|Figure 1. An IR image of a freshwater cup and a saltwater cup after an ice cube was added to each.|
Will an ice cube melt faster in freshwater or saltwater? Why do we salt the road in water? How does an iceberg melt and how might it affect the ocean currents? All these curious questions are wonderful for students to explore. And they are very easy to do.
However, the science behind these questions are not that easy. To explain the results, we will probably need some reasoning at the molecular level, which is not at all easy for lower-grade students. But that is what we hope them to learn. These explorations require not only hands-on but also minds-on, which is why they are so great.
|Figure 2. An IR image take after four|
minutes showing the convection in
the freshwater cup.
They are not obvious at first glance and often can be counterintuitive. If you search "ice melts slowly in saltwater
" in Google, you can find a lot of discussions and debates. Many students and teachers were confused by what they observed in such a simple system as an ice cube floating in a cup of saltwater. Most of the discussions were, however, merely theoretical.
Had they had an IR camera, the thermodynamic processes would have been much more obvious. Figures 1-4 show a series of IR images taken to reveal what happened in the two cups after an ice cube was added.
Obviously, ice molt faster in freshwater because cold molten water can sink to the bottom and warmer water at the bottom is pushed to rise. This process, called convection, runs continuously to carry heat from the whole cup to melt the ice cube.
|Figure 3. An IR image taken after|
nine minutes showing the cooling
effect at the bottom as indicated by
the greenish halo.
In the case of saltwater, the cold water just sat at the top. The only explanation of this is that saltwater is denser so molten freshwater from the ice cube cannot sink, even if it is colder. Somehow, saltwater provides greater buoyancy.
Figure 4 shows that sixteen minutes later, the cold front still had not reached the bottom. This means that not only convection slowed down but also conduction was very slow.
|Figure 4. 16 minutes later...|
Recall our finding that a cup of saturated saltwater can spontaneously develop a temperature gradient from bottom up
. This experiment provides a direct evidence that supports the theory that the temperature gradient can be created by the salinity. However, this evidence is not decisive, as the phenomenon reported here happens in an unsaturated solution whereas the small temperature gradient only exists in a saturated solution.
The puzzle still remains unsolved.
This is the fifth follow-up of the blog article: "A perfect storm in a cup of salt water?" This investigation focused on the relationship between the salinity gradient and the temperature gradient. Is the temperature gradient caused by the salinity gradient, or the other way around? Both arguments seem to make some sense. On the one hand, one can argue that the salinity gradient stops the convection. On the other hand, warmer water tends to dissolve more salt. So we are in a chicken-egg situation.Let's do an experiment to explore a bit further. I prepared two cups of saturated saltwater. One open and the other sealed. I let them sit overnight and then checked the salinity and temperature distribution the next day using Vernier's salinity sensor and temperature sensor. I did this by moving the salinity sensor and the temperature sensor together up and down in the saltwater. Figure 1 shows the results for the open cup.
Figure 1. The salinity gradient and
temperature gradient observed in an
open cup of saturated saltwater.
To measure the data for the closed cup, I first removed the seal and then quickly did the measurement. Since the salinity and temperature gradient would take some time to readjust after the seal was removed, we can pretty much assume that the results I got approximately reflect what would have been measured if the seal had not been removed. Figure 2 shows the results. The comparison of the results shows that the salinity gradient is about the same for the open and closed cup--the bottom is about 1.3 ppt saltier than the top, but the temperature gradients are quite different--the open cup measured about three times as large as the closed cup (0.3°C vs. 0.1°C). Due to the evaporative cooling effect, the overall temperature of the open cup is at least 0.5°C lower than the closed one.
Figure 2. The salinity gradient and
temperature gradient observed in
a closed cup of saturated saltwater.
Note: The measurement was done
shortly after removing the seal.
Hence the results can be regarded
as approximately those of the
sealed cup as the gradients will
take a longer while to establ
What do these results suggest? A weak temperature gradient may exist in a closed system that does not have the driving force of evaporative updraft.
This is the third followup of the blog article: "A perfect storm in a cup of salt water?"
I woke up last night with a perfect explanation for the mysterious temperature gradient observed in a saturated salt solution. It is the recrystallization of salt at the bottom of the cup that releases the heat.
Since water molecules are constantly evaporating from the surface of the solution, a corresponding amount of ions must return to the crystal form at the same time--because a reduced amount of water in a saturated solution in the cup cannot take them any more. This most likely occurs at the bottom since the surface of the precipitate already provides a perfect ground of crystal growth. When ions adhere to the surface of a crystal, heat is released. The amount of released heat is approximately equal to half of the cohesive energy of the salt crystal (because it is a surface adhesion), which may be quite high because of the strong electrostatic attractions in the ionic crystal. The released heat transfers to the solution near the bottom and, together with the evaporative cooling effect on the surface, creates the temperature gradient we observed. The entire process runs continually across the solution because of the diffusion of water molecules and ions driven by their concentration gradients: the concentration of water/salt becomes lower/higher at the surface when water evaporates.
There are four evidences that support this theory:
- The temperature gradient disappears when we sealed the cup, because that stopped the evaporation at the surface as well as the recrystallization at the bottom.
- We observed no temperature gradient in an unsaturated solution because there is no recrystallization process.
- The temperature hiked when the sensor touched the salt deposit at the bottom.
- This temperature gradient lasts for a long time because this process will continue until all the water molecules evaporate.
Now, how can we make use of this effect to produce clean energy? As we produce sea salt by using solar energy to evaporate the water in brine anyway, might it be possible to harvest the energy released from the crystallization process? This seems like a stone that kills two birds: generating electricity while producing salt.
The diagram above illustrates the energy cycle of a saltpan/ionic power plant combo. This design is based on a chain reaction that involves two phase changes in a salt solution to convert solar energy into electricity through the ionic potential.
PS: I found in Wikipedia the concept of solar pond
that uses a large pool of saltwater to collect solar energy. I think its mechanism is different from what I discussed above. I have had no luck reproducing the effect of a solar pond in a cup yet.
This is the second followup of an earlier blog article "A perfect storm in a cup of salt water?
I did an experiment to investigate the relationship of the salt concentration with the mysterious temperature gradient in a cup of salt water. The experiment was to measure the top-bottom temperature differences in three cups of salt water: low-concentration, medium-concentration, and saturated solution. In the saturated solution, there is a salt precipitate at the bottom of the cup. In all measurements, a fast-response temperature sensor was moved up and down in a cup. And the solutions had existed for over 100 hours to ensure that the salt was completely dissolved and the systems had reached thermal equilibrium with the environment.
The results shown in the graph above clearly indicate that the temperature gradient exists only in the saturated solution. The two unsaturated solutions exhibit no appreciable temperature gradients and measure approximately the same temperature with plain water.
The results were confirmed by an IR image shown above (from left to right: low-concentration, medium-concentration, and saturated).
This experiment suggests that there is probably no ion gradient in an unsaturated solution. An unsaturated salt solution has the same temperature everywhere and that temperature is the same as that of the plain water, whatever its concentration is. I originally expected that an unsaturated solution would have a temperature gradient more or less proportional to the salt concentration, as would a colligative property. This surprising result made me think that the prime suspect is the salt precipitate at the bottom of the cup. We know there is a lot going on on the surface of the precipitate layer. The dissolving and crystallization never cease. It is just that the two processes reach a dynamic equilibrium--the rate of dissolving becomes the same as the rate of crystallization. Sort of like what is shown below:
Let's think a bit more about the meaning of this experiment. Notice that the temperature curve of the saturated solution lies entirely between that of the ambient temperature and that of the pure water temperature in the graph. This means that the existence of the precipitate somehow weakens the evaporative cooling effect, and probably the evaporation process itself. Why would the evaporation of water at the top of the solution slow down in the presence of some precipitate at the bottom? Exactly how does this process contribute to the temperature gradient existing in the solution?
We can plausibly reason that the rate of evaporation decreases because the ions somewhat act as binding agents that hold the water molecules more tightly through the strong electrostatic attractions. This is known as the water shell effect—water molecules are attracted to an ion and form a dynamic shell around it. As a result, it is more difficult for water molecules to leave the solution to evaporate. But this picture cannot explain why there is virtually no difference between the temperature of a cup of pure water and the temperature of a cup of unsaturated salt water.
It seems the mystery is far from being uncovered. While clarifying a few things, this experiment makes the phenomenon more baffling. Stay tuned for our next investigation.
In terms of its other implications, there is one thing that we can rule out now. There is no such effect in the ocean, as sea water is not saturated.
This is the first followup of the blog article last week "A perfect storm in a cup of salt water?".
Several people including Bob Tinker, John Loosmann, and Einar Berg suggested that it was the evaporation of water that drives the observed persisting temperature gradient. It turned out that they were right. After sealing the salt cup with plastic wrap and leaving the three cups for 24 hours, their thermogram shows the sealed cup vanishes from the infrared image (see the image to the right--the sealed salt water cup is in the middle, which is invisible). This means that the temperature everywhere in the cup is the same as the ambient temperature. The infrared image also shows the baking soda cup, which has not been sealed, continues to show a temperature gradient.
Now, we have to explain why the pure water cup shows a cool infrared signature. So I added a sealed pure water cup. The thermogram to the right shows that the sealed pure water cup vanishes in the infrared image (the sealed water cup is on the right of the thermogram), whereas the open pure water cup shows a cooler image for the part filled with water. Interestingly enough, the upper part of the cup that does not have water contact is constantly almost 1°C warmer than the filled part. This temperature gradient is clearly shown in the infrared image below. Why is the temperature gradient across the water line on the surface of the plastic cup so sharp?
Now go back to the evaporative updraft force. At this point, we only know that cutting off evaporation shuts down the energy loop. We still do not know how what happens under the water line in the salt water cup. The following graph clearly shows that this effect exists in not only a salt solution, but also a baking soda solution and a sugar solution.
We can suspect that the ion concentration gradient is another driving force for this energy circuit. This will be our next step of investigation. Stay tuned.
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°C 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