Melting ice cubes
Skills and concepts that students must have mastered
How the activity is situated in the course
Content/concepts goals for this activity
Higher order thinking skills goals for this activity
Other skills goals for this activity
Description and Teaching Materials
(per group of 2-4 students):
- 1 clear plastic cup filled with room-temperature salt water (35psu or higher), marked as salt water
- 1 clear plastic cup filled with room-temperature fresh water, marked as fresh water
- 2 ice cubes
- liquid food dye either in drop bottle or with a pipette
DescriptionBefore 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.
DiscussionThe 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)
This activity can be used for many different purposes (outreach, teaching majors, teaching minors, ...) and using many different methods (inquiry-based, free activity, ...). For details see the presentation attached to this activity.
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.
In interested groups modifications of the experiment are sometimes suggested, for example which of the ice cubes would melt faster if the ice cubes were held at the bottom of the beakers (and so the one in freshwater would be cooled by the melt water and the one in salt water would be surrounded by "warmer" waters, because the melt water would raise to the surface of the beaker). 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 one 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.
This experiment can be used in many different ways depending on the audience you are working with. For an audience with little knowledge about physics, you might want to start with a very structured activity, much like the one described in the slides attached to this 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!). Or you could just hand out the materials and let them design an experiment to find out as much as possible about density without telling them what they are looking for. More details here: http://mirjamglessmer.com/2013/09/07/melting-ice-cubes-one-experiment-many-ways-post-34/
This experiment is also very well suited in a Problem-Based Learning setting, as I describe here: http://mirjamglessmer.com/2015/06/05/oceanography-pbl-workshop-and-case/
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. See this link of a time lapse movie of the experiment with dyed ice cubes (and please feel free to bring it to your class to show, or to use as a backup plan! https://vimeo.com/139232370)
Depending on the audience I use this experiment with, the learning goals are very different. Therefore, not 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.
References and Resources
- http://mirjamglessmer.com/2013/09/07/melting-ice-cubes-one-experiment-many-ways-post-34/ for different ways to use this experiment as described above, or
- http://mirjamglessmer.com/2015/06/05/oceanography-pbl-workshop-and-case/ on how to construct a Problem-Based Learning course around the experiment