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Lesson 4:

Mixtures and Separations

Concepts conveyed | Materials | Procedure | Benefits | Resource

Concepts conveyed:

The purpose of this classroom demonstration and activity is to explain what a mixture is and to show how dye mixtures can be separated into their components. The separation processes shown here are paper chromatography and the use of Sep-Pak mini-columns.

Materials:

(for one demonstration)

  • 2 to 6 small filter papers (one for each student)
  • 2 to 6 small strips of filter paper (about 0.5 cm x 2 cm)
  • 3 water soluble ink pens (preferably black, green, and red)
  • 1 Waters C18 Sep-Pak mini-column
  • water (enough for four elutions and for the paper chromatography)
  • methanol (enough for two elutions)
  • 1.5 mL of ordinary grape soda
  • 3 mL of water/acetone (a 88/12 volume/volume ratio is critical)
  • 2 to 6 Petri dishes with the bottom filled slightly with water (one for each student)
  • 1 overhead projector
  • 4 test tubes

Procedure:

Classroom Activity–Paper Chromatography

Allow each student to take a small filter paper, strip of filter paper, a petri dish with the bottom covered with water. Instruct them to poke a small hole in the center of the paper with a pen or pencil. Ask the students to take one of the ink pens provided for them and draw a small design around the hole in the center of the paper. Young children are often very creative about this. At the very least, draw a circle around the central pinhole with a radius that is at least 0.5 centimeter. (If the radius is too large, however, it will take a long time to see the separation effect). Make sure that the ink is allowed to dry. Then ask the students to role the filter strip lengthwise into a tiny tube or wad, which will serve as the wick, and insert one end of it into the small hole from the backside of the ink-designed filter paper so that it is snug. Place the other end of the wick into the water-filled petri dish so that the ink-designed filter paper rests upon the rim of the petri dish and so that it does not get wet (with the design facing upward, away from the petri dish). The water will start to move up the wick and wet the filter paper. The water will then move radially outward from the wick toward the edges of the filter paper. Observe what occurs when the water works its way past the ink and discuss this with the students. Note: What should occur is a separation of the different colors contained in each ink pen. Tell the students that this is called paper chromatography. Chromatography literally means “the writing of colors.” It involves the separation of the components in a mixture based on their relative affinity between the stationary phase (the filter paper) and the mobile phase (the water)

Classroom Demonstration–Separation of Dyes in Grape Soda

Prior to class, flush Sep-Pak mini-column with water, then methanol, and again with water. Then, in class, pass 1.5 mL of grape soda through the column and wash twice with water. Ideally, the clear solution that results should taste like the original grape soda. However, do not allow anyone to taste the solution because of the presence of methanol which is toxic and was used to pre-treat the columns. The students could smell the clear liquid which will have the aroma of grape soda. At this time, the dyes should be absorbed by the first part of the cartridge and not extractable with water. There may appear to be two dyes, red and blue, present at this time. Elute the red dye by injecting the 3 mL of water/acetone mixture into the column. Try not to contaminate the red dye with the remaining blue dye. This can be done by watching to make sure that the dye coming off the column is pure red and not purple. When this occurs, this small amount of mixed dyes should be collected in a separate test tube and discarded. Collect the rest of the eluent, which will be clear, in a test tube. Elute the blue dye with methanol and collect in a separate test tube. Project the colors of the solution on an overhead projector by pouring the red and blue dye into separate Petri dishes. After the activity and the demonstration, discuss with the students the results and additional methods of separation.

Benefits:

  • The instructor can use this activity as a means to check attendance. Students can be asked to hand in their filter papers with their signatures on it.
  • The instructor can return the filter papers to the students during the following class period to help them remember the concepts that were taught.
  • The demonstration may be used in conjunction with classroom lectures on either mixtures or separations or both.

Resource:

  • Durham, B., “Separation of Dyes in Grape Soda”, in Chemical Demonstrations for Grade Schools, Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, 1996.