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MCAT

Unit 3: Lesson 1

Chemical and physical sciences practice passage questions

Purification of caffeine

Problem

Caffeine is most valued for its ability to temporarily ward off drowsiness and restore alertness. It belongs to a large class of compounds known as alkaloids, compounds that contain mostly basic nitrogen atoms. These are of plant origin and can be extracted from tea bags, for example, through a series of purification steps.
Figure 1. Caffeine Molecule
start underline, start text, E, x, t, r, a, c, t, i, o, n, space, P, u, r, i, f, i, c, a, t, i, o, n, end text, end underline
After brewing tea bags in water, the liquid extract is filtered to remove any insoluble material. The remaining supernatant can then be separated through liquid/liquid extraction. The two solvents used in this extraction step are water and dichloromethane.
The two solutions then separate into two layers; a brown aqueous solution and an organic layer with dichloromethane. Evaporation of the organic solvent yields a caffeine compound with impurities such as green chlorophyll.
start underline, start text, S, u, b, l, i, m, a, t, i, o, n, space, P, u, r, i, f, i, c, a, t, i, o, n, end text, end underline
Further purification of caffeine is possible through sublimation if impurities are nonvolatile. Crude material is placed on the bottom of a sublimation chamber below a cool surface test tube. Impure caffeine extract is then heated to sublimation. A vacuum is applied to the airtight environment by sucking all of the air out of the chamber through the vacuum/gas line as shown in Figure 2.
left parenthesis, 1, right parenthesis Cooling water goes in test tube.
left parenthesis, 2, right parenthesis Cooling water goes out of test tube, generating a cold surface.
left parenthesis, 3, right parenthesis Vacuum/gas line creates a near zero pressure environment. It constantly removes gas it is exposed to from the system.
left parenthesis, 4, right parenthesis Crude caffeine material is placed inside the base of the sublimation apparatus.
left parenthesis, 5, right parenthesis When heated, sublimed caffeine gas rises from the base of the apparatus, forming solid crystals when it makes contact with the cold surface test tube.
left parenthesis, 6, right parenthesis Solid non-volatile impurities are left at the base of the apparatus after sublimation.
left parenthesis, 7, right parenthesis External heating drives the sublimation process. The gaseous caffeine will be separated from the less volatile impurities and then forms crystal caffeine deposits along the cool surface. The caffeine deposits are then collected for the next step in processing.
Figure 2. Sublimation apparatus
This vacuum is key to the process of sublimation. For a substance to change phase into a gas, its molecules must reach a vapor pressure equal to that of its external air pressure. Because a vacuum lowers the air pressure, the caffeine can reach this vapor pressure at a lower temperature and bypass the liquid phase.
Figure 3. Caffeine Phase diagram with the arrow indicating the path of the caffeine sublimation.
How would the caffeine sublimation phase diagram be affected if the vacuum in the apparatus allowed more air pressure in the chamber?
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