Advanced Placement (AP) Chemistry Practice Exam

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What characteristic of a solvent affects the retention factor in chromatography?

Viscosity of the solvent
Temperature of the solvent
Polarity of the solvent
Density of the solvent

The correct answer is: Polarity of the solvent

The retention factor in chromatography is significantly influenced by the polarity of the solvent used in the process. In chromatography, the separation of compounds occurs based on their interactions with the stationary phase and the mobile phase, which in this case is the solvent. When the solvent's polarity matches the polarity of the compounds being separated, the compounds will have a higher affinity for the mobile phase and will therefore travel further up the chromatographic medium, resulting in a higher retention factor. Conversely, if the solvent is less polar than the compounds, the compounds will interact more with the stationary phase, leading to a lower retention factor. This aspect of polarity is crucial because it determines how much time a compound spends in the mobile phase versus the stationary phase, ultimately influencing how effectively different substances can be separated based on their chemical nature. Thus, the choice highlighting polarity as a defining characteristic directly correlates to the behavior of substances during chromatography.