Preface: I am proposing a challenge based on this mystery. If you wish to know more about this challenge, please be sure to read the Challenge section found at the end of this blog post.
Congratulations to Grazyna Zreda and Alfredo Tifi who both solved Chemical Mystery #8. While neither Grazyna nor Alfredo figured out exactly how I pulled off this trick, they both determined that I was making use of the “salting out” phenomenon. In the “salting out” experiment, a water-soluble ionic salt is added to a mixture of water and a water-soluble organic liquid. If enough salt is added, the mixture separates into two layers: one rich in water, and the other rich in the organic liquid.1 You can see how this works (and also the solution to Mystery #8) in the video below:
In Mystery #8 I used acetone as the water soluble organic liquid and table salt as the ionic substance. I first mixed acetone, water, and two different dyes without adding any salt. The yellow dye was obtained from yellow food dye, while the blue dye was obtained from blue glitter. The dye on blue glitter dissolves very well in acetone, but not so well in water. The other yellow dye dissolves very well in water, but not so well in acetone.
Acetone and water dissolve well in one another due to hydrogen bonding interactions between the oxygen atom on acetone molecules and the O-H bond on water molecules (Figure 1).
Figure 1: Representation of a hydrogen bond (yellow dashed line) formed between a molecule of acetone (lower molecule) and a molecule of water (upper molecule). Image made using Odyssey modeling software.
All four of these components mixed very well together (acetone, water, blue dye, yellow dye) to form a green colored solution results. When a lot of table salt was added, the green solution separated into two layers: a blue colored, acetone rich layer on top and a yellow-colored, salt-water rich layer on bottom. How did this occur?
When the salt dissolved into the mixture, the resulting Na+ and Cl- ions interacted very strongly with water molecules through ion-dipole forces (Figure 2). These ion-dipole interactions attracted water molecules much more strongly than the acetone-water hydrogen bonds. As a result, the ion-dipole forces pulled water molecules away from acetone molecules and the liquids separated into the two separate phases. The yellow dye, which dissolves better in water than in acetone, ended up in the salt water layer. The blue dye, which dissolves better in acetone, ended up in the acetone layer.
Figure 2: Representation of a chloride ion (green) interacting with six water molecules through ion-dipole forces (yellow dashed line). Image made with Odyssey modeling software.
What is interesting about this project is that one can use many different combinations of dyes, organic liquids, and salts to achieve different effects. For example, Graznya Zreda “solved” this mystery by reporting that she mixed yellow food dye, water, blue food dye (in place of the blue dye found on glitter), isopropyl alcohol (in place of acetone), and potassium carbonate (in place of salt). Upon mixing these items a beautiful green solution was observed; adding potassium carbonate separated the mixture into the blue and green layers (Figure 3).
Figure 3: Experiment carried out by one of Grazyna Zreda's students. Left to right: Test tubes containing yellow food dye in water and blue food dye in 70% isopropyl alcohol; Mixing the yellow and blue fluids to form a green solution; Addition of potassium carbonate to form a green solution; separation into blue and yellow layers upon dissolution of potassium carbonate.
Grazyna and I began communicating on Twitter about these experiments, and one afternoon we even spent an hour or two “together”, electronically messaging back and forth about various experiments we were trying. This was really a lot of fun! Throughout our combined efforts, we discovered some really cool things. First, green food dye alone (in place of both blue and yellow) can be used in Grazyna’s version of this experiment! That’s because green food dye contains a combination of blue and yellow food dye. Second, using different blends of organic liquid and ionic salts with blue food dye and purple “fall color” food dye resulted in completely different results (Figure 4).
Figure 4: Color combinations achieved using blue food dye, purple “fall color” food dye in conjunction with (Left) acetone, salt, and water; (right) isopropyl alcohol, potassium carbonate, and water.
Challenge: Finally, here is a challenge I am proposing for you and your students based on this experiment: See if you can create layers that display your school colors by modifying this experiment using different combinations of ionic salts, dyes, miscible organic liquids and water. If you achieve this, see what other color arrangements you can create. I'm particularly interested in seeing a purple/green combination! I would love to hear from you regarding different combinations you are able to create. Of course I would also be interested in hearing about your various successful recipes if you are willing to share them! Be creative…dyes can come from a surprising number of different sources…like glitter, of all things!
Reference: Shakhashiri, Chemical Demonstrations Volume 3, p. 266 – 268.