Experiment: Ice cubes melting in fresh water and salt water

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.

Context

Audience

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.

 

Goals

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

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

Description

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.

Discussion

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 EMSEA14, people asked what would happen if the ice cubes were held at the bottom of the beaker.
    • 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!

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Dyed ice cubes about to be put into fresh water (left) and salt water (right)
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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.
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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!

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Cold (dark purple) ice cubes held by warm (white-ish) fingers over room-temperature (orange) cups with water
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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.
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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.

Assessment

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.

 

Equipment

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:

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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…

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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:

I have also written about it a lot on my blog, see posts tagged “melting ice cubes experiment“.

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.

Building a miniature well

Groundwater dynamics in the kitchen.

This activity is suitable for young children who wonder where the tap water comes from. All you need is some sand, an empty toilet paper roll, and some water.

First, you need to build your well. You could dig a hole into a sand-filled bucket and then put in the toilet paper roll, or you can just set the roll into a bucket and then fill sand around it (which is what I did).

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Put your empty toilet paper roll in a larger vessel and fill the space around the roll with sand.

Next, you “let it rain” on the sand to replenish the ground water.

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Let it “rain” to replenish the groundwater.

After a while, water starts collecting in the well, and the water level rises. It looks pretty yucky at first, clearly the sand I got from the playground is pretty dirty.

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After a while, the water starts rising up in the well.

Things to discuss:

  • How will different sands / soils influence the water quality in your well?
  • What could you do to improve the water quality?
  • What effect will torrential downpours (like what I did above) have on the water quality compared to nice and slow summer rain?

And then if you want to go there, you could discuss pollutants in the soil that will have an effect on water quality etc..

And you could actually try different sands / soils on water that you either color, or in which you dissolve other things, or in which you suspend things. But I was too lazy to do this for this post. But I might come back to this experiment for nicer pictures in natural light, but you might have to wait for that until next summer :-(

I really like this demo, it is quick and easy and nice if you don’t feel like digging a massive hole in your garden (which I did not).

Guest post: The mystery of the cold room

Guest post by Kristin Richter!
Today I’m excited to bring to you a guest post from Innsbruck, Austria, written by my friend Kristin Richter. Kristin ran the oceanography lab in Bergen before I took over, and she is a total enabler when it comes to deciding between playing with water, ice and food dye, or doing “real” work. Plus she always has awesome ideas of what else one could try for fun experiences. We just submitted an abstract for a conference together, so keep your fingers crossed for us – you might be able to come see us give a workshop on experiments in oceanography teaching pretty soon! But now, over to Kristin.
A little while ago, I made an interesting experience while presenting some science to students and the general public on the “Day of Alpine Science”  in Innsbruck using hands-on experiments. Actually, my task was to talk about glaciers but being a physical oceanographer I felt like I was on thin ice. Well, glaciers, I thought, hmmm … ice, melting ice, going into the sea, … sea, … sea ice! And I remembered how Mirjam once showed a nice experiment to me and some friends about melting ice in fresh and salt water. And suddenly I was all excited about the idea.
To at least mention the glaciers, I planned to fill two big food boxes with water, have ice float (and melt) in one of the tanks and put ice on top of a big stone (Greenland) in another tank filled with water to show the different impact of melting land ice and sea ice on sea level. Since melting the ice would take a while (especially on a chilly morning outside in early April) I would have enough time to present the “actual” experiment – coloured ice cubes melting in two cups of water – one with freshwater, and the other one with salt water.
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Melting ice. A comparison of sea ice and glaciers melting’s impact on sea level, ice cubes melting in fresh and salt water on the right. Photo by “Forschungsschwerpunkt Alpiner Raum”, University of Innsbruck.

As we expected many groups with many students, I needed a lot of ice. I told the organizers so (“I need a lot of ice, you know, frozen water”) and they said no problem, they will turn on their cooling chamber. The day before, I went there and put tons of water into little cups and ice cube bags into the chamber to freeze over night.The next morning – some hundreds of students had already  arrived and were welcomed in the courtyard – I went to get some ice for the first group. I opened the cooling chamber,… and froze instantly. Not so very much because of the cold temperature but because I was met by lots of ice cube bags and little cups with… water. Like in LIQUID WATER! Cold liquid water, yeah, but still LIQUID! Arrrghhhh, my class was about to begin in a few minutes and I had NO ICE. “Ah, yes”, volunteered the friendly caretaker, “come to think of it, it is just a cooling chamber!”I started panicking, until a colleague pointed out the Sacher Cafe (this is Austria after all) and their ice machine across the road. I never really appreciated ice machines, but that one along with the friendly staff saved the day. Luckily, I brought some colored ice cubes from at home – so I was all set to start.

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Ice melting in fresh water (left) and salt water (right). Photo by “Forschungsschwerpunkt Alpiner Raum”, University of Innsbruck.

And the station was a big success, the students were all interested, asked many questions and were excited about the colored melt water sinking and not sinking. :-)  I even managed to “steal” some students from the neighboring station of my dear meteorology colleagues. That was something I was particularly proud of as they could offer a weather station, lots of fun instruments to play with and a projector to show all of their fancy data on a big screen. (Actually, I also abandoned my station for a while to check out their weather balloon.)

Anyway, I had a lot of fun that day and could definitely relate to Mirjams enthusiasm for this kind of teaching. I can’t wait for the next opportunity to share some of those simple yet cool experiments with interested students. I will bring my own ice though!

 

Fjord circulation

Tank experiment on a typical circulation in a fjord.

Traditionally, a fjord circulation experiment has been done in GEOF130’s student practicals. Pierre and I recently met up to test-run the experiment before it will be run in this year’s course.

This is the setup of the experiment: A long and narrow tank, filled with salt water, a freshwater source at one end and an outlet at the other end. This sets up a circulation from the head towards the mouth of the fjord close to the surface, and a deep return flow.

Watch the movie below to see how different circulations are set up depending on the depth of the freshwater source. As in the picture, velocity profile 1 is for the case where freshwater is being added close to the surface, and in case 2 the freshwater is being added deeper down.

Progressive waves on a rope

Visualization of progressive waves: wave form and energy move forward while the rope itself stays in place.

When I talked about waves in GEOF130 recently, in order to explain the concept of progressive waves, I showed a drawing from one of the textbooks, where someone was moving a rope such that waves traveled on the rope. The idea was to show that for progressive waves the wave form and energy travel, while the matter itself stays more or less in place, only moving up and down or in circular orbital motions.

The look I got from one of the students for showing that drawing confused me a bit and I am still not sure whether it was a “I have no idea what you are trying to tell me!” or a “Duh! Are we in kindergarden?”, but I think it was probably closer to the former. So from now on I will carry a piece of rope on me to show this in lectures and to have students try themselves.

A wave shape traveling forward on a rope, while the rope itself stays in place.

I filmed a quick video because it was difficult to watch the wave while exciting it myself, but it turns out it is even more difficult to hold a camera more or less steady while exciting waves at the same time, plus the movement is pretty quick even for a camera as awesome as mine. Anyway, if you want to procrastinate learn more about waves, watch this!

Measuring temperature.

Students build thermometers.

As described in this post, I like to have students build “instruments” to measure the most oceanographic properties (temperature, salinity and density). I find that they appreciate oceanographic data much more once they have first-hand experience with how difficult it is to design instruments and make sense of the readings. Over the last two days I described the experiments for salinity and density, today it’s temperature.

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Students building thermometers.

Measuring temperature is probably the most difficult of the three properties. Firstly, there are lots of technical difficulties to be overcome. How can we seal the mouth of the bottle around the straw in a way that it is really water tight? How much water do we have to fill in the bottle? Does it matter if there are air bubbles trapped? What if the water level when we fill the bottle is not visible because of the seal? If the straw is clogged up with modeling clay, will we still be able to use it in the instrument? How long does the straw have to be above the seal in order to avoid water spilling out when the temperatures we try to measure become too hot?

