Help with Chem lab question?

Question

We did a lab where we used chromatography paper to separate the colors from different colored ink pens. Then we had to calculate the different Rf values for each of the colors we saw.

Ex. The color red separated into a neon orange, light orange, and pink. We calculated the Rf value for each of the 3 colors.

The question we were asked was: Why is it impossible for the Rf value to be greater than 1?

P.S. If you give me an answer with a mathematical argument could you provide an explanation to help me understand

Thanks.

Answer 1

Okay, check this out:  the Rf is a ratio, a fraction.  The distance the solvent moved is always put on the bottom.  Nothing can move faster than the solvent, since the solvent is what is carrying the components (the colors in your case) up the chromatography paper.  

A rough analogy: The solvent is a car carrying passengers; the colors are the passengers.  No passenger can move faster than the car it's in.  Make sense?

So that's the bottom number in the ratio.  The top number is how far the component moves.  Since nothing moves faster (or farther, therefore) than the solvent, this number will always be less than or equal to the bottom number.  This means that the decimal form of the ratio (arrived at by dividing the top number by the bottom number) will always be less than one.

If the solvent moves 2.4 cm and the orange spot moves 2.3 cm, the Rf of the orange spot is 2.3/2.4 = 0.96.  That's a very high Rf value, which goes to show that even if the component moves alllllmost as far as the solvent and has a really large Rf, the decimal is still less than one.  if the component moves the same distance as the solvent, then the ratio will be exactly one.  This happens sometimes, and can be fixed by changing th solvent.

So the ratio cannot be greater than one because no component of a mixture can move faster up chromatography paper than the solvent does.  Top number = smaller than the bottom number; ratio < 1.

Answer 2

Because Rf is a ratio of distances that the components move compared to the solvent front.  The spots cannot move faster than the solvent front, since it is the solvent that is moving it along the support!

It can move as FAST as the solvent (in theory), so that would be an Rf of 1, and of course it can go slower, Rf < 1, but not faster.