More details on the structure of fresh water and salt water ice.
Fresh water and salt water ice have very different structures as I already discussed in this post.
In the image above you see that the structures are very different. Whereas fresh water ice is clear and transparent, salt water ice has a porous structure and is milky.
The pores can be made visible by dropping dye on the ice cubes, as we did in class on Tuesday. For salt water ice, dye penetrates into the ice cube along the brine channels; the ice cube seems to be soaking up the dye like a sponge and becomes colored through and through. In case of the fresh water ice, dye cannot penetrate because the crystal structure is so regular and tight, and the dye just comes off the ice.
Different didactical settings in which the “ice cubes melting in fresh and salt water” experiment can be used.
In part 1 and 2 of this series, I showed two different ways of using the “ice cubes melting in fresh water and salt water” experiment in lectures. Today I want to back up a little bit and discuss reasons for choosing one over the other version in different contexts.
Depending on the purpose, there are several ways of framing this experiment. This is very nicely discussed in materials from the Lawrence Hall of Science (link here), too, even though my discussion is a little different from theirs.
1) A demonstration.
If you want to show this experiment rather than having students conduct it themselves, using colored ice cubes is the way to go (see experiment here). The dye focuses the observer’s attention on the melt water and makes it much easier to observe the experiment from a distance, on a screen or via a projector. Dying the ice cubes makes understanding much easier, but it also diminishes the feeling of exploration a lot – there is no mystery involved any more.
2) A structured activity.
Students are handed (non-colored) ice cubes, cups with salt water and fresh water and are asked to make a prediction about which of the ice cubes is going to melt faster. Students test their hypothesis, find the results of the experiment in support with it or not, and we discuss. This is how I usually use this experiment in class (see discussion here).
The advantage of using this approach is that students have clear instructions that they can easily follow. Depending on how observant the group is, instructions can be very detailed (“Start the stop watch when you put the ice cubes in the water. Write down the time when the first ice cube has melted completely, and which of the ice cubes it was. Write down the time when the second ice cube has melted completely. …”) or more open (“observe the ice cubes melting”).
3) A problem-solving exercise.
In this case, students are given the materials, but they are not told which of the cups contains fresh or salt water (and they are instructed not to taste). Now students are asked to design an experiment to figure out which cup contains what.
This is a very nice exercise and students learn a lot from designing the experiment themselves. However, this also takes a very long time, more than I can usually afford to spend on experiments in class. After all, I am doing at least one hands-on activity in each of the lectures, but am still covering the same content from the text book as previous lecturers who used their 180 minutes per week just lecturing. And I am considering completely flipping my class room, but I am not there yet.
4) An open-ended investigation.
In this case, students are handed the materials, knowing which cup contains fresh and salt water. But instead of being asked a specific question, they are told to use the materials to learn as much as they can about salt water, fresh water, temperature and density.
As with the problem-solving exercise, this is a very time-intensive undertaking that does not seem feasible in the framework we are operating in. Also it is hard to predict what kind of experiments the students will come up with, and if they will learn what you want them to learn. On the other hand, students typically learn much more because they are free to explore and not bound by a specific instruction from you.
Visualization of how much salt is actually contained in sea water.
When preparing “sea water samples” for class, it is always astonishing to me how much salt I have to add for normal open-ocean salinities. Time and time again it looks like it should be way too much, but then when tasting it, it tastes salty, but like the ocean and not like brine.
Since it is still astonishing to us, Pierre and I thought, it would probably be a good thing to show to our students. 0.18 teaspoon full of salt corresponds to only 1 gram of salt (averaged over several non-scientific internet sources, but well within the measurement error of my kitchen scales [and yes, I know the trick of measuring the weight of several spoons and then dividing by the number, but thanks!]).
What I want to do in the lecture is have the students estimate how much salt they need for a 35 psu liter of water. And not estimate by weighing (because I want each of the students to be able to touch the salt, but at the same time don’t want salt all over the lecture theatre), but visually estimate.
The little jar in the picture above contains 10 grams of salt. So in order to have students estimate how much salt they would need for a liter of 35psu water, we filled 12 of those little jars with 10 grams each and handed them to the students. Obviously we didn’t tell the students how much salt was contained in a jar!
Knowing that there are 10 grams of salt in each of the jars, it is pretty obvious that we need three and a half of those little jars for 35 grams of salt. When we did this in the lecture on Tuesday – and again, the students were not told how much salt was in one jar! -, the first person who answered guessed “four”. And then someone actually said “three and a half”. Oh well, lucky guess or great skill? I was hoping for answers like “maybe one of those jars”, because that would be closer to my own intuition. I guess next time I’ll be framing it differently. Maybe use something with one liter volume and put 35 grams in it? Or ask them to tell me in teaspoons? Does anyone have a good idea that they would like to share with me?
Sea ice and fresh water ice have distinctly different properties that can easily be investigated even in big class rooms.
In “on how ice freezes from salt water” I talked a bit about how dye was rejected when I tried to produce colored ice cubes for another experiment. But even non-colored ice that were made out of fresh water or salt water shows distinctly different structures.
On the left, you see that the surface is very smooth apart from a couple of cracks. The red food dye that was dripped on the ice cube comes right off, like water off a duck’s back. On the right, the food coloring is not rolling off, instead it is creeping into all the little brine channels, hence nicely showing a web of pores all throughout the ice cube.
In class, students get clear ice cubes, and before they put them in the cups, I ask them to make a prediction. Which of the ice cubes will melt faster, the one in fresh water or the one in salt water? Everybody has to make a prediction. And having run this experiment with 100+ people by now, I can tell you: Approximately 5% predict the right outcome. And that is not 5% of the general population [edit: this used to say “5% of the general circulation”!], that is 5% of people who were either attending my class or a workshop on oceanography with me, who were attending a workshop on teaching oceanography, or my nerdy friends. So don’t be sad if you get it wrong – you are in good company.
So now that everybody has made a prediction, the ice cubes go into the cups with fresh water and salt water. In the beginning, the excitement is usually moderate. After all, you are staring at a plastic cup with an ice cube floating in it. But then, after the first minute or so, there is no denying any more: The ice cubes have started melting. And one of them is melting a lot faster than the other one. The one in fresh water is melting a lot faster than the one in salt water! How can this be? At this point, students typically start secretly (because remember – no tasting in the lab!) tasting the water in the cups to make sure that they didn’t actually swap the cups. After all, it should be the ice cube in the salt water melting faster, shouldn’t it?
But no, it is true: The ice cube in fresh water is melting faster than the one in salt water. But how??? Enter food coloring.
If at this stage one or two drops of food coloring are dripped on the ice cubes, this dye helps visualize the circulation similarly to the dyed melt water I showed you the other day [which, incidentally, one of the student groups yesterday observed without food dye or me prompting. Great job!].
And now the whole thing makes much more sense: In the fresh water case, melt water is denser than the water in the cup and sinks to the bottom of the cup. As it is sinking away from the ice cube, it is being replaced with warmer water from the cup. Hence the ice cube is always floating in relatively warm water which helps it melt.
In salt water, on the other hand, the melt water forms a layer on top of the water in the cup. Even though it is very cold, it is still less dense than the salty water in the cup. The ice cube is more and more surrounded by its own melt water and not by the warmer water in the cup as was the ice cube in the fresh water. Therefore, the ice cube in the fresh water is melting faster than the one in salt water!
This experiment is easy to run in all kinds of settings. However it helps if the student groups are spaced out enough so that the instructor can reach all of the groups and listen in on the conversations to get a feel of how close to a solution the students are, or chat to the students to help them figure it out.
There will be two follow-up posts to this one: One about different didactical settings, and one different contexts this experiment can be used in.
I’ve been wondering how to best show how sea ice freezes for quite a while. Not just that it freezes, but how brine is rejected. By comparing the structure of fresh water and salt water ice, one can get an idea of how that is happening (and I’ll write a post on that after we have done this experiment in class). But I accidentally stumbled upon a great visualization when preparing dyed ice cubes for the melting ice cube experiment (see this post) when all my ice cubes came out like this:
Instead of being nicely homogeneously colored, the color had concentrated in the middle of the ice cubes! And since the dye acts in similar ways to salt in the ocean (after all, it IS a salt dissolved in water, even though not the same as in sea water), this is a great analogy. It is even more visible when the ice cubes have started to melt and the surface has become smooth:
Clearly, when forming, the ice crystals have been rejecting the dye! In the ocean, due to cooling happening from above, ice would freeze downward from the surface, under the influence of gravity the brine channels would be vertical, and brine would be released in the water underneath. In my freezer, however, cooling is happening from all sides at once. There is a tendency for the dye to be rejected towards the bottom of the ice cube tray under gravity, but as ice starts forming from all sides, the dye becomes trapped and concentrated in the middle of the forming ice cube. Can you see the little brine channel leading to the blob of color in the middle?
