Passage VII

Students were given 12 known compounds and 4 unidentified compounds (Unknowns A-D) to analyze.

Experiment 1

Students placed a solid sample of each known compound in a separate capillary tube (a thin glass tube). Samples were then heated, 4 at a time, in a melting point apparatus (see Figure 1).

The samples were monitored through the eyepiece. When liquid first appeared in a sample, the temperature was recorded. When no solid remained in the sample, the temperature was again recorded. The compounds were placed in Groups I-IV based on similarity in melting point (MP).

Experiment 2

For each compound in a group, students dissolved a sample and placed a small spot of the solution 2 cm from the bottom of a 13 cm tall and 7 cm wide thin-layer chromatography (TLC) plate. Each spot was circled with a pencil and allowed to dry. The plate was placed in a tank containing a small amount of a particular solvent. When the solvent had moved up to 1 cm from the top of the plate, the plate was removed and allowed to dry. Four distances were measured, in cm, under UV light: distance a for each spot and distance b (see Figure 2).

The R_f value of each compound was then calculated by dividing distance a by distance b. The results of Experiments 1 and 2 are shown in Table 1.

Experiment 3

The techniques from Experiments 1 and 2 were used to analyze Unknowns A-D (see Table 2).