Key concepts
Chemistry
Molecules
Diffusion
Gradient
Solutions

Introduction
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!

Background
Many of the foods we eat are colored with artificial dyes. Some of these dyes will easily dissolve in water. This means that when foods containing the dyes are placed in water, the molecules that make up the dye will break away from the food and start to spread out in the water. The process where molecules spread out like this is called diffusion. The molecules tend to spread from areas where there are a lot of that type of molecule (a high concentration) to areas where there are fewer of that type of molecule (a lower concentration). Eventually the molecules all spread out enough that the concentration is equal everywhere. This is called equilibrium. The molecules, however, do not stop moving completely at that point. They will still bounce around randomly, and diffusion will still occur—but much more slowly.

Does all that sound confusing? Don't worry! You experience molecular diffusion every day with your sense of smell. Have you ever noticed how a smell will seem stronger when you are very close to the source or when the source of the smell just started out (for example, pulling a batch of fresh-baked cookies out of the oven)? This is because initially there is a higher concentration of odor-causing molecules in the air close to the source. Gradually these molecules will diffuse through the air into the rest of the room so you can detect the smell from farther away. In this test you will see diffusion in action instead of smelling it—by using liquids instead of gases.
 

Materials

  • Hard-shelled colored candies, such as M&Ms or Skittles
  • Small plate
  • Glass or measuring cup
  • Warm tap water
  • Dish towel or paper towels
  • Spoon
  • Sugar
  • Workstation than can tolerate spills and color dyes
  • Food coloring (optional)
     

Preparation

  • 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.
     

Procedure

  • 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. 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?
  • Extra: Watch the plate for about five to 10 minutes. Do the colors continue to diffuse at the same rate they did initially?
  • Extra: 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?
  • Extra: Try the test with cold water instead of warm water. Do the colors diffuse at a different rate?
  • Extra: 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?

 

Observations and results
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. Remember what you read in the background about gradients. 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 random motion of the molecules—but this process is much slower than the initial diffusion caused by the sugar gradient.

Cleanup
You probably don't want to eat mushy candy, so dispose of any remaining candy or water in the trash. Use a towel to wipe up any spilled water.

More to explore
Candy Chromatography: What Makes Those Colors?, from Science Buddies
Find the Hidden Colors of Autumn Leaves, from Scientific American
Chromatography: Be a Color Detective, from Scientific American
Astonishing Diffusion, from Science Made Simple
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies