Category Archives: hands-on activity (easy)

Tasting sea water reloaded

Internal waves in a bottle

Internal waves are shown in simple 0.5l bottles.

Waves travel on the interface between fluids of different densities and the phase speed of those waves depends on the density difference between the two fluids.

MVI_4505_02

Internal waves on the interface between water (dyed blue) and white spirit.

The simplest way to demonstrate this in class can be seen below – two 0.5l plastic bottles are used, one half-filled with water, the other one filled with half water, half vegetable oil. Waves can very easily be excited by moving the bottles, and it is clearly visible that the waves at the interface between water and oil are a lot slower than the ones on the interface between water and air.

For showing this experiment to larger audiences when people can’t play with the bottles themselves, it really helps to color either the water or the oil layer for greater contrast. See here for different combinations that we tried in connection to forskningsdagene in Bergen.

Incidentally, those internal wave bottles are a great toy. If you don’t have one available but wish you had a paper weight as awesome as mine on your desk, here is a movie for you:

How mountains form

A simple DIY tidal model

On the structure of fresh water and salt water ice

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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.

Fresh water ice (on the left) and salt water ice (on the right).

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.

Investigating fresh water and salt water ice cubes in class. Already in this photo the difference is clearly visible, and it is even more obvious when you pick up the cups and look at the ice cubes from the side.

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.

Melting ice cubes – what contexts to use this experiment in (post 4/4)

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What contexts can the “ice cubes melting in fresh water and in salt water” experiment be used in?

As you might have noticed, I really like the “ice cubes melting in fresh water and in salt water” experiment. Initially, I had only three posts planned on the topic (post 1 and 2 showing different variation of the experiment and post 3 discussing different didactical approaches to the experiment), but here we are again. Since I like this experiment so much – here are suggested contexts in which to use the experiment.

1) The scientific method.

No matter what introductory class you teach, at some point you will talk about the scientific method. And what is better than talking about the scientific method? Correct, having students experience the scientific method! This experiment is really well suited for that, because you can be fairly sure that most students will come up with a hypothesis that their experiment will not support.

2) Laboratory protocols.

For courses that include a laboratory component (like mine does), at some point you will have to talk about how to document your experiments. Again, since the hypothesis will typically not be supported by the results of the experiment, this is a great example on how important it is to write down the hypothesis and how you are planning on testing it, and then noting all the observations, not only the one that are along the lines of what you suspected. Also recording the little errors that occur along the way (“someone swapped the cups with the ice cubes, so we are not sure any more which one is which”) is very important, and if you have a class doing this experiment, you can be sure that at some point someone will make a mistake, not write it down and then be very confused afterwards. Great teaching and learning opportunity!

3) Different teaching methods.

If you are teaching about didactical models, this experiment is very well suited for this, too (see my post 3 on the topic and the Lawrence Hall of Science resource). Just have different people work on the experiment using the different methods and then discuss what and especially how people learned using those methods. The Lawrence Hall of Science resource mentions a fourth method (and I didn’t want to give the impression that I am recommending it, therefore I omitted it in my post 3) – the “read and answer” method, where students read about density, stratification and density-driven circulation and then answer questions like “what is density?” or “what is thermohaline circulation”. Again, not recommended for your oceanography class, but adding this option might be very relevant if you are teaching students or educators how to (not) teach.

4) Oceanography and climate

Yes, this is probably the main reason why you are doing this experiment in class. Now you can talk about salt in the ocean. About density-driven currents (and are there other things that drive currents apart from density differences?). About the importance of ocean currents, heat transport, the global overturning circulation, fresh water and many more.

Can you think of more contexts for this great experiment? Let me know! (Depending on your browser, you can comment on this post in the “leave a reply” box below or, if you don’t see that box, by clicking the speech bubble next to the title of the post.)

Melting ice cubes – one experiment, many ways (post 3/4)

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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.

Demonstration of melting ice cubes. The melt water is clearly marked by the dye. This makes it a good demonstration, but diminishes the satisfying feeling of discovery by the observer, because the processes are clearly visible right away rather than having to be explored.

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.

How much salt is there in sea water?

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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.

A teaspoon full of salt corresponds to approximately 5 grams. That means that for typical open-ocean salinities, you have to add 7 teaspoons full of salt to a liter of water.

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.

10 grams of salt in a little plastic jar.

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!

12 x 10 grams of salt. It does look like a lot more, doesn’t it?

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?

Properties of sea ice and fresh water ice

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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.

Ice formed from fresh water (on the left) and salt water (on the right). Note the small pores in the salt water ice cube – those are the channels that form when brine is rejected.

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.

I first saw this experiment when Angelika Renner from the Norwegian Polar Institute in Tromsø visited my GEOF130 class last year. She says that she got the idea from the APECS book [link*], that, btw, provides many great ideas for outreach projects.

* I’m not affiliated, nor do I get money for recommending this book. It’s just a great resource that I think everybody should be aware of!

[edit 11.9.2013: new post on the same topic here: http://mirjamglessmer.com/2013/09/11/on-the-structure-of-fresh-water-and-salt-water-ice/]

Ice cubes melting in fresh water and salt water (post 2/4)

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The “ice cubes melting in fresh water and salt water” experiment the way I usually use it in class.

— Edit — For an updated description of this experiment please go to this page! — Edit —

You might remember the “ice cubes melting in fresh water and salt water experiment” from a couple of days ago. Today we are going to talk about it again, but with a little twist on it. See, when I showed you the experiment the other day, I used dyed ice cubes, so the melt water was colored and it was easy to track. Doing that, I focussed you attention on the melt water. This is not how we do it in class.

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.

Dyed ice cubes melting in fresh water (left) and salt water (right). Edited on Sept. 14th, 2014. Since this seems to be the most popular post on this blog I thought people might appreciate a better picture… And if you are really curious go check out the newer posts on the topic, a lot has happened over the last year!

Glasses filled with fresh water and salt water, and one ice cube in each. Drops of food dye have been added on the ice cubes to visualize the circulation. The left glass is homogeneously pink, whereas the right glass has a pink layer on top and only little pink below that layer.

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.

Sketch showing the explanation for why the ice cubes melt faster in fresh water than in salt water.

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!

The experiment run in the lecture theater.

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.

Adventures in Oceanography and Teaching

Dr. Mirjam S. Glessmer