|MadSci Network: Chemistry|
Susie: 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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