What is the function of ethanol in RNA extraction?

Answer 1

This wash step allows you to centrifuge the sample and collect a "clean" RNA pellet, after discarding the supernatant that contained contaminating salts and proteins.

When isolating and purifying RNA, 75% ethanol is used as a wash solution because RNA is a precipitate (solid) in this percentage of ethanol, while most proteins and salts remain in solution (are soluble). At a lower % ethanol, both the RNA and the proteins would be soluble, so you would not be able to separate them. At a higher % ethanol, both the RNA and salts would remain in the pellet, so you would not be able to separate the salts from your RNA.

Prior to the wash step, you probably added 100% ethanol to your sample, so the final total concentration of ethanol was 75%. This step is where the RNA precipitates out of solution. You would then centrifuge the sample and discard the supernatant, as above. In the wash step, you are merely using the same solution (75% ethanol) to wash the RNA pellet you created in the previous step.