I must say, when I first took the ice cubes out of the freezer I was pretty annoyed because they weren’t homogeneously colored. But now I appreciate the beauty of the structure in the ice, and you can bet I’ll try this again with bigger ice cubes!
Experiment to visualize the effects of density differences on ocean circulation.
This is the first post in a series on one of my favorite in-class experiments; I have so much to say about it that we’ll have to break it up into several posts.
Post 1 (this post) will present one setup of the experiment, but no explanations yet.
Post 2 will present how I use this experiment in GEOF130, including explanations.
Post 3 will discuss how this experiment can be used in many different setups and
Post 4 will discuss different purposes this experiment can be used in (seriously – you can use it for anything! almost…).
So, let’s get to the experiment. First, ice cubes are inserted into two cups, one filled with fresh water at room temperature, the other one filled with salt water at room temperature. In this case, the ice cubes are dyed with food coloring and you will quickly see why:
As the ice cubes start to melt, we can see the dyed melt water behaving very differently in fresh water and salt water. In fresh water, it quickly sinks to the bottom of the cup, whereas in salt water it forms a layer at the surface.
After approximately 10 minutes, the ice cube in freshwater has melted completely, whereas in salt water there are still remains of the ice cube.
Why should one of the ice cubes melt so much faster than the other one, even though both cups contained water at the same (room) temperature? Many of you will know the answer to this, and others will be able to deduce it from the different colors of the water in the cups, but the rest of you will have to wait for an explanation until the next post on this topic – we will be doing this experiment in class on Tuesday and I can’t spoil the fun for the students by posting the answer today already! But if you want to watch a movie of the whole experiment: Here it is!
(Yes, this really is how I spend my rainy Sunday mornings, and I love it!)
– I first saw this experiment at the 2012 Ocean Sciences meeting when Bob Chen of COSEE introduced it in a workshop “understanding how people learn”. COSEE has several instructions for this experiment online, for example here and here. My take on it in the “on the Cutting Edge – Professional Development for Geoscience Faculty” collection here.
Preparations for experiments to be shown at the science fair “forskningsdagene” are under preparation.
Forskningsdagene, a cooperation between research institutes and schools, science centers and other educational places, will take place next month in Bergen. This year’s topic is ocean and water, and many interesting activities are being planned.
Today Kjersti, Martin and I met up to test which dyes and liquids are best suited for internal wave experiments. Since the target group on at least one of the days are school kids, conventional substances (like potassium permanganate as dye or white spirit as one of the liquids) might not be the best option. Instead, we went for food coloring and vegetable oils.
In the end, we came up with many different options and decided that we should probably bring all the bottles so people can play with them, too. And we should found a company that sells these bottles as nerdy paper weights. I have had one on my desk for a year now and I’m still playing with it, as is pretty much everybody who comes to my office.
But of course the best option wasn’t mentioned until afterwards: Oil and balsamic vinegar! Thanks, Jenny!
A hands-on activity in which students use real data to find similarities in the sea surface height and the ocean depth along satellite tracks.
In yesterday’s GEOF130 class, we explored how the sea surface height and the ocean depth are related. All we needed: Sticky notes, scissors and this work sheet (as always – leave a comment if you want more details!).
When I went and bought the scissors, the lady asked me if I was a kindergarden teacher. I said no, I teach at the university. And that was the end of that conversation…
Hands-on activity to better understand the concept and consequences of heat capacity. Also a great party trick.
Imagine you take a balloon. Any kind of normal balloon. You blow it up. You hold it over a candle flame. What do you think will happen?
Yes – it will burst pretty instantly.
Now imagine you are taking a new balloon. You fill it with water (or, in our case, you fill it about half with water and half with air). You hold it over the flame. What will happen now?
You even take a second candle.
You wait some more.
What happens? Nothing.
And why not? Because water has a much higher heat capacity than air. Meaning you have to put a lot of energy into a small volume of water to warm it up, about 4 times more than you would have to add to a similar volume of air. So the balloon does not get hot quickly, hence the plastic doesn’t get weakened enough for the balloon to burst. In fact, it did not only not get hot quickly, it did not get hot enough at all within the attention span of a typical student or instructor. So, because my students asked nicely, I decided to demonstrate what happens when the balloon is half filled with water, but the flame is directed to an area of the balloon that is not in direct contact with the water. If you can’t imagine what happens, check it out here (if you CAN imagine what happens, I’m sure you will check it out, too…).