Chromatography is used to separate mixtures of substances into their components based on small differences in solubility of different molecules. To begin the process, you must dissolve a substance (called the mobile phase) into a second substance (called the stationary phase). The mobile phase flows through the stationary phase and carries the components of the mixture with it, which usually travel at different rates. Molecules that are the most soluble will move the furthest and the least soluble will travel the shortest and are measured in retention time.

In paper chromatography, the stationary phase is the paper and the mobile phase is the water. If using the same color but with different brands, it is easy to see that the same color is not always made of the same mixture. A black from Brand X may contain more red compared to Brand Y which may have more green or blue.

Determining the components of a mixture are important because it allows scientists to know what is in a mixture and how to recreate it (such as in medicine if a new mixture is found in nature) or how to alter it (such as if a mixture is toxic to people, chromatography can help determine what is making it toxic).

Key Terms

Chromatography – process for separating components of a mixture

Mobile Phase – the substance that moves and is dissolved into the stationary phase

Stationary Phase – the substance in which the mobile phase is carried through; the substance that stays still through the process

Soluble – able to be dissolved. (Think of sugar in water)

Solute- substance being dissolved. (The sugar being dissolved in water)

Solvent- substance that is dissolving another substance. (The water that sugar is being dissolved in)

Retention Time – the amount of time it takes for a substance to travel a certain distance

Capillary Action – the ability for a liquid to flow in narrow spaces without the assistance, or even in opposition to, external forces like gravity. (Think of it like a soda straw that would draw up the soda because it was attracted to the sides of the straw)

Supplies

- Paper Towels OR Coffee Filters

- Cup

- Tape

- Scissors

- Water Based Markers

- Recommended the same color of 2 different brands

- Water

Step 1: Cut the Coffee Filters or Paper Towels Into Long Strips.

The best size for the strips is approximately 4 cm X 25 cm. This paper will act as the medium that the dissolved molecules of ink will have to travel through.

Step 2: Draw a Starting Line for Your Color Pigments.

Draw a line with a pencil about 1 ½ inches (4 cm) from the bottom across the filter/paper towel strip. To understand which molecules are more soluble we need to have a starting point so we can measure which molecules travel the farthest.

Step 3: Choose 2-3 Markers and Draw Small Concentrated Dots of Ink Onto the Pencil Line.

Make sure to space them out by at least 1 cm. Let dry for at least 30 seconds. It’s highly recommended to use the same color but with different brands.

Step 4: Fill the Bottom of the Cup With Water

Fill the bottom of the cup with enough water to submerge the bottom of the filter/paper towel by ½ inch (13 mm). The water will act as our solvent (dissolving agent) for our solute (material being dissolved).

Step 5: Place the Filter/paper Towel Into the Water.

Be sure NOT to submerge the ink into the water. Secure the filter/paper towel to the side with a tape

Step 6: Observe the Movement of the Water and Color Pigments

Watch the water go up the paper and pull the different components of the color upwards. For example, black ink is made up of many other colors combined to make it black. By performing the chromatography we are separating out each color that goes into making it look black. The upward movement of color pigments is due to the pigments traveling with the water. Capillary action pulls the water up due to attractive forces within each molecule of the paper. Plants transport water from the ground to their highest leaves with this same process of capillary action. Notice how some colors travel farther up the paper than others. By traveling farther this means that a certain color pigment was more soluble in the water and was carried farther.