Melting ice cubes
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.
Undergraduate course in physical oceanography, but also in outreach activities with school children or the general public.
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 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
density of cold vs fresh, warm vs cold water; density-driven currents
Higher order thinking skills goals for this activity
usage of the scientific method: formulation of hypotheses, testing, evaluating and reformulating.
Other skills goals for this activity
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), 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 below.
Presentation "melting ice cubes" (Acrobat (PDF) bytes Jun25 13)
Teaching Notes and Tips
Students usually assume that the ice cube in the salt water will melt faster than the one in fresh water, "because salt is used to de-ice streets in winter".
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
References and Resources
This activity has been discussed before, for example here: