And now we’ve reached the heights where there is fresh snow! And also where there are wild rivers. Don’t know which I find more exciting :-)
Ok, yeah, it’s definitely the rivers that I find more exciting. Also, isn’t it funny that at first glance, the river seems to be flowing left-to-right, because that’s the direction in which the waves break? It’s flowing right-to-left, though, and the waves are breaking in an upstream direction because the water is just flowing so fast, it’s ripping the carpet out from underneath their feet, so to speak.
And now we have arrived in winter wonder land :-)
…where there is some new ice on the more sheltered parts of the lakes, too!
And now we on our way back down on the west side of the mountains. Snow only on the higher peaks, not at ground level any more.
And now it’s properly overcast and occasionally also snowing!
But let’s end this part of the journey with this beautiful outflow of the lake, and the standing waves that look as if they were braided into each other. So pretty!
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.
It’s a little more difficult to see in the picture below, where a storm drain drips into the fjord. Can you make out how the wave rings are spreading through the thin ice?
And one thing that I found super fascinating was that ducky ice-breaking: You see the open water bit just behind it, and there are a lot of waves. But then you also see those waves spreading into the ice! Poor ducky must have put a lot of energy into this…
My favourite picture, though is the one below: A crisscross of waves and an ice floe that moves (obviously) with both wave fields!
This even works if there is more ice than just one lonely flow:
One thing I really like is watching ice form. Well, maybe not watching the actual freezing (don’t have the attention span for that) but looking at all the different stages.
At first, you have all the small, individual needles that still slush around.
Then, the needles start sticking together, and if there is a little wave action, the ice breaks apart into individual pancakes, which bump into each other and start piling up along the edges.
If the wave action isn’t too bad, those pancakes can freeze together, forming a closed ice cover.
Sometimes you can also see several stages at once, like in the picture below: Open water out on the fjord, slush a little further in, then smaller pancakes and then larger pancakes. Not very surprising: Most wave action in the middle of the fjord, and the further you get towards the shore, the more waves have been dampened by the ice, so the larger the ice floes can grow.
Here you can actually see waves going through the ice-covered area, giving you an idea of how flexible the pancakes still are. More on that in the next post… ;-)
Here you see all the stages in one pic again. Together with the raising sun it makes for very pretty pictures! :-)
One of my favourite phenomena right now is desublimition, or deposition: The phase transition of water vapour to ice that doesn’t go through the liquid phase. It happens when moist air is cooled below the dew point and condensation doesn’t occur spontaneously: When the supercooled water vapour then gets in touch with a cold surface, it turns to ice immediately. And the results are incredibly beautiful!
These pictures are all from a trip I took with my godson and his family to Möhne Reservoir, the largest artificial lake in western Germany. You can see we were actually on a shore: What a surreal mixture of shells, leaves and frost flowers.
Frost flowers on ice cream. You must have seen them before: They sometimes occur when you’ve had some ice cream, put the left-overs back in the freezer, and take them out again. And there you have it: Water-ice crystals all over your lovely ice cream! Completely annoying because, obviously, they only taste like water and mess up your whole ice cream experience (or is that only me)?
Frost occurs when water vapour freezes without going through the liquid phase. Look at the awesome crystals!
Once I started thinking about the process that formed the ice and realised that those were actually frost and not just ordinary ice crystals, they all of a sudden stopped being annoying and instead became something that I kinda look forward to finding when I open a tub of my frozen blended strawberries. Because the structures are different every time, and really really pretty! And also how awesome is it to know that those ice crystals formed from water that wasn’t even liquid? Yes, this is the kind of stuff that makes me happy! :-)
Using the “melting ice cube” experiment to let future instructors experience inquiry-based learning.
I recently (well, last year, but you know…) got the chance to fill in for a colleague and teach part of a workshop that prepares teaching staff for using inquiry-based learning in their own teaching. My part was to bring in an experiment and have the future instructors experience inquiry-based learning first hand.
So obviously I brought the ice cubes melting in fresh water and salt water experiment! (Check out that post to read my write-up of many different ways this experiment can be used, and what people can learn doing it). On that occasion the most interesting thing for me was that when we talked about why one could use this — or a similar — experiment in teaching, people mainly focussed on the group aspect of doing this experiment: How people had to work together in a team, agree to use the same language and notation (writing “density of water at temperature zero degree Celsius” in some short syntax is not easy when you are not an oceanographer!).
And this experiment never fails to deliver:
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 documentation 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 EMSEA14, people asked what would happen if the ice cubes were at the bottom of the beaker. Today, 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 blog post.
So yeah. Still one of my favorite experiments, and I LOVE watching people discover the fascination of a little water, ice, salt and food dye :-)
Photo taken by Ulrike Bulmann
Photo taken by Ulrike Bulmann
Btw, when I gave a workshop on active learning last week and mentioned this experiment, people got really really hooked, too, so I’ll leave you with a drawing that I liked:
Frost flowers! I learned about those in the context of Arctic and Antarctic ice formation. I kinda assumed that ice flowers only formed in salt water, because I remember hearing about how the ice needles that form wick up brine and that, due to their large surface (which you will remember noticing in the last post where we looked at them forming on trees), they are super important in the air-sea exchange of all kinds of stuff, like for example bromine. So imagine my excitement when I saw them growing the other day!
Frozen Schlei river in Schleswig
Frost flowers are really pretty by themselves, but they also tell us a lot about recent weather conditions. For example, they only form when the air is A LOT colder than the water/ice surface. Do you know the snowy ice crystals you sometimes find on the inside of ice cream containers when you’ve opened and refrozen them? Yep – same thing! I even suspect that the ice crystals I was talking about in this post are also frost flowers.
I find it really fascinating how they are distributed over the larger surface of the Schlei river.
Schlei river in Schleswig coated in frost flowers
Here, for example, you see them forming on the edges of ice that has been broken up by some mechanical process. Judging from their placement, I would suspect that they only formed after the ice was broken and some of the pieces tilted up.
Cracked ice and frost flowers
Here, they were probably everywhere, but then the ice got broken up and some parts submerged. When the water there refroze, no snow flowers formed, just “normal” ice. However, the existing snow flowers seem to have continued growing!
Ice with frost flowers. Partially submerged and then refrozen into “normal” ice
The really interesting thing is that frost flowers don’t actually form from the water that is freezing below, but from water vapour in the air. Which, btw, explains why they can form on benches, ice cream lids or trees (all of which would be really difficult if they could only form on open water surfaces).
Ice with frost flowers. Partially submerged and then refrozen into “normal” ice
Above you see a larger part of the Schlei’s surface: Seems like there used to be frost flowers everywhere, but when the ice broke up, some of it got pushed out of the water, and as such preserving the frost flowers and letting them continue to grow. Meanwhile, other parts got flooded and only normal ice formed there. Maybe because the temperature gradient at that point wasn’t large enough any more?
Isn’t this just beautiful??? I could watch this all day, every day.
Frozen Schlei river in Schleswig with frost flowers
But let’s look at some more details. No idea why that patch of frost flowers formed there! But they seem to always start in small patches, which eventually grow together if the conditions are stable enough over long enough periods of time.
Frost flowers on ice
Here, we see the opposite situation to the one a couple of pictures up: “Normal” ice had formed, and then was broken up. And then, when the crack froze over, frost flowers formed!
What happens when water vapour freezes to ice without going through the liquid phase? Frost flowers!!!
That’s when trees suddenly look like this:
Btw – the stem of that tree is painted white! That’s just to confuse you a little but…
But let’s take a closer look. This is what the branches look like: Tiny ice needles growing on the individual pine needles! And the orientation of the image below is correct. They are growing to the side!
You can clearly see them all growing to one direction, to one side!
When you take off a bit of frost, this is what it looks like. Needles, but with a fractal 3D structure! Since what happened here (water vapour freezing without becoming liquid in between) is basically snow forming on the surfaces down here instead of in the clouds up above, it isn’t too surprising that snow is exactly what the frost bits feel like.
A piece of frosting. This picture isn’t blurry – the ice needles have a fractal 3D structure!
Look below, you can clearly see the frost only growing to one side (and this picture is the right way up, too!):
Frosting on tree branches
Doesn’t it make you want to sit there and just watch?
What a nice picknick spot!
Although every time the slightest of breezes comes, this is what happens:
Tree being de-frosted by wind
Also really cool: These plants growing on a balcony behind a glass railing. Only the tips have been frosted!
Plants on balcony with frosted tips
And if you were wondering what this post has to do with oceanography, check out the image below. Can you spot it?
Frozen Schlei river in Schleswig
Can you spot it now? No, not my niece (although she is pretty cool, too!), the frost flowers!
For most of my readers it might be pretty obvious what the movement of floating ice says about the flow field “below”, but most “normal” people would probably not even notice that there is something to see. So I want to present a couple of pictures and observations today to help you talk to the people around you and maybe get them interested in observing the world around them more closely (or at least the water-covered parts of the world around them ;-)).
For example, we see exactly where the pillars of the bridge I was standing on are located in the river, just by looking at the ice:
What exactly is happening at those pillars can be seen even more clearly when looking at a different one below. You see the ice piling up on the upstream side of the pillar, and the wake in the lee. Some smaller ice floes get caught in the return flow just behind the pillar. Now imagine the same thing for a larger pillar – that’s exactly what we saw above!
And then we can also see that we are dealing with a tidal river. Looking at the direction of the current only helps half of the time only, and only if we know something about the geography to know which way the river is supposed to be going.
But look at the picture below: There we see sheets of ice propped up the rails where the rails meet the ice, and more sheets of ice all over the shore line. As the water level drops due to tides, newly formed ice falls dry and that’s all the sheets of ice you see on land.
The bigger ice floes in the picture have likely come in from the main arm of the Elbe river.
Small port on a tiny bay on the Elbe river in Hamburg. Look at the sheets of ice on shore!
It is actually pretty cool to watch the recirculation that goes on in all those small bays (movie below picture). Wouldn’t you assume that they are pretty sheltered from the general flow?