Top pic: Ice cubes (left) and leftover uranine solution from when I was preparing those ice cubes Bottom left pic: Melting the ice cubes in a water bath to see if they start glowing again when melting. Bottom right pic: Yes, they do! :-)
Why do I find this exciting? Because that means that a phase change of the water switches fluorescence as a tracer of that water on and off. Or the other way round: Seeing water fluoresce (or not) tells you what state the water is in without having to figure out anything else about that water.
…and funny how that the apprenticeship as chemical technician that I started in parallel to finishing high school (and didn’t finish because other stuff — oceanography — became more interesting) does come into play in the most unexpected moments :-)
I had frozen a second tray* with ice cubes dyed with uranine (you know, the green stuff I found in the lake near my house the other day?). Like the other ones, they clearly froze back-of-the-freezer-forward, and the most interesting thing to me: Only the parts that weren’t frozen were fluorescing in the fridge!
For comparison, here a picture of the leftover water (so same dye concentration as the ice cubes) in the beaker, and the tray with frozen ice cubes next to it. You can see the beam of the UV lamp on the table* so you believe me that the ice cubes were lit with UV light, too, they just don’t fluoresce!
*Ailin and Steffi, did you see I got you a gold colored ice cube tray? :D Although, had I thought that I would be looking at the ice cubes themselves this much, I would probably have gotten two whites instead of a white and a gold one… But now you at least have a pretty ice cube tray in the lab :-)
**and a stain that I am only now noticing, but it wasn’t me; I was working super carefully since today I am not dressed for the lab and I don’t want that stuff on my clothes!
Top pic shows the ice cubes “in situ”, clearly showing the cold back wall of the freezer where they were sitting.
Bottom left pic shows a top view of those ice cubes and it is very obvious that they have been starting to freeze from the back wall of the freezer forward: The upper row of ice cubes in the pic has formed clear ice in the direction towards that wall and has pushed the dye forward, whereas the bottom row in the pic is still not completely frozen and ice cubes seem to be freezing from all sides towards the middle and not as distinctly from back to front.
Bottom right pic: The rest of the water I prepared for the ice cubes that I left sitting on the counter for future use — still looks well mixed, no sinking of the dye to be observed!
And with these exciting updates I’ll leave you for now, so start playing with your own ice cubes! :-)
But the dyed ice cubes tell interesting stories all by themselves, too!
Salt water doesn’t freeze
“Salt water doesn’t freeze”? Then how do we get sea ice in the Arctic, for example?
When freshwater freezes, the water molecules arrange in a hexagonal crystal structure. If there is salt (or anything else) in the water, however, the ions don’t fit into the regular structure. Ice freezes from the water molecules, and all the disturbances like salt get pushed in the last remaining bits of liquid water, which therefore gets higher and higher concentrations of whatever was dissolved in it. As those little pockets with high concentrations of salt get cooled further, more and more water molecules will freeze to the surrounding freshwater ice, leading to even higher concentrations of salt in the remaining liquid water. So the freshwater is freezing, while rejecting the salt.
Of course if you cool for long enough, also the last bit of remaining water will freeze eventually, but that takes surprisingly long (as you can try by freezing salt water in some of the cups ice cube trays and freshwater in others, for comparison. Also the structures of freshwater vs saltwater ice look very different and are interesting to look at, see how here).
When the ocean freezes, this rejection of high-salinity water leads to interesting phenomena: Even when you melt it again to include all the pockets of high salinity water, sea ice will have salinities way lower than the water it froze from. This is because of a process called brine release. Since you are cooling the ocean from above, sea ice also forms from the surface downwards. This means that it is easy for the salty water to be pushed, “released”, or “rejected”, downwards, into the liquid ocean below. That ocean will then of course get more salty right below the ice!
In the picture below you see something similar happening in the left pictures. Instead of salt, I have used blue food dye for visualization purposes. In the top left, you see an ice cube exactly as it looked when I took it out of the ice cube tray it froze in, and in the bottom left you see the same one after I let it melt a little bit so the surface got smoother and it got easier to look inside (a lot more difficult to hold on to, though!).
Do you see how the top part of the ice cube is pretty much clear, while the bottom part is blue? That’s because it froze top-to-bottom and the dye got pushed down during the initial freezing process!
Stuck in an ice cube tray
Something else that you see in the top left picture is the effect of the ice cube being stuck in the ice cube tray as it froze: Pores filled with blue dye that had nowhere to escape!
