How can you find out whether the dye in spinach is a pure substance?

Answer

Hi Nick,

A substance is “pure” when all the molecules that make up it are identical to each other. That is of course a theoretical definition, which is not feasible in practice.

In the case of spinach, I can assure you that the green dye is not a pure substance, but a mixture of various chlorophylls and carotenoids. This can be investigated with a technique called chromatography. When you finely rub some spinach leaves (or, if necessary, a cube of frozen spinach) together with an organic solvent, such as acetone, the dyes dissolve in the acetone. A little of that solution is now placed on the top of the column (this is a glass tube, closed at the bottom with a tap, and filled with an adsorbent). The different molecules in the mixture will now have a different affinity for the adsorbent (the column fill) and when we flush the column with more solvent, different molecules will move through the column at different speeds. You can compare this with an obstacle course. Thus, different molecules will leave the column at different times.

In this way a complex mixture can be separated into its pure components.