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Why does ethanol only deflected slightly when a positively charged rod is placed next to a stream of it?
I assume you mean when compared with water.
Indeed, a stream of ethanol is deflected slightly less than a stream of water because ethanol has a smaller net dipole moment than water.
A net dipole moment is a measure of how , or for that matter, a molecule is. In other words, for a polar molecule, a net dipole moment expresses the magnitude of the difference between the partial positive part of the molecule and the partial negative part of the molecule.
The bigger the net dipole moment, the more polar a molecule will be. The more polar a molecule is, the more it'll be attracted to the charged particles in the rod.
Check out video of a stream of water being bent - a plastic ruler is used in the demo. The stream of water bends due to the polarity of water molecules. Video from: Noel Pauller
The partial negative part of the ethanol molecule, i.e. the oxygen from the ##"-OH"## group, will be attracted to the positively charged particles in the rod, causing the stream to be deflected.
However, because ethanol has a smaller dipole moment than water - approximately 1.68-1.69 D, compared with 1.85-1.87 D for water, this deflection will not be as significant as the deflection of a water stream.
This happens of course because the charge separation in the ethanol molecule is smaller than the charge separation in the water molecule ##->## the attraction to the positively charged particles in the rod will be weaker.