Advanced Placement (AP) Chemistry Practice Exam

What does paper chromatography primarily separate?

1. Gases based on their molecular weight

2. Mixtures based on solubility differences

3. Liquids based on density

4. Solids based on melting points

The correct answer is: Mixtures based on solubility differences

Paper chromatography primarily separates mixtures based on solubility differences. This technique utilizes a stationary phase, usually a sheet of paper, and a mobile phase, which is a solvent that travels up the paper by capillary action. Different components of the mixture will dissolve to varying extents in the solvent, allowing them to migrate at different rates along the paper. Those that are more soluble will travel further, while less soluble components will not move as far. This differential movement is what leads to the separation of the mixture into its individual constituents, making solubility the key factor in the process. The other options do not accurately describe the purpose of paper chromatography; gases are not separated based on molecular weight, liquids are not distinguished by density, and solids are not sorted by melting points in this method. This specificity is what makes paper chromatography a valuable technique in analytical chemistry, particularly for assessing complex mixtures.