Tuesday, October 26, 2010

An infrared view of a popular chemistry experiment

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 google "ice melts slowly in saltwater," you can find a lot of discussions--and debates as well. 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 based on theoretical deductions.

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.

The IR images show that 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 that counters the thermal 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.

PS: Sprinkling some salt to an ice cube seems to accelerate the melting process. This seems to be in contradiction with the observation that ice melts more slowly in saltwater. This is where a lot of people are confused. The physics behind the two processes is different, even though they involve exactly the same chemical ingredients--just water in two different phases and salt. 

Monday, October 25, 2010

Which colors absorb more light energy?

Figure 1. A page with some color
strips under a table lamp. Click the
image to enlarge it to see the details.
We all know black objects absorb more light energy than white ones. What about red, green, blue, and any other colors? With an affordable infrared (IR) camera, this is very easy to figure out. (Update in 2015: There are now a few IR cameras that are priced under $300, such as FLIR ONE and SEEK THERMAL)

Use your word processor to draw and print some strips in any color you want on a page, as shown in Figure 1. Put the page under a table lamp (or sunlight) and let the light shine on it for 10 seconds. Then aim an IR camera at the paper. Figure 2 shows the results.

Figure 2. An IR image showing the
amount of light energy absorbed by
the color strips.
Obviously the black strip absorbed the most. But the red, blue, and green ones did not absorb much. Interestingly, the dark gray and purple ones seemed to have absorbed more energy than I would imagine.

I have to admit that I didn't know how other colors absorb light energy before doing this experiment. With an IR camera, you can easily check it out just on your own like what I did--for any color and any comparison.

If you have heard that Steve Chu, our Energy Secretary, has been serious about advising people to paint their roofs with light colors and Mayor Michael Bloomberg has agreed to answer the call in New York City, you may find this little experiment worth your while--you may pick a color that does not absorb a lot of energy yet it will be more colorful than white.

Updates in 2013: Links to my YouTube videos about this experiment:

Sunday, October 24, 2010

Visualizing convection without using ink

Figure 1. A top view of a floating
ice cube.
If you have done a convection demo using a container of water and some ink, you may have had to change the water after each demo since the ink had diffused everywhere, which may make the convection pattern less easy to observe. Depending on the size of your container, that is some work to do and some water and ink to waste.

Here is a greener and better way to do it--using an infrared (IR) camera. An IR camera shows hot and cold (typically) in red and blue colors, which can be considered as "IR ink" that can be seen only through an IR camera. With the tool, all you can do is to add some ice cubes or hot water to a container of water every time you need to do a demo. There is no need to change the water.
Figure 2. A side view of a floating ice
cube showing "cold fingers." 

One thing to notice is that you should not use a glass container--because it reflects off IR rays that will get into the image. A clear plastic one is the best as it does not reflect much and it allows you to observe what happens inside (if anything visible) with naked eyes.

Figure 3. A view from another side
showing the the cooling at the
Figure 4. An IR image after hot water
was added to room temperature water
in a container showing hot water
tended to float atop.
Figure 5. An IR image of a fish tank
showing a clear pattern of
temperature stratification.

Friday, October 22, 2010

Salinity gradient vs. temperature gradient

Figure 1. The salinity gradient and temperature 
gradient observed in an open cup of saturated 
This is the fifth follow-up of my 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 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 establish.
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 re-adjust 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.

What do these results suggest? Is it possible that a weak temperature gradient exists in a closed system that does not have the driving force of evaporative updraft?

Tuesday, October 12, 2010

Visualizing vapor pressure lowering

Figure 1. Two shallow plastic containers. 
The left one holds a lot of salt and the 
right one is plain water. A small amount 
of water was added to the left one.
This is the fourth followup of the blog article: "A perfect storm in a cup of salt water?" that started this journey of discovery.

The vapor pressure lowering is an effect that says the water vapor pressure above saltwater is lower than that above freshwater. This is more generally described by Raoult's Law, which states that the vapor pressure of an ideal solution depends on the vapor pressure of each chemical component and the mole fraction of the component present in the solution. Since the sodium and chlorine ions hardly evaporate, the vapor pressure above saltwater comes from the evaporation of water molecules.

The molecular mechanism behind the vapor pressure lowering is easy to understand--the ions stay in the way of water molecules and slow down the rate of their evaporation and, in the case of salt, they even act to attract the water molecules and prevent them from leaving the solution.
Figure 2. An IR image of the two shallow
containers right after water was added to
the salt one on the left.

Let's try to use infrared (IR) imaging to visualize this process. Prepare two plastic containers like the ones shown in Figure 1. Add plenty of salt to one of them and some water to the other. Then add some water to the salt one. Figure 2 shows an IR image just after water was added. The image shows that the system absorbed heat while salt was being dissolved.

Figure 3. An IR image after half an hour
showing that the evaporative cooling
effect of the saltwater container is weaker
than the pure water one.
Let the containers sit for about half an hour and then take another IR image. Figure 3 shows the result. Interestingly, the colors reversed. Now, the saltwater container appears to be warmer than the pure water one.

Figure 4. An IR image after a few hours
showing that the contrast of colors
became greater.
Wait for a few hours and then come back to take an IR shot. Figure 4 shows that the temperature difference became greater.

How to interpret these results? There are two mechanisms that cause the temperature difference.

One is the vapor pressure lowering mentioned above. Using a Vernier relative humidity sensor, one can confirm that the humidity above the saltwater is lower than that above the freshwater. This means that the evaporation weakens above saltwater, which reduces the cooling effect.

The other is the crystallization of salt that releases heat. The evaporation of every water molecule weakens the ability of the solution to hold ions. As water constantly evaporates, a corresponding amount of ions must return to the crystalline form--mostly at the bottom because the contact area with the wall is much smaller compared with the contact area with the bottom. This process releases heat at the bottom. Since the saltwater is very shallow, the heat conduction may happen fast enough so that the crystallization heat will pass to the surface of the saltwater--even if convection may be insignificant with such a shallowness--and make it even warmer on top of the weaker evaporative cooling effect. This effect, which is totally based on molecular reasoning, is yet to be confirmed by an experimental method.

The vapor pressure lowering process and the crystallization process in this system are intertwined. If evaporation slows down (absorb less heat) due to salt, crystallization slows down (release less heat) too. The small amount of crystallization heat transfers to the surface and slightly increases the evaporation rate, which in turn causes slightly more ions to crystallize. The two processes manage to keep the saltwater container warmer than the freshwater container. But we still don't know which process contributes more. The question is, without the crystallization heat, can the IR image of the saltwater be as warm as it appears to be? How can we separate the two effects? Sealing the containers to stop evaporation doesn't work because that will stop crystallization as well.

Why do I insist on the theory of crystallization heat? Not only because it is logical. If we look at Figure 2, we will see that the effect of heat of solution is pretty significant. In order for salt to dissolve in water, some heat needs to be absorbed. Now, when the reverse process has to happen, i.e., when the salt ions have to return to the solid form, the same amount of heat must be released--in a much slower pace because of the low rate of evaporation (compared with the rate of dissolving). This is just simply the rule of energy conservation at work. The chemical potential must act like a spring. Energy is stored when it is "compressed" and released when it "bounces back."

Most likely, I now think this mysterious effect in a cup of saltwater is an orchestration of many physical and chemical effects. The salt gradient in a saturated solution is yet another mystery to be uncovered: the salinity gradient exists only in a saturated solution but not in any unsaturated solution.

A small cup of saltwater may contain a lot of physical chemistry! Stay tuned for more follow-up experiments.