I was bothered by an experiment I did recently about the temperature distribution in a cup of salt solution. I added a few spoons of table salt and baking soda in two cups of water to create two saturated solutions. Then I left them sit there for a few days, along with a cup of plain water. When I came back and aimed my infrared camera at them, I saw something quite puzzling: in the two cups of solution, the bottoms were always about 0.5 warmer than the tops (see the IR image above)! In contrast, a cup of plain water did not show this temperature difference--the temperature was the same everywhere just as expected.
Exactly what kind of chemical force sets up this temperature gradient? We all know that warmer water should rise and colder water should sink, and eventually the convection stops and the temperature becomes the same everywhere. But this is apparently not true in the presence of salt solute. I feel this has to do with gravity. It must be gravity that causes a concentration gradient of the solute, which in turn results in the temperature gradient. But I am not sure how exactly this happens. I have no idea what energy source feeds this temperature gradient. Don't forget that the cup material tends to eliminate it through heat conduction and the air through convection. There must be an invisible hand that counters all these thermodynamic forces. This seems pretty amazing to me.
To make sure that this is not an effect of infrared radiation, I confirmed the result by sticking a sensitive temperature probe into the solution and moved it up an down for a few times. The image below is the 60-second result recorded by the temperature probe, which clearly agrees with the IR image.
This is an example that, once again, shows the power of infrared imaging. I would not have noticed there was such a temperature gradient in a solution without my infrared camera. The infrared camera, in just one simple shot, captured the salient and subtle details that reveal very complex physics, which I still do not understand.
What is the significance of this result rather than a tempest in a teacup? Might the temperature gradient be used to generate a voltage gradient, which in turn generates electricity? In other words, might this be some kind of battery that is a 100% clean energy source?
The ocean is a gigantic solution of salt. Half Celsius of temperature difference in the ocean translates into an enormous amount of energy. Might there be such an effect in the ocean?
1) Evaporation is a driving force
2) The temperature gradient exists only in a saturated solution
3) Mystery solved?
4) Visualizing vapor pressure depression
5) Salinity gradient vs. temperature gradient
6) An evidence from an ice cube