Silica turning transparent during column chromatography with chloroform Initially, my column had been loaded with n-hexane and ethyl acetate, however I ended up having to use a more polar solvent to elute my product. It was my first time running chloroform through a column packed with silica and I noticed that as the chloroform made its way down the column, the silica turned from white to transparent. 

My initial hypothesis was that chloroform and silica must have the same refractive index. I looked up the values, and found them both to be ca. 1.45 (see references), so I am inclined to believe this explanation. 

However, if someone here could weigh in, and either confirm/deny this. (Or, alternatively, venture a new hypothesis). 

I am guessing no chemical transformation takes place when silica looses its opalescence, because if that were the case then the whole procedure becomes completely useless. However, I could be wrong.

References: 

i. http://refractiveindex.info/?shelf=main&amp;book=SiO2&amp;page=Malitson

ii. https://en.wikipedia.org/wiki/Chloroform_(data_page)
You are correct, sir!  (or madame)  The silicic acid becomes translucent (not transparent) in chloroform because their refractive indexes are similar.  This is a chance to check for air bubbles in the column, which reduces efficiency.  Also, any material still on your column will probably not have the same RI, and will be visible as a white or colored band, probably at the top of the column.