Here is a fun project you can try with leftover candy you have. You will make some amazing art using nothing but colored candy and hot water—and a little bit of science. Save some of your candy and get started!
Time: 10 minutes
Key Concepts: Chemistry, molecules, diffusion, gradient
- Hard-shelled colored candies, such as M&Ms® or Skittles®
- Small plate
- Glass or measuring cup
- Warm tap water
- Dishtowel or paper towels
- Workstation than can tolerate spills and color dyes
- Food coloring (optional)
- Arrange pieces of candy in a circle around the inner rim of the plate. Use at least two different colors, alternating them in groups of two or three.
- Fill a measuring cup or glass with warm tap water.
- Slowly pour warm tap water into the middle of the plate, until it partially covers the candy (or fully, depending on how deep the plate is).
- Watch the plate closely for a few minutes. What happens?
- Empty and dry off the plate.
- Make a circle of candy around the plate again.
- Put a small pile of sugar (about a quarter teaspoon) directly in the middle of the plate.
- Slowly pour warm tap water near the center of the plate (but not directly onto the pile of sugar).
- Watch the plate closely for a few minutes. Question: What happens this time? Is it different from what happened the first time?
- Clean off the plate and repeat the test. Try different color patterns and/or arranging the candy (and sugar) in different shapes. What patterns and artwork can you make?
When you pour water onto the plate, the candy’s colored coating starts to dissolve in the water. As a result, you can see the colored dye diffuse towards the center of the plate. This diffusion can result in amazing, colorful rainbow patterns, as the colors remain mostly separate at first instead of bleeding together. You might have seen a “pie wedge” type pattern with different slices of each color you arranged around the rim.
When you put sugar in the middle of the plate something strange happens. The dye seems to hit an invisible wall in the water and stop diffusing at first, then diffuses much more slowly. This occurs because in addition to colored dye, the candy’s coating also contains sugar. Both the sugar and the dye dissolve into the water, forming a mixture called a solution. With your first test the sugar wants to spread out from where there is a high concentration of sugar (right next to the candy) to where there is a lower concentration of sugar (the middle of the plate). The whole solution (sugar and dye molecules) moves along this gradient from high concentration to low concentration. Although you can’t see the sugar from the candy coatings, you can see the colored dye. With your second test, however, there is already a lot of sugar in the middle of the plate—there is not a steep sugar gradient between the rim of the plate and the center. This prevents the sugar (and the dye) from spreading towards the center as rapidly. You might wonder “well, there are still no dye molecules in the middle of the plate, so shouldn’t the dye molecules keep diffusing, even if the sugar doesn’t?” That’s a good question! Different types of molecules diffuse at different rates in water. In this case the solution with the sugar molecules diffuses much faster than the dye molecules would on their own. This explains why, if you drop some food coloring into still water, it diffuses much more slowly than the dye from the candy. The food coloring does not contain any sugar.
If you watch the plate for a while, you might have noticed that the colors remain separated for quite some time. This can seem surprising—you might expect the colors to all blend together and turn a muddy brown. Remember, however, that it’s the sugar gradient that drives the diffusion. After the whole plate is filled up with colors, the sugar concentration is the same everywhere—there is no sugar gradient between the different colors. So the colors will continue to diffuse slowly due to the random motion of the molecules—but this process is much slower than the initial diffusion caused by the sugar gradient.
For Further Exploration
- Watch the plate for about five to 10 minutes. Do the colors continue to diffuse at the same rate they did initially?
- Leave the plate out all day and check on it periodically or let it sit overnight. How long does it take the colors to blend completely?
- Try the test with cold water instead of warm water. Do the colors diffuse at a different rate?
- Drop some food coloring into a plate of still water and watch how it diffuses. How is it similar to or different from the way the dye diffused from the candy? Can you figure out why?