why viscosity of ethanol decreases after 40%ethanol?

Answer 1

this question may appear ambiguous to the inexperienced. the question asks for an explanation for the decrease, which implies the asker's initial expectation for an increase. the questioner assumed that viscosity increased with ethanol concentration, but facts led to the conclusion that this trend is only obeyed up to the 40% conc level, after which viscosity decreases with ethanol concentration.

why the bell curve? that is what you are supposed to answer, Radagast.

40% ethanol (w/w in water, which is the obvious solvent) happens to be the point at which there is a certain ratio of ethanol to water molecules. this special ratio is apparently the point at which there are maximum interactions (H-bonding) between ethanol and water such that the fluid becomes more viscous than water or ethanol alone.

by increasing ethanol conc from 0-40% you will thus see an increase in viscosity. after 40%, the ratio of water molecules apparently increases, so the fluid viscosity will tend toward the viscosity of ethanol (which is still higher than pure water).

using arbitrary values, viscosity starts at 1 for pure water, increases to 10 for 40% ethanol, then decreases to 2 for 100% ethanol.

Answer 2

Your question is poorly formed. Do you mean after it increases above 40% or after it drops below 40%. And I presume you mean in water.