Chromatography Flashcards – AQA GCSE Combined Science: Trilogy (8464) — Chemistry — Higher

How is a substance’s $R_f$ value calculated from a chromatogram?

$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$

Measure the substance’s distance from the origin to the centre of its spot.

How can $R_f$ values help identify a compound?

Compare its measured $R_f$ value with known values for the same solvent.

How can changing the solvent affect a compound’s $R_f$ value?

The same compound can have a different $R_f$ value in a different solvent.

How do the spot patterns of a pure substance and a mixture differ in a solvent system?

A pure substance produces one spot; a mixture or impure substance produces more than one.

How should measurements and final answers used to determine $R_f$ be reported?

To an appropriate number of significant figures.

What is paper chromatography used for?

Separating the substances in a mixture for further analysis.

What are the stationary and mobile phases in paper chromatography?

The paper is the stationary phase; the solvent is the mobile phase.

Where is the sample placed in paper chromatography, and in which direction does the solvent move?

The sample is placed near the bottom of the paper, and the solvent moves up through it.

Why do substances in a sample move at different speeds during paper chromatography?

They distribute differently between the mobile and stationary phases.

How does a substance’s attraction to the solvent versus the paper affect how far it moves?

A substance carried more strongly by the solvent moves further; one held more strongly by the paper moves less far.

How do the components of a mixture appear after separating in paper chromatography?

They form distinct spots at different distances.