Skittle Chromatography

Age

Elementary School, Middle School, High School

Format

Hands-on

Materials

   Skittles (M&M’s work fine)
   TLC Plates
   Water (Vinegar or alcohol work as well)
   Small containers (Centrifuge tubes work fine)
   Coffee Stirrers or Capillary Tubes (Either one works well)
   Blow-dryer (if needed)
   Large containers
   Pencil
   4 beakers for different stages of chromatography 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Always wear safety goggles and a lab coat. Do not get prepared solution in the eyes and as good as the solution may smell, do not drink or eat it. Nothing in the laboratory should be eaten or drank unless specified by the coordinator.

Preparation

If you are doing this for a huge show, follow the following directions. Scale it down for a smaller show. Use your discretion.

Take a 3lb bag of Skittles and separate them by color. Put each color into its own large container Add enough water to cover the top of the Skittles or until desired concentration Let set for about 1hr. Any more than that and the Skittles themselves start to dissolve Make several elutions of each color in smaller containers Cut-out as many TLC plates as needed. Make sure that these plates are the right size for what you want to do.

This concludes the physical steps for preparation. Preparation for this demo takes about 2 hours and is a little messy.

Demonstration

Make a line about 1/2-1” from the bottom the TLC Plate with a pencil. Take a capillary tube or coffee stirrer and suck some of the color out of the centrifuge tubes and put one drop of each color on the TLC Plate equally spaced along the line. Take a beaker and put a small amount of water (or other solvent). Place the prepared TLC Plate into the beaker and allow the water to push the color up the plate. When the solvent is about 1/2” from the top, the chromatography is finished. You will see what base colors make up the 5 colors in the Skittles.

What to Say

Have each kid smell the tubes so that they know that the colors are actually from the skittles. Then have the kids try and explain what they think will happen. Then, after you have prepared the plate and allowed it to start separating, look at a plate that is half done and have the kids speculate on what they think is happening. Explain that the color is more charged than the plate and water holds onto the color tightly and therefore moves up the plate. This phenomenon is called polarity. Then, look at a finished plate, and have the kids speculate on what they see. Explain that the colors of the skittles are made up of many colors. Purple is made up on Blue and Red, Orange is its own color, Red is its own color, green is made up of blue and yellow, and yellow is its own color.

Why It Is

TLC is a simple, quick, and inexpensive procedure that gives the chemist a quick answer as to how many components are in a mixture. A TLC plate is a sheet of glass, metal, or plastic which is coated with a thin layer of a solid adsorbent (usually silica or alumina). A small amount of the mixture to be analyzed is spotted near the bottom of this plate.

The TLC plate is then placed in a shallow pool of a solvent in a developing chamber so that only the very bottom of the plate is in the liquid. This liquid, or the eluent, is the mobile phase, and it slowly rises up the TLC plate by capillary action.

As the solvent moves past the spot that was applied, an equilibrium is established for each component of the mixture between the molecules of that component which are adsorbed on the solid and the molecules which are in solution.

In principle, the components will differ in solubility and in the strength of their adsorption to the adsorbent and some components will be carried farther up the plate than others. When the solvent has reached the top of the plate, the plate is removed from the developing chamber, dried, and the separated components of the mixture are visualized. If the compounds are colored, visualization is straightforward.

The strength with which an organic compound binds to an adsorbent depends on the strength of the following types of interactions: ion-dipole, dipole-dipole, hydrogen bonding, dipole induced dipole, and van der Waals forces. With silica gel, the dominant interactive forces between the adsorbent and the materials to be separated are of the dipole-dipole type.

Highly polar molecules interact fairly strongly with the polar Si—O bonds of these adsorbents and will tend to stick or adsorb onto the fine particles of the adsorbent while weakly polar molecules are held less tightly.

Weakly polar molecules thus generally tend to move through the adsorbent more rapidly than the polar species. The stationary phase is a powdered adsorbent which is fixed to a aluminum, glass, or plastic plate. The mixture to be analyzed is loaded near the bottom of the plate. The plate is placed in a reservoir of solvent so that only the bottom of the plate is submerged.

This solvent is the mobile phase; it moves up the plate causing the components of the mixture to distribute between the adsorbent on the plate and the moving solvent, thus separating the components of the mixture so that the components are separated into separate "spots" appearing from the bottom to the top of the plate.

In Thin Layer chromatography, the stationary phase (the adsorbent: silica gel or alumina) is polar, and the polarities of both the component of the mixture and the solvent used as the mobile phase are the determining factors in how fast the compound travels.

TLC is usually used only to analyze mixtures: to determine the number of components and to see if a desired component is present.

Real Life Examples

One main real-life example of chromatography is to determine what colors make up various colors of ink using paper chromatography. Also, you can use chromatography to determine what compounds make up a mixture. Many biochemists and organic chemists use chromatography for this reason. Biochemists can use chromatography to analyze proteins and substrates and organic chemists use it to determine the composition of various compounds. TLC Chromatography has also been used to analyze pesticides. TLC can also be used in immunological approaches with antibody-antigen complexes. It can be used to identify lipophilic and hydrophilic compounds in animal parasites and the hosts they infect. Determining the structure of bacterial membranes has been an important innovation with the use of TLC and column chromatography.