Had I taken out those ice cubes earlier, when they had just frozen half way through, we would have found a clear ice layer floating on a cold, blue ocean. Maybe I should do that next time!
Checking on the temperature distribution of your freezer
Something else fun we can observe from the right pictures: Here, the dye was concentrated towards the center of the ice cube rather than the bottom! How did that happen?
My theory is that those ice cubes were located in an area of the freezer that was cooling from all sides (more or less) equally, whereas the ones shown on the left must have been placed somewhere where cooling happened mainly from the top.
So if you ever want to know where the cooling in your freezer happens, just put lots of dyed little water containers everywhere and check from which side the dye gets rejected — that’s the cooling side! Actually, I might check that for the freezer below just for fun. Would you be interested in seeing that done?
Now it’s your turn!
Let’s look back at the ice cubes I froze yesterday in the picture above. I’ve now written about a lot of things I see when I look at them. What else do you see? Do you think it’s interesting to use with kids, for example? I’ve used those experiments with first year university students, too, I think there is plenty to observe and explain here!
Oh look, a plume of (almost) un-dyed water hitting the green lake!
I am really fascinated by the balance between green water leaking out of the pipeline and into the rain drainage, the rain falling on the lake, and the rain water coming into the lake through the rain drainage system. Right now, the water coming out of the drainage is a lot less green than the water in the lake, which is itself being diluted by rain. So much so that you can see a clear plume entering before it is mixed so much, entraining so much lake water, that you loose track of it in the green.
This makes me think about all kinds of stuff: how long between it raining on the catchment area that drains into the lake and the water actually reaching it? And how large might the catchment area be relative to the area of the lake (i.e. how large are the respective influences on the color)? So much entertainment just stemming from a little green dye :)
Before I start gushing about my awesome new UV lamp (thanks for encouraging that purchase, Uta! :-)), some other updates on the state of green in the park across the road from my house (don’t know what I am talking about? Check out previous posts on the fluorescent dye tracer).
The lake is still bright green and very well mixed, similar to what it looked like in this post. But what is a lot easier to see now is the green water coming out into the Kiel fjord. It was very hard to see on the pictures I took the other day on our fluorescent night walk, and I didn’t see any by eye the first couple of days, but for the last days it has been clearly visible:
It’s still a lot clearer by eye than on the pictures, but even in these pictures you see the plume going out of the storm drain, don’t you?
In other news: my UV lamp arrived today and I am so excited!
So here is a water sample I took out of the green stream, photographed in normal daylight and then lit by my UV lamp. Pretty cool, ey? :-)
Who wants to come fluorescent water-spotting with me? :-)
Remember I said that there were weird and wonderful things going on when I last ran the melting ice cubes in salt and fresh water experiment? It is really difficult to see in the picture below (sorry!) but you can probably spot the ice cube floating at the surface and the melt water sinking down, inducing some turbulence? And then there is a small ice bit a bit to the right of the center of the picture. And that ice bit is floating upwards.
Dyed ice cube floating at the surface, and small ice bits floating up
Watch the melting ice cubes video below to see all the thing in action, it is visible really well as soon as the picture is moving:
So what is going on there? I think the solution to this riddle lies in me forcing ice to freeze even though it contains more salt (or in this case, red food dye) than it is happy with. Remember how dyed ice cubes look?
Ice cubes frozen from colored water
So basically there is dye trapped in the middle of the cube, because cooling is happening from all sides, hence ice is starting to form from all sides, pushing the dye to the center of the ice cube. In the ocean, cooling would of course only happen from above, so salt is being rejected as brine.
Anyway, since I wanted to dye the ice cubes to make things more visible for this blog, I am adding a dissolved substance to the water that would usually not be there. Hence I am making the ice slightly denser than it would otherwise be. So when small ice bits chip away from the main cube (which still contains large parts of pure fresh water ice from the sides of the cube where, during the freezing, the dye could still be rejected; and which therefore still floats), they are denser than the water and sink. But as they melt, the dye washed out, and eventually the remaining ice is fresh, hence less dense, enough to float up again.
The whole thing looks pretty fascinating.
What do you think, is that the correct explanation? Or can you come up with a better one? Let me know!
P.S.: Everybody I showed this video to was fascinated by how the little piece of ice is floating up. But what I find a lot more fascinating is how it came to be at the bottom of the beaker in the first place! After all, ice is supposed to float on water (or drift up again if pulled down and then released) but how did it get down there???