Then, there are many problems connected to the actual measurement. If we lift up the thermometer (and hence squeeze the plastic bottle) – how does that influence our reading? Since we have half a liter of water in the thermometer, are we actually measuring the temperature of the water sample, or are we influencing it while trying to measure? How do we come up with a scale for our temperature measurements had I not supplied (mercury-free) thermometers to calibrate the new thermometer with? So many questions to think about and discuss!

Measuring salinity

Students evaporate water to measure the salinity of a water sample.

As described in this post, I like to have students build “instruments” to measure the most oceanographic properties (temperature, salinity and density). I find that they appreciate oceanographic data much more once they have first-hand experience with how difficult it is to design instruments and make sense of the readings. Today I’m presenting two groups that focused on salinity, while yesterday’s group was measuring density.

Students evaporate water to measure the salinity of a water sample.

The students in the course I currently teach were determined to not only evaporate some water to qualitatively look at how much salt was dissolved in the sample, they wanted to do it right. So they set out to measure the vessel, the sample and the remaining salt. But since measuring salinity is really pretty difficult, they ran into a couple of problems. First – my scales were nowhere near good enough to measure the amount of water they could fit into the evaporation cup with any kind of precision. Second, even the amount of water that they could fit took a lot longer to evaporate (or even boil) than anticipated. Third, they realized that even though they could see salt residue in the end, this might not be all the salt that had been there in the beginning, plus there was grime accumulating at the base of the cup, so weighing the cup in the end might not be the best option. But they still learned a lot from that experiment: For example that once the (small quantity of) water was boiling, it became milky very quickly and then turned to crystallized salt almost instantly. Or that in order to use this method, a tea candle is not as suitable as a heat source as a lighter (and there might probably even be even better ones out there).

P.S.: In this course, none of the groups set the wooden tongs on fire! :-)

Measuring density

Students build a device to measure density.

As described in this post, I like to have students build “instruments” to measure the most oceanographic properties (temperature, salinity and density). I find that they appreciate oceanographic data much more once they have first-hand experience with how difficult it is to design instruments and make sense of the readings.

Students designing a hydrometer.

Measuring (relative) density is by no means easy. There are a lot of conceptual things to understand when developing hydrometers – one question that comes up every time is what to mark. The first idea tends be to mark displacement on the cup – either a zero-mark without the hydrometer in the cup and then the water level once the hydrometer is floating in the water, or the level of the bottom of the hydrometer. Which makes it fairly hard to measure other samples than the ones the instrument was initially calibrated with.

The student group working on the task was faced with – and overcame – many difficulties. The straw was top-heavy, so in order to float more or less vertically, it had to be cut down. They had four different test solutions, but they did not know the densities of those solutions, only their salinities, hence they developed their own density scale relative to which they measured the density of their solution. Pretty ingenious!

Tasting sea water reloaded

Doing the “tasting sea water” activity again with a different group of students.

A very good introduction to the concept of salinity is the “tasting sea water” activity. Last time I ran that activity, students were very quick to correctly connect the samples with the correct sampling locations without much discussion going on. This time round, though, there was a lot of discussion. Students quickly sorted samples in order of increasing salinity, but there was no agreement to be found on whether the Baltic or the Arctic should be fresher. Since I only pointed to a location and didn’t specify the depth at which the sample had been taken, some students argued that the Arctic was very fresh at the very top, whereas the Baltic was brackish. Others said that the Baltic was a lot fresher than any oceanic location.

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Students tasting four different samples of “sea water” with salinities corresponding to Arctic sea ice, the Baltic sea, the open ocean and the Mediterranean. Samples have to be associated with locations on a map.

In another group, there was a big discussion going on about how in marginal seas, evaporation or precipitation can dominate.

It is always great to see how much you can discuss and learn from an activity as simple as this one!

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.

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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: