NOVA Online | Shackleton's Antarctic Odyssey

Icebergs Ahead!

Lesson Objectives

By the end of this activity, students will be able to:

understand how icebergs are suspended in water.
understand why ice floats.
describe the melting process of an iceberg.
differentiate between the floating behavior of ice and cork.

Related National Standards

National Science Education Standards (National Research Council)
Grades 5-8
Science Standard B: Physical Science
Properties and changes of properties in matter

A substance has characteristic properties, such as density, a boiling point, and solubility, all of which are independent of the sample.

Materials Needed for Each Group

2 8-ounce paper cups

1 glass measuring cup

4 glass beakers (400 ml size)

water

mud or fine clay

graphite or charcoal dust

cork stopper

access to a freezer

balance

Estimated Time to Complete Lesson

The paper cups should be filled as described in Procedure Step #1 the day before the activity is done in class and placed into a freezer overnight. The activity itself should take one class period.

Teaching Strategy

Background information
One milliliter (cubic centimeter) of liquid water weