The Density Bottle experiment has inspired many colorful variations over the years.1-6 I recently came across yet another colorful variation that I’d like to discuss. Check it out in the video below (Video 1)
Video 1: Bi-Color Science Experiment, Tommy Technetium YouTube channel.
In this demonstration, neon purple food dye, water, acetone, and salt are mixed. The mixture separates into a two-layer system with a pink acetone layer on top and a blue saltwater layer on the bottom. This phenomenon, known as the “salting out” effect, commonly happens when salt is added to a mixture of water and an organic solvent, causing the layers to separate.
The pink-blue color separation observed here has been reported previously,2 but I would like to add some details on how the color separation occurs. The neon purple food dye used in the experiment (Video 1) contains a mixture of Blue Dye #1 (Figure 1, left) and Red Dye #3 (Figure 1, right).
Figure 1: Chemical structures of (left) Blue Dye #1 (MW = 792 g mol-1) and (right) Red Dye #3 (MW = 880 g mol-1).
During the salting out process, the layer into which dyes dissolve depends on several factors, with the most important being the size of the dye.7-9 This is because for a dye molecule to dissolve, it must “make room” for itself by “pushing apart” water molecules. This process is endothermic because there is an energy cost for separating the hydrogen bonding between water molecules. Since larger molecules must separate more water molecules to dissolve, this energy cost increases with molecular size. However, this energy cost can be offset by the release of energy through hydrogen bonding interactions between the water and the dye molecule.
Now we are ready to understand how the blue dye ends up in the water layer, but the red dye does not. First, the blue dye (MW = 792 g mol-1) is 10% smaller than the red dye (MW = 880 g mol-1), so the energy cost for dissolving it in water is lower than that of the red dye. Second, the blue dye contains over twice as many atoms (11) capable of accepting hydrogen bonds from water, as compared to the red dye (5) (to see this, count the number of nitrogen and oxygen atoms on each molecule). So the blue dye can release more energy through hydrogen bonding than the red dye when it dissolves in the water layer.
It must be stressed again that the dissolution of dyes into various layers is complex and depends on a wide variety of factors, not just molecular size and hydrogen bonding capabilities.7-9 Nevertheless, the color separations observed when various dyes are included in salting out experiments can often be described using these two characteristics.6
One final note: beginning January 2027, the United States will no longer allow Red Dye #3 in food products.10 Thus, if you want to try this experiment in your classes, you might want to stock up on purple food dye that contains Red Dye #3. I have observed that a mixture of Blue Dye #1 and the more common Red Dye #40 does result in a pink-red color separation, but the color separation is not as sharp.
As always, if you try this experiment or any variations, I’d love to hear your results.
Happy Experimenting!
References
- Kuntzleman, T., Chemical Mystery #8: Go Blue! November 2016.
- Kuntzleman, T., Solution to Chemical Mystery #8: Go Blue! November 2016.
https://www.chemedx.org/blog/solution-chemical-mystery-8-go-blue
- Kuntzleman, T., The Density Bottle Strikes Again, December 2017.
- Webb, J. and Rothenberger, O., The Salting-Out Effect: A Colorful Demonstration That Leads to Student - Teacher Activities, April 2023.
- Fleming, D., Battle of the Forces(link is external), March 2023.
- Kuntzleman, T. Exploring Color Separation in Salting-Out Experiments May 2024.
https://www.chemedx.org/blog/exploring-color-separation-salting-out-expe...
- Endo, S.; Pfennigsdorff, A.; Goss, K.-U. Salting-Out Effect in Aqueous NaCl Solutions: Trends with Size and Polarity of Solute Molecules. Environ. Sci. Technol. 2012, 46, 1496-1503.
- Hyde, A. M.; Zultanski, S. L.; Waldman, J. H.; Zhong, Y.-L.; Shelvin, M.; Peng, F. General Principles and Strategies for Salting-Out Informed by the Hofmeister Series. Org. Proc. Res. Dev. 2017, 21, 1355-1370.
- De Stefano, C.; Lando, G.; Malegori, C. Oliveri, P. Sammartano, S. Prediction of water solubility and Setschenow coefficients by tree-based regression strategies. J. Molec. Liq. Dev. 2019, 282, 401-406.
- https://cen.acs.org/food/food-ingredients/FDA-bans-red3-food-drugs/103/w...
Safety
Safety: Video Demonstration
Safety: Video Demonstration
Demonstration videos presented here are not meant as tools to teach chemical demonstration techniques. They are meant as a tool for classroom use. The demonstrations may present safety hazards or show phenomena that are difficult for an entire class to observe in a live demonstration.
Those performing the demonstrations shown in this video have been trained and adhere to best safety practices.
Anyone thinking about performing a chemistry demonstration should first read and then adhere to the ACS Safety Guidelines for Chemical Demonstrations (2016) These guidelines are also available at ChemEd X.
General Safety
General Safety
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies
NGSS
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.
Matter and its Interactions help students formulate an answer to the question, “How can one explain the structure, properties, and interactions of matter?” The PS1 Disciplinary Core Idea from the NRC Framework is broken down into three subideas: the structure and properties of matter, chemical reactions, and nuclear processes. Students are expected to develop understanding of the substructure of atoms and to provide more mechanistic explanations of the properties of substances. Chemical reactions, including rates of reactions and energy changes, can be understood by students at this level in terms of the collisions of molecules and the rearrangements of atoms. Students are able to use the periodic table as a tool to explain and predict the properties of elements. Using this expanded knowledge of chemical reactions, students are able to explain important biological and geophysical phenomena. Phenomena involving nuclei are also important to understand, as they explain the formation and abundance of the elements, radioactivity, the release of energy from the sun and other stars, and the generation of nuclear power. Students are also able to apply an understanding of the process of optimization in engineering design to chemical reaction systems. The crosscutting concepts of patterns, energy and matter, and stability and change are called out as organizing concepts for these disciplinary core ideas. In the PS1 performance expectations, students are expected to demonstrate proficiency in developing and using models, planning and conducting investigations, using mathematical thinking, and constructing explanations and designing solutions; and to use these practices to demonstrate understanding of the core ideas.
*More information about this category of NGSS can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions.
"Matter and its Interactions help students formulate an answer to the question, “How can one explain the structure, properties, and interactions of matter?” The PS1 Disciplinary Core Idea from the NRC Framework is broken down into three subideas: the structure and properties of matter, chemical reactions, and nuclear processes. Students are expected to develop understanding of the substructure of atoms and to provide more mechanistic explanations of the properties of substances. Chemical reactions, including rates of reactions and energy changes, can be understood by students at this level in terms of the collisions of molecules and the rearrangements of atoms. Students are able to use the periodic table as a tool to explain and predict the properties of elements. Using this expanded knowledge of chemical reactions, students are able to explain important biological and geophysical phenomena. Phenomena involving nuclei are also important to understand, as they explain the formation and abundance of the elements, radioactivity, the release of energy from the sun and other stars, and the generation of nuclear power. Students are also able to apply an understanding of the process of optimization in engineering design to chemical reaction systems. The crosscutting concepts of patterns, energy and matter, and stability and change are called out as organizing concepts for these disciplinary core ideas. In the PS1 performance expectations, students are expected to demonstrate proficiency in developing and using models, planning and conducting investigations, using mathematical thinking, and constructing explanations and designing solutions; and to use these practices to demonstrate understanding of the core ideas."