How Does the Rainbow Candy Experiment Work?

Have you seen the rainbow candy experiment? It's a very simple experiment that involves pouring water into a plate that has M&M's candies or Skittles arranged in a pattern. Very curious shapes of sharply divided regions form spontaneously. How does this happen?! 

Arrange some M&M’s candies (Skittles also work well) on a Styrofoam plate and pour in some water. A beautiful pattern results (Video 1)!

Video 1: Colorful Candy Experiment in Hot and Cold Water, Tommy Technicium YouTube Channel, accessed 2/7/2021

This fascinating experiment was first described by Elizabeth Sumner Wafler (using Gobstoppers instead of M&M’s) in 2001.¹ One of the most striking features of this experiment is the sharp lines of division that form, separating each color into its own discrete region. It was first suggested that wax coating on the candies inhibited the spread of colors.¹ I later did some experiments that called this this hypothesis into question. I repeated the experiment using candies that contained no wax, and yet saw the same non-mixing effect.² Later experiments indicated that the dyes on the candies were negatively charged, which led to the proposal that negative charge repulsion could cause each color of dye to remain in its own region.³ However, candies completely rinsed of dye are also observed to cause the non-mixing effect (Video 2), indicating that something other than the dyes in the candies contribute to the sharp divisions observed.

Video 2: Colorful Candy Chemistry, Tommy Technicium YouTube Channel, accessed 2/7/2021

Many discussions regarding this experiment ensued on ChemEdX, and varied explanations for the formation of distinct regions of color were offered.⁴ Ultimately, water stratification was landed upon as the generally agreed upon mechanism for the effect. During water stratification, regions of water with different densities form layers, with each layer acting as a barrier to mixing. The water layers become arranged according to density, with the more dense layers lying beneath the less dense layers.

I have been a bit skeptical of the water stratification hypothesis. This is probably because this experiment is carried out in very shallow water on a flat plate, and it is very difficult to observe different layers form in this set up. As a result, it was difficult for me to envision a mechanism whereby different layers with different densities could spontaneously form. Further, I couldn’t visualize how such layers might prevent mixing by various regions of color. I recently struck up a conversation about this experiment with my friend Jerry Bell from the Wisconsin Initiative for Science Literacy. Our conversations inspired me to try out several experiments on this colorful system. In the end, I became convinced that the water stratification hypothesis is correct. Video 3 illustrates some of the experiments I tried.

Video 3: Colorful Candy Designs, Tommy Technicium YouTube Channel, accessed 2/7/2021

I think it would be helpful to share some of Jerry’s commentary on Video 3 that he shared with me after watching it:

“The spreading of the color from the candy in water is mass flow driven by gravity as the dense dissolving sugar coating solution falls to the bottom of the container (obvious in the aquarium, but also true in the saucer). When it hits bottom it has to change direction from vertical to horizontal and spreads. It continues to spread at a rate that is enormously faster than diffusion as long as there is sugar dissolving to continue the driving force. When two of these layers of moving sugar solution meet, they stop moving, because they are essentially iso-dense from the different candies. The sugar cube experiment demonstrates that a solution that is more dense, because sugar dissolves much faster, will actually push a less dense layer back. Once stopped, the only way that mixing can occur is by diffusion, which is much slower than most of us realize, because we don’t really meet diffusion in our everyday life.”⁵

Sometimes, dazzling artistic interfaces form between various regions of color when carrying out this experiment in an aquarium. I often observe a separation of color reminiscent of the interface that results when a more dense cold front of air sinks beneath a less dense warm air front (Figure 1). There are some aspects of this experiment that I’d still like to explore. For example, I tend to notice that the dye from the red M&M’s tend to dissolve into the water more quickly than the other colors. What’s going on with that? Do all the colors mix at different rates? As always, please do let me know if you observe anything new if you investigate this interesting phenomenon.  

Happy experimenting!

Acknowledgement: I wish to thank Jerry Bell for patiently sharing his knowledge with me, eventually helping me understand this beautiful experiment a little bit better. I also thank the members of the Wisconsin Initiative for Science Literacy for helpful discussion.

References:

1. Wafler, E.S. Inspired Inquiry, Science and Children, 2001.

2. Kuntzleman, T. S., Fun with M&M’s, Chemical Education Xchange, 2013.

3. Kuntzleman, T. S., Solution to the M&M Mystery?, Chemical Education Xchange, 2014.

4. Barth, B.; Farmer, W.; Jacobsen, E. C.; Kuntzleman, T. S.; Langella, M. M.; Maw, M.; Tifi, A.; VonBokel, J. Comments section associated with references 2 and 3.

5. Bell, J. Personal correspondence with the author, January, 2021.