As I mentioned before, Kristin Richter and I are running the workshop “Conducting oceanographic experiments in a conventional classroom anywhere” at the European Marine Science Educator’s Association Meeting in Gothenburg, Sweden. There is quite an active Twitter crowd around, so you can follow the storyfied meeting or look out for #EMSEA14 on Twitter.
Our workshop has been represented quite well there, too, so I’ll just post a couple of my own pictures here.
Plus there are a lot of post dealing with the exact same experiment after the cut below. And there are two more posts on this exact experiment coming up that are scheduled already, one tomorrow, the other one in two weeks time. And thanks to a very nice family of participants I already have plenty of ideas of how to modify this experiment in the future!
[edit: There finally is a picture of me in the workshop, too, to show that I actually did contribute and not just leave it all to Kristin:
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.
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?
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???
Or why you should pay attention to the kind of salt you use for your experiments.
The melting ice cubes in salt and fresh water is one of my favorites that I haven’t written about in a long time, even though (or possibly: because) I wrote a whole series about it last year (see links at the end of this post).
Now that the EMSEA14 conference is almost upon us and Kristin and I busy preparing our workshop, I thought I’d run the experiment again and – for a change – take the time to finally know how much time to schedule for running the experiment. This is the experiment that I have run most often of all in all kinds of classes, but there you go… Usually I have more time than just 30 minutes, and there is so much other content I want to cover in that workshop!
There are a couple of things that I learned running this experiment again.
It takes at least 10 minutes to run the experiment. My water was slightly colder than usual room temperature, my ice cubes slightly smaller, though. And those 10 minutes are only the time the ice takes to melt, not the time it takes to hand out the materials and have the groups settle down.
There is a reason it is always recommended to use kosher salt for these kind of experiments. Look at the picture from one of the old posts in comparison to the ones from today: The iodized salt containing folic acid I had in my kitchen dissolves into really milky water. I really should have walked the two extra meters to get the good salt from my oceanography supplies in the other room!
Some food dyes are the devil. My whole kitchen is red. Plus the ice cubes didn’t freeze nicely (for a post on ice cubes freezing from salt water click here), the ice chipped when I tried to get the cubes out of the ice cube tray. I definitely can’t have that mess at a workshop. So here is another argument for using non-dyed ice cubes! The more important argument being that you think more if the cubes are not dyed and you don’t immediately see the explanation…
But it is always a fun experiment to run, and there are always new things to spot. Watch the video below and see for yourself! (Explanations on the weird phenomena coming up in a future post!)
I know I’ve said it before about another experiment, even today, but this is my absolute favorite experiment and I still get endlessly fascinated. I’ve written about salt fingering before, and given tips on run the experiment, but today we tried a different setup.
Removing a barrier between waters of different densities and watching what happens. (deutscher Text unten)
Today, one of the groups performed a classical experiment (shown for example here) – but the awesome thing is that they came up with the planning pretty much by themselves in order to determine the effects of temperature and salinity on density. They compared water of the same temperature, but one fresh and one salty; warm salty vs cold fresh water; and cold salty vs warm fresh water. They predicted the outcome correctly, and we are showing two movies below: One normal movie and one in slow motion. Enjoy!
Heute hat eine Gruppe ein klassisches Experiment reproduziert. Allerdings haben sie es quasi selbstständig entwickelt.
Um den Effekt von Temperatur und Salzgehalt auf die Dichte zu bestimmen, werden zwei Wassermassen in einen Tank gefüllt, durch ein Wehr getrennt. Das Wehr wird herausgezogen und die dichtere Wassermasse schichtet sich unter die weniger dichte. Die Gruppe hat drei Fälle verglichen: Wasser gleicher Temperatur mit und ohne Salz; warmes salziges Wasser mit kaltem süßen; und warmes süßes Wasser mit kaltem salzigen. Der Film unten zeigt eine Zeitlupe der Bewegung.
I got a lot of weird looks when I excitedly told people about two years ago that I had just bought a lava lamp. But what’s not to love about them? Plus they are great for teaching. These days kids don’t know them any more, so they are missing out on a really nice mental image of how convection works. Be it in the Earth’s mantle or in the ocean…
When I moved into my new flat, for the first week I only had an inflatable air bed and my lava lamp in my living room (oh, and a lot of boxes of course). So I have spent a lot of time looking at how the flow changes over time.
Today, all I want to share is this 9 minute movie of the lava lamp. But I’m working on a post where I’m discussing the temporal development of the flow. Sounds interesting? Stay tuned! :-)
After the frustrations of taking pictures of steam in my last post, I decided that I could use the very same cute mug to show other stuff, too. I know it has been done over and over again, but we have new students every year, don’t we, so someone has to keep telling the old stories, right?
So. When should you pour the milk into your tea? Right away or a little later?
The answer, as you know, is “it depends”.
Do you want your tea as hot as possible? Then put the milk in right away and it won’t cool the tea down as much. Want the milk to cool down the tea as much as possible? Then wait for as long as you can before pouring it in.
The explanation behind this is of course that the cooling due to evaporation is happening faster the larger the temperature difference between the tea and the surrounding air. If you let it sit without milk, due to the larger temperature difference it cools down faster than if you poured in the cold milk, thus cooling it closer to room temperature, and then waited.
And there are even occasions when you would you put milk into the cup before adding the tea: If you have delicate china and don’t want to risk ruining it by pouring in almost boiling tea. Plus allegedly that way the milk doesn’t scald and form those weird flakes?
Ha, this is a bad pun. We are modeling the Denmark Strait Overflow – but in a non-numerical, small-scale-and-playdough kind of way.
More than a year ago, Kjetil and I ran that experiment with a group of high-school students and when writing a post about a much more sophisticated version of this experiment I realized I never documented this one in the first place. So here we go!
Dye is added to the “northern end” of the tank (i.e. the end where the water is being cooled by a sport’s injury cooling pack). As the water cools, it becomes denser and fills up the reservoir on the northern end until it spills over the clay ridge.
This is a very simple demonstration of how overflows actually work.
Is there an equation of state for hypersaline water at very cold temperatures?
A friend of mine is looking to calculate changes in density of a hypersaline Antarctic lake from summer to winter. Apparently, this lake is about 10 times saltier than the ocean and often cools down to -17C at the bottom.
My own spontaneous answer was that I am not aware of such an equation of state, and that I doubt that there is a lot of empirical data in that property range. Plus from talking to Dead Sea researchers while working on double diffusion, I know that measuring salinities that are that high is not at all easy – the constancy of composition of sea water breaks down (at least in the Dead Sea) which has consequences for the measurement methods that can be used, and in any case CTDs aren’t calibrated for those salinities. But I am hoping that the collective wisdom of my readers will come up with a better answer.
So, dear readers. Do you know of an equation of state that applies to that range of properties, or do you have any other comments on the issue? Please leave a comment below or get in touch with me! That would a) really help my friend, and b) help satisfy my curiosity :-)