Eddies, those large, rotating structures in the ocean, are pretty hard to imagine. Of course, you can see them on many different scales, so you can observe them for example in creeks, as shown below:
Eddies in the Pinnau river, and their dark “shadows”.
If you can’t really spot them in the image above, check out this post for clues and a movie.
So how can you create eddies to observe their structure?
Dye spiral caused by an eddy in a jar
I took a large cylindrical jar, filled it with water, stirred, let it settle down a little bit and then injected dye at the surface, radially outward from the center. Because the rotating body of water is slowed down by friction with the jar, the center spins faster than the outer water, and the dye streak gets deformed into a spiral. The sheet stays visible for a very long time, even as it gets wound up tighter and tighter. And you can see the whole eddy wobble a bit (or pulsate might be the more technical term) because I introduced turbulence when I stopped stirring.
Watch the movie below if you want to see more. Or even better: Go play yourself!
P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 27th, 2015.
I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.
Explore how melting of ice cubes floating in water is influenced by the salinity of the water. Important oceanographic concepts like density and density driven currents are visualized and can be discussed on the basis of this experiment.
This hands-on experiment is suited for many different audiences and can be used to achieve a wealth of different learning goals. Audience ranges from first-graders over undergraduates in physical oceanography to outreach activities with the general public. Depending on the audience, this activity can be embedded in very different contexts: For children, either in their physics teaching to motivate learning about concepts like density, or in the context of learning about the climate system and ocean circulation. For college/university students the activity can either be used in physics teaching to get a different view on density; in oceanography/Earth science to talk about ocean circulation and processes that are important there; to motivate the scientific process; or to practice writing lab reports (you can be sure that students will at some point be tasting the water to make sure they didn’t accidentally swap the salt water and fresh water cup – a great teachable moment for a) Never putting anything in your mouth in a laboratory setting, and b) Always documenting exactly what you are doing because stuff that you think you will definitely remember obviously isn’t remembered that clearly after all). For the general public, this is typically a stand-alone activity.
Skills and concepts that students must have mastered
It helps if the concept of density is known, but the experiment can also be used to introduce or deepen the understanding of the concept.
How the activity is situated in the course
I use this activity in different ways: a) as a simple in-class experiment that we use to discuss the scientific method, as well as what needs to be noted in lab journals and what makes a good lab report, or density-driven circulation; b) to engage non-majors or the general public in thinking about ocean circulation, what drives ocean currents, … in one-off presentations.
Content/concepts goals for this activity
Students learn about concepts that are important not only in physical oceanography, but in any physical or Earth science: density in general; density of water in particular, depending on the water’s temperature and salinity; how differences in density can drive currents both in the model and in the world ocean; how different processes acting at the same time can lead to unexpected results; how to model large scale processes in a simple experiment. After finishing the activity, they can formulate testable hypotheses, are able to reason based on density how a flow field will develop and they can compare the situations in the cups to the “real” ocean.
Higher order thinking skills goals for this activity
Students learn about and practice the use of the scientific method: formulation of hypotheses, testing, evaluating and reformulating.
Other skills goals for this activity
Students practice writing lab reports, making observations, working in groups.
Description and Teaching Materials
(per group of 2-4 students):
1 clear plastic cup filled with room-temperature salt water (35psu or higher, i.e. 7 or more tea spoons of table salt per liter water), marked as salt water (optional)
1 clear plastic cup filled with room-temperature fresh water, marked as fresh water (optional)
2 ice cubes
liquid food dye either in drop bottle, with a pipette or with a straw as plunging syphon
Before the experiment is started, students are asked to make a prediction which ice cube will melt faster, the one in salt water or the one in fresh water. Students discuss within their groups and commit to one hypothesis.
Students then place the ice cubes into the cups and start a stop watch/note the time. Students observe one of the ice cube melting faster than the other one. When it becomes obvious that one is indeed melting faster, a drop of food dye can be added on each of the ice cubes to color the melt water. Students take the time until each of the ice cubes has melted completely.
The ice cube in the cup containing the fresh water will melt faster, because the (fresh) melt water is colder than the room-temperature fresh water in the cup. Hence its density is higher and it sinks to the bottom of the cup, being replaced by warmer waters at the ice cube. In contrast, the cold and fresh melt water in the salt water cup is less dense than the salt water, hence it forms a layer on top of the salt water and doesn’t induce a circulation like the one in the fresh water cup. The circulation is clearly visible as soon as the food dye is added: While in the freshwater case the whole water column changes color, only a thin meltwater layer on top of the salt water is colored (for clarification, see images in the presentation below)
Teaching Notes and Tips
Students usually assume that the ice cube in salt water will melt faster than the one in fresh water, “because salt is used to de-ice streets in winter”. Have students explicitly state their hypothesis (“the one in salt water will melt faster!”), so when they measure the time it takes the ice cubes to melt, they realize that their experiment does not support their hypothesis and start discussing why that is the case. (Elicit the misconception, so it can be confronted and resolved!)
My experience with this experiment is that all groups behave very consistently:
At least 80% of your audience will be very sure that the ice cube in salt water will melt faster than the one in fresh water. The other 20% will give the correct hypothesis, but only because they expect a trick question, and they will most likely not be able to come up with an explanation.
You can be 100% sure that at least in one group, someone will say “oh wait, which was the salt water again?” which hands you on a plate the opportunity to say “see — this is a great experiment to use when talking about why we need to write good documentations already while we are doing the experiment!”
You can also be 100% sure that in that group, someone will taste the water to make sure they know which cup contains the salt water. Which lets you say your “see — perfect experiment to talk about lab safety stuff! Never ever put things in your mouth in a lab!”
You can also be sure, that people come up with new experiments they want to try.
At a workshop on inquiry-based learning, people asked what the dye would do if there was no ice in the cups, just salt water and fresh water. Perfect opportunity to say “try! Then you’ll know! And btw — isn’t this experiment perfect to inspire the spirit of research (or however you would say that in English – “Forschergeist” is what I mean!). This is what you see in the pictures in this blogpost.
It is always a good idea to have plenty of spare ice cubes and salt/fresh water at room temperature ready so people can run the experiment again if they decide to either focus on something they didn’t observe well enough the first time round, or try a modified experiment like the ones described above.
A reviewer of this activity asked how easily students overcome the idea that water in the cup has to have just one temperature. In my experience this is not an issue at all – students keep “pointing” and thereby touching the cups, and in the thin-walled plastic cups I typically use the temperature gradient between “cold” melt water and “warm” salt water is easily felt. The (careful!) touching of the cups can also be explicitly encouraged.
Different ways to use this experiment
This experiment can be used in many different ways depending on the audience you are working with.
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. And remember in order for demonstrations to increase the learning outcome, they need to be embedded in a larger didactical setting, including forming of hypotheses before the experiment is run and debriefing afterwards.
Structured activity: For an audience with little knowledge about physics, you might want to start with a very structured activity, much like the one described above. 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”).
Problem-solving activity: Depending on your goals with this experiment, you could also consider making it a problem-solving activity: You would hand out the materials and ask the students to design an experiment to figure out which of the cups contains fresh water and which salt water (no tasting, of course!). This is a very nice exercise and students learn a lot from designing the experiment themselves.
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, so maybe give it a try?
Problem-based learning: This experiment is also very well suited in a Problem-Based Learning setting, both to work on the experiment itself or, as we did, to have instructors experience how problem-based learning works so they can use it in their own teaching later. Find a suggested case and a description of our experiences with it here.
Inquiry-based learning: Similarly as with Problem-Based Learning, this experiment can be used to let future instructors experience the method of inquiry-based learning from a student perspective. For my audience, people teaching in STEM, this is a nice case since it is close enough to their topics so they can easily make the transfer from this case to their own teaching, yet obscure enough that they really are learners in the situation.
Pro tip: If you are not quite sure how well your students will be able to cope with this experiment, prepare ice cubes dyed with food coloring and use them in a demonstration if students need more help seeing what is going on, or even let students work with colored ice cubes right from the start. If ice cubes and hence melt water are dyed right away, it becomes a lot easier to observe and deduct what is happening. Feel free to bring the photos or time lapse movie below as a backup, too!
Dyed ice cubes about to be put into fresh water (left) and salt water (right)
When the ice cubes start melting, it becomes very clear that they do so in different manners. In the left cup, the cold meltwater from the ice cube is denser than the lukewarm water in the cup. Hence it sinks to the bottom of the beaker and the water surrounding the ice cube is replaced by warmer water. On the right side, the lukewarm salt water is denser than the cold melt water, hence the cold meltwater floats on top, surrounding the ice cube which therefore melts more slowly than the one in the other cup.
The ice cube in the fresh water cup (left) is almost completely gone and the water column is fairly mixed with melt water having sunk to the bottom of the beaker. The ice cube in the salt water cup (right) is still a lot bigger and a clear stratification is visible with the dyed meltwater on top of the salt water.
And here a time-lapse movie of the experiment.
Another way to look at the experiment: With a thermal imaging camera!
Cold (dark purple) ice cubes held by warm (white-ish) fingers over room-temperature (orange) cups with water
After a while, both cups show very different temperature distributions. The left one is still room temperature(-ish) on top and very cold at the bottom. The other one is very cold on top and warmer below.
When you look in from the top, you see that in the left cup the ice has completely melted (and the melt water sunk to the bottom), whereas in the right cup there is still ice floating on top.
Depending on the audience I use this experiment with, the learning goals are very different. Therefore, no one assessment strategy can be used for all different applications. Below, I am giving examples of what are possible ways to assess specific learning goals:
– Students apply the scientific process correctly: Look at how hypotheses are stated (“salt melts ice” is not a testable hypothesis, “similar-sized ice cubes will melt faster in salt water than in fresh water of the same temperature” is).
– Students are able to determine what kind of density-driven circulation will develop: Suggest modifications to the experiment (e.g. ice cubes are made from salt water, or ice cubes are held at the bottom of the cups while melting) and ask students to predict what the developing circulation will look like.
– Students can make the transfer from the flow field in the cup to the general ocean circulation: Let students compare the situation in the cup with different oceanic regions (the high Arctic, the Nordic Seas, …) and argue for which of those regions displays a similar circulation or what the differences are (in terms of salinity, temperature, and their influence on density).
In general, while students run the experiment, I walk around and listen to discussions or ask questions if students aren’t already discussing. Talking to students it becomes clear very quickly whether they understand the concept or not. Asking them to draw “what is happening in the cup” is a very useful indicator of how much they understand what is going on. If they draw something close to what is shown on slide 28 of the attached slide show, they have grasped the main points.
Don’t worry, it is totally feasible to bring all the equipment you need with you to run the experiment anywhere you want. This is what we brought to EMSEA14 to run the workshop three times with 40 participants each:
What we brought to EMSEA14 to run workshops on the ice cubes melting in fresh and salt water experiment
In one big grocery bag:
4 ice cube trays
4 ice cube bags (backup)
2 thermos flasks (to store ice cubes)
1 insulating carrier bag (left)
4 empty 1.5l water bottles to mix & store salt water in
1 tea spoon for measuring salt
500g table salt
21 clear plastic cups for experiments
10 clear plastic cups to hand out ice cubes in
11 straws (as pipettes)
1 flask of food dye
11 little cups with lids to hand out food dye in
nerves of steel (not shown :-))
And if you are my friend, you might also get the “ice cube special” — a pink bucket with all you will ever need to run the experiment! Below is what the ice cube experiment kit looks like that I made for Marisa, with labels and everything…
An “ice cube experiment” kit that I made for a friend. Want one, too?
References and Resources
This activity has been discussed before, for example here:
P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 4th, 2015.
I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.
Guest post by Susann Tegtmeier (written two months ago, I just never got around to posting it. Sorry!)
No one likes clouds when they bring rain, but what if you could make your own? Making a cloud inside a bottle will help us to understand how they are formed in the atmosphere. The experiment demonstrates how changes in air pressure, temperature and volume are related and how these changes can lead to the sudden appearance of tiny water droplets, or in other words, lead to the formation of a cloud.
You can do the experiment alone at home, in front of a classroom or as a hands-on experiment with all your students. I have chosen the latter option as part of my ‘Introduction to meteorology’ lecture for the first-year students in the Bachelor program ‘Physics of the Earth System’. For this class, Mirjam and I received funding from our university’s PerLe project for teaching innovations. We use the PerLe funding to consolidate the student’s physical-based understanding of the climate system through various experiments, exercises and discussions.
For the experiment you need an air-tight, transparent container that you can pump up with air (in order to increase the pressure inside the bottle). We made a simple version using materials from home including a plastic water bottle supplemented with valve from a bike tire that is attached between the bottle and the cap. Furthermore you need a pump (in our case a bike pump), water and matches.
Picture by Susann Tegtmeier
During the first round of the experiment, the students pumped up the bottles enhancing the pressure inside. During our discussion before the experiment, the students assumed correctly that the bottles would warm due to the enhanced pressure under a constant volume. By putting their hands around the bottles, it was possible for the students to feel that indeed the air inside the bottles was warming. When opening the valve slowly the opposite effect could be noticed and the bottles cooled very quickly. While the temperature change is small, it turned out to be quite fascinating and memorable for the students to see and feel the ideal gas law, they learned about earlier in class, in real life action.
During the second round of the experiment, the pumping up of the bottles was repeated, but this time with a small amount of water in the bottles. Since warm air can take up more water vapor than cold air, some of the water in the bottle was evaporated during the increase of pressure and temperature. While we discussed this effect during the experiment, it was, of course, not possible to observe the formation of the invisible water vapor. The next step of the experiment, the opening of the valve and the accompanying cooling of air, can theoretically lead to the condensation of the above discussed water vapor back to water. However, to the surprise of the students, no condensing little water droplets could be seen in the bottles.
Picture by Susann Tegtmeier
In order to lift the mystery, we carried out the third part of the experiment. With the bottle open, we lit a match and a moment later threw the blown out, smoking match into the bottle. Now the bottle needs be closed quickly before the same action (pumping of bottles and opening of valve) can be repeated. Only in this last round of the experiment, the expected water droplets became visible while the air was cooling. The reason is that small condensation nuclei are necessary for water vapor to condense and form water droplets. The experiment demonstrates this effect quite nicely in the bottle, but it also holds on large scales for the formation of atmospheric clouds.
The ‘Cloud in a bottle’ experiment is a perfect class room exercise, as it leads the students within 30 min from the basic, physical principles of the ideal gas law to one of the big climate effects, the aerosol – cloud interaction.
My friends know me well. Especially A&I, which was proven again when they sent me the link to an article about two things that I am mildly obsessed with: Latte and double-diffusive mixing.
My obsession with latte is a fairly recent thing, but I have been known to blog about interesting convection pattern in it (for example here). The obsession with double-diffusive mixing, however, is well documented for more than the last 12 years (for example when I am writing experimental instructions, poems or scientific articles about it).
The double-diffusive process that I have been most concerned with is salt fingering, because it is oh-so-pretty, and also fool-proof to create for teaching purposes (when you know how to do it).
Diffusive layering I seem have to be a little frustrated with, at least in teaching (but reading back this post now, it turns out that that was entirely my own fault and not my students’. Oh well, you live and learn! Isn’t this exactly the kind of stuff that makes for great teaching portfolios? ;-)).
And it also turns out that I did the experiments themselves all wrong. According to the article “laboratory layered latte” by Xue et al. (2017). I should not have been trying to carefully stratify a tank in order to see diffusive layering. Instead, I should just have quickly poured the lower density fluid into the higher density one, and layers would have formed by themselves!
So there is one thing that you won’t see any time soon:
Yep. Me drinking latte from any kind of vessel that doesn’t let me look at the stratification! I don’t know how I could ever have fallen into the trap of missing out on observing fluid dynamics while having my early morning coffee in the office. Now I urgently need a nice glass mug!
And you should go check out the article, it’s a really nice read. My new ambition in life: Write a fluid dynamics research article that applies the FD to some really cool, yet mundane, every day thing. Are you in, Elin? :-)
Xue, Nan and Khodaparast, Sepideh and Zhu, Lailai and Nunes, Janine K. and Kim, Hyoungsoo and Stone, Howard A., Laboratory layered latte.Nature Communications 8(1), 2017
For a popular science presentation on climate change, I needed a simple illustration for how ice cores can be used as archives of past climates. Luckily, my sister and family were excited to do some early Christmas baking for climate science!
And playing with food colors is always fun…
I think I had too much fun playing, actually, the “ice core data” would have been a lot easier to interpret if the different layers were just laying flat!
I should probably noticed here already that the color pattern wasn’t as regular as it should have been for easy interpretation of the core data later…
But it was fun! And they rose beautifully even though we were a little afraid that the time between mixing in the baking powder and actually baking the muffins was kinda long (because we had to mix in all the different dyes…)
The really difficult part, it turns out, was the coring itself. I had wider-than-usual straws, but instead of just cutting out the core, it was really difficult to have them pierce through the crust, and they compressed the core much more than I had hoped.
You can kind of see where the core goes in the cross section, and how the different colors correspond to their old locations inside the cupcake. But somehow this worked much better in my imagination than it did for real!
And I have a new-found appreciation for food bloggers. It’s really difficult to take good pictures of food!
But in case you were wondering: They taste just like boring, non-rainbow muffins. And my niece liked them! :-)
What keeps you entertained at conference dinners is probably different for different people, but we quite enjoyed watching how the candles placed closer to the door to the balcony burned a lot faster (and a lot more messy) than those on other tables…
How much salt is there in sea water? What concentration do you need before crystals start forming? What will those crystals look like? I am sure those are the kind of questions that keep you awake at night!
Of course this can easily assessed experimentally. On a visit to the University of Bergen’s Centre for Science Education just now, I was shown the result of such an experiment: A litre of water was mixed with 35 grams of salt to simulate sea water with its typical salinity. Below, you see what the beaker looked like after sitting out for three months.
You can see that salt crystals are forming at the walls of the beaker, but that their structure depends on depth below the initial water level (see the 1000 ml mark on the beaker).
When there is still a lot of water in the beaker, crystals look like ornate flowers. Then, the less water is left in the beaker, the more square the crystals become. And at the bottom of the beaker, you see the typical salt crystals you would expect.
Actually, even though they look like the kind of salt crystals I would expect, apparently someone who knows about crystallography commented that there must be other stuff in there than just cooking salt since the crystals don’t look the way they should. I need to read up on this! :-)
Anyway, this is an experiment that I want to do myself, so maybe in three months time there will be more pictures of this!
Thanks for a very nice lunch, Olaug, Frede, Andreas, Morven and Elin! Looking forward to working with you a lot more in the future! :-)
P.S.: with this blog post I am testing to blog pretty much “real time” from my mobile phone, so if you notice anything odd, please let me know!
The other day I was sitting in my conservatory with a friend when I had to take the photo below:
Can you see how one bottle refracts light and the other one does not? What does this tell us about whether there is water in either of those bottles? I met most “normal” people wouldn’t even notice a difference.
I know, I’m a nerd, but I have so much fun “discovering” small stuff like that! :-)