetone molecules (as compared to water molecules) from the straw surface.

Be sure to drop me a line in the comments if you use this experiment in your classroom. If you do so, use caution when waving around the straw dipped in acetone, because droplets of acetone are often flicked off the straw in many directions. Thus, you should stand a safe distance from observers when conducting this demonstration.

Let me know if you fool your students!   

References  

  1. http://pubs.acs.org/doi/pdf/10.1021/ed400578q
  2. http://pubs.acs.org/doi/pdf/10.1021/ed076p1201
  3. http://pubs.acs.org/doi/pdf/10.1021/ed100831p
  4. https://en.wikipedia.org/wiki/File:Heat_of_Vaporization_(Benzene%2BAcetone%2BMethanol%2BWater).png The values cited above were taken at 300 K.