Ice From the Ocean vs Ice From the Sky – Can You Tell the Difference?

Ice that forms in the polar oceans is an important driver behind the global climate. This ice is physically different from frozen precipitation in a number of different ways. In this brief inquiry activity, students make qualitative observations about two types of ice cubes and deduce ice composition based on their observations. This activity may serve as an introduction to a variety or chemical/physical/environmental science concepts.

Objectives

Students will be able to:

• Understand the physical differences between sea ice and freshwater ice.
• Discuss the formation of each type of ice and what leads to these differences.

Lesson Preparation

• For a class of 30 students, working in pairs, assemble the materials listed.
• Prepare the day before by preparing a saltwater solution with the proportion of 3.5 grams of salt per 100mL of water. Fill two of the four ice cube trays with saltwater solution. Fill the remaining two trays with freshwater. Place all trays in the freezer.

Procedure

• At the beginning of the activity, - without telling students which is which - issue one “sea” ice cube, one “sky” ice cube, and some paper towels to each student pair. Ask them to handle each cube, comparing and contrasting appearance, hardness, weight, temperature (by feel), texture, smell, and lastly, taste (but only if ice cube trays were clean). Have them record their observations down.
• Issue each student pair a dropper of food coloring. Reminding them that food coloring stains skin and clothing, ask them to carefully drip one or two drops on each ice cube and note the differences they see.

Extension

This inquiry is appropriate for all grade levels and is intended to be a suitable introduction to any number of chemical, physical, and environmental science lessons. Some examples:

• Chemistry/Physics – solutes like NaCl depress the freezing point of fluids so the two types of ice cubes should register two different temperatures when measured with precise thermometers. Additionally, “sea” ice has greater porosity because solutes stay in solution as solute-free ice forms around them. This creates microscopic “brine channels” which cause the sea ice to be weaker than freshwater ice when the two are compressed. The freezing process also leaves a small amount of liquid brine in the bottoms of these ice cube trays. The increased concentration of NaCl can be measured with a refractometer or other spectroscope.

• Environmental/Earth Science – Thermohaline circulation (the global ocean conveyor) is partially driven by salty water in the polar seas sinking to the ocean floor and pulling warmer surface waters towards the poles. This is demonstrated by way of the brine that finds its way to the bottom of the ice cube tray.

Resources

Lesson Materials attached.

Assessment

Students are assessed on their deductive reasoning skills via verbal questioning and written assessment. See Lesson Materials.

Author/Credits

Created by Timothy Dwyer from experiences and photographs taken during the Polar Gigantism in Antarctica 2016 expedition. All photos are copyright Timothy R. Dwyer (PolarTREC 2016), courtesy of ARCUS.