Draw the molecular interactions associated with the macroscopic observation of water creeping up chromatography paper. What do I draw?

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caledon | High School Teacher | (Level 3) Senior Educator

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The absorption of liquids, typically water, is dependent upon the chemical structure of the object that it interacting with the water. We normally gauge the absorbance of solids, and some solids are much more absorbent than others.

What you might begin by drawing is, as the question states, a macroscopic view(macro = big scopic = view...so the "big picture" of paper absorbing water i.e. discoloring as the water moves upward). Then you might draw a "zoom" view that goes to the molecular scale, showing the way that the water molecules are interacting with the paper molecules.

Paper is typically composed of cellulose, lignin and a few other varieties of plant tissue, but unless you've been given specific molecules to work with, you can probably just summarize them by drawing glucose (it's a bit easier to draw).

There are two ways in which the paper actually absorbs the water molecules;

  1. Every object has some space between its atoms; the water molecules are able to fill in this space because they're much, much smaller, and they're also in liquid form, which means they aren't stuck in a particular shape or orientation relative to other water molecules, whereas the paper molecules are pretty much locked into their existing positions.
  2. Water molecules are, famously, polar. The difference in electronegativity between the hydrogen and oxygen molecules creates an imbalanced distribution of charge, meaning that parts of the water molecule are positively charged, and parts are negatively charged. This allows the water molecule to have electromagnetic interactions with other polar molecules, rather than just floating around inert. Parts of the glucose/cellulose molecules are polar as well, which provides locations for the water molecules to interact with them. Once the paper molecules are saturated with water molecules, the strength of their interactions with other paper molecules is diminished; this is why soggy paper tears easily.

So, if I was you, I would begin with the macroscopic view, and then zoom in to show the water and paper molecules, including their partial charges. The water molecules are attracted both to the empty space and to the polar charges on the paper, and so they "climb" via the interactions of the polar bonds with the paper and with each other, piling up one on top of another until they find a nice spot on a paper molecule to bond with. There's a nice diagram of this about 55 seconds into the video link I included below.

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