The Item:

Please answer the following questions by yourself.

1. Were the black pen lines on strips A, B, C, D and E made by the same pens? Using your results, explain why you think the lines were or were not made by the same pens.

2. Suppose that one paper strip was marked with two of the inks used in this experiment, and the two ink lines on the strip were drawn right on top of each other. Design and discuss an experiment that could be used to identify these two inks.

Item Description:
This event is designed to illustrate how ink can be separated into its component chemicals using paper chromatography. Five unknown samples of ink on pre-labeled chromatography strips were provided to students. The students use these strips and water as a solvent to separate each ink into its component colors by paper chromatography.

Question 1 assesses whether the student can correctly compare the chromatography strips to determine whether any of the five inks are from the same pen, and explain their answer in terms of their experimental results (i.e., the colors obtained on the chromatography strips.)

A good response correctly interprets the results from the chromatography event to correspond with the labels of strips which show identical patterns and explains that none of the inks are identical. The response needs to discuss that the component chemical of identical inks are the same color and have the same mobilities.

Question 2 assesses whether the student can apply their knowledge of paper chromatography to design an experiment which will identify an unknown mixture of inks.

A good response uses the principles of paper chromatography in their experimental design to separate the components of the two unknown inks (using same conditions as previous described). The response should indicate that once the component colors of the unknown mixture are determined, this strip can be compared with previously run samples.

Summary:

Criterion 1:
Response indicates correctly which of the five inks are identical
OR if none are identical correctly indicates this.

Criterion 2:
Response shows that identical inks will separate into the same colors.

Criterion 3:
Response indicates understanding that each chemical (or ink component) has a characteristic mobility on the paper strip OR that the distance moved or rate of movement can be used to identify ink components.

Criterion 4:
Response indicates that identification of a mixture of unknown inks can be made by chromatographing these inks under the same conditions as the previous 5 inks and making comparisons.

samples of the 5 inks can be mixed and each run in parallel with the unknown.