Photosynthesis is the process in which plants convert light energy from the sun to chemical food energy. To absorb the light, leaves use brightly colored pigments with chlorophyll being the most important one.
There are two main types of chlorophyll: chlorophyll A which is bluish-green, and chlorophyll B, which is yellowish-green. During most of the growing season, leaves contain more chlorophyll than any other pigment, making them appear green.
In the fall, chlorophyll begins to break down, and the other pigments, which have been there all along, are finally revealed. Yellow leaves have pigments called xanthophyll, orange leaves have a pigment called carotenoids. Anthocyanins give leaves their intense red and purple pigments.
These pigments, however, aren’t present in the leaves during the summer and are only made toward the end of summer.
Chromatography is the separation of a dissolved mixture by passing a through filter paper through which different parts of the mixture will move at different rates. The pigments that were more soluble in the solvent (alcohol) moved further up the paper than the less soluble pigments. Using the green leaf as an example, the blueish-green chlorophyll A was less soluble than the yellowish green chlorophyll B and thus it didn’t move as far up the paper.