Chromatography and Rf Values Flashcards

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What is chromatography used for in chemistry?

Chromatography is used to separate mixtures and help identify the substances in them.

In paper chromatography, what is the stationary phase?

The stationary phase is the chromatography paper. The coloured chemicals stick to this paper.

In paper chromatography, what is the mobile phase?

The mobile phase is the solvent, such as water or ethanol. It moves up the paper.

Why do you draw the start line in pencil, not pen, in paper chromatography?

Pencil does not dissolve in the solvent. Pen ink would move and confuse the results.

Why must the spots of mixture be above the solvent level at the start?

If the spots are under the solvent, they will wash off. They must start above it to travel up.

What is the solvent front in paper chromatography?

The solvent front is the top line reached by the solvent on the paper.

How can chromatography show if a coloured substance is pure?

A pure substance gives one spot on the paper. A mixture gives two or more spots.

Briefly describe how to set up a paper chromatography experiment.

Draw a pencil line and add small spots of the mixture. Put the paper in a beaker with solvent below the line and let the solvent soak up the paper.

What does Rf value mean in chromatography?

Rf (retention factor) is a number that shows how far a spot moves compared to the solvent.

How do you calculate the Rf value of a spot?

Rf = distance moved by the spot ÷ distance moved by the solvent front, both measured from the start line.

How can you use Rf values to help identify an unknown substance?

Run the unknown and known samples in the same solvent. If they have the same Rf value, they may be the same substance.

What is the range of possible Rf values, and what do high or low values show?

Rf values are between 0 and 1. A low value means the spot stayed near the start. A high value means it moved close to the solvent front.