The green-leaf extract solution discolors as soon as it touches chalk powder, but the chalk becomes green. Molecules of all the components that make up chlorophyll are extracted from the leaf by precipitating on the surface of chalk’s particles. In general, uptaking of dissolved substances, vapors, or gases by the surface of solids or liquids is called adsorption by chemists, and the material that uptakes – adsorbent. All [tag]chromatographic[/tag] methods that separate mixtures, ultrafine purify compounds as well as analyze them are based on this phenomenon.

Captured by the surface of adsorbent’s solid body, molecules can transfer back into solution – eluent, and then again get adsorbed and so on, changing its state infinite number of times. Between the solvent (benzene in Tsvet’s experiments) and the adsorbent (chalk powder) there is an equilibrium: almost all of the molecules are on the surface of the chalk particles, but in the solvent there is almost none. But this “almost” is the main idea behind the chromatographic effect.

A few molecules present in the solution are carried down the tube with the flow of a solvent. However, on the way, they again slowly precipitate on other chalk particles, but instead of them the new molecules get transferred into the solution. The solvent uninterruptedly flows from the top of the tube. The amount of absorbed substance in the upper part of the tube is decreasing, whereas at the bottom, it is increasing more and more. Gradually, a colored layer in the circular form moves through the adsorbent down the tube.

Molecules with different composition or structure precipitate (adsorb) on to solid surfaces differently. Some bind a bit more than the others, but some stay longer in the bound state and less in the solution; whereas, others slightly longer linger in the solution and get faster carried away by the solvent’s flow.

That’s why the colorful mixture of various substances gradually gets separated into individual components. And each component is concentrated in its own layer. Moving with the different speed along the column, these layers separate from each other and form a chromatogram. Each individual color band corresponds to some single chemical compound.