MadSci Network: Chemistry

Re: what processes propell alcohol boats?

Date: Fri Nov 17 21:27:45 2000
Posted By: Sojo Luis E., Staff, Reseach and Development, Axelson Biopharma Research
Area of science: Chemistry
ID: 970458631.Ch


I believe that in part you have the answer to your question.  The mixing 
of water and alcohol results in a temperature increase that in some cases, 
depending on the mixing proportions, can result in up to few degrees.  
This change in temperature is a direct result of the dissolution process, 
with a reorganization of the intermolecular hydrogen bonds between water 
and alcohol molecules from their state before mixing.  This release is 
related to the Enthalpy of dissolution of the particular alcohol in 
water.  The resulting temperature rise is translated into kinetic energy 
of both alcohol and water molecules right at the point of mixing.  These 
molecules are in fact moving faster that those water molecules located in 
front of the aluminum boat.  The result is a “molecular” push from the 
stern of the boat.  This is a similar effect to the solar windmills.  I am 
sure you probably have seen these devices in novelty stores.  There are 
made up of a horizontal helix place on a vertical axis.  The helix has 
flat (parallel to the axis) paddles.  One side of each paddle is painted 
black and the opposite side in painted white in such a way that if you see 
the paddles from one side you would only see either black of white.  The 
helix is placed inside transparent plastic case to avoid wind contact.  As 
light (heat) is shone on the device and you can see that the helix starts 
to rotate moving as if it was been pushed from the black side of each 
paddle.  The explanation, as in the alcohol-propelled boat, is that air 
molecules next to the black surface are hotter that those near the white 
surface.   That is to say that the molecules next to the black surface 
have more kinetic energy and bump into the paddle side with more force 
that those next to the white surface with the resulting circular motion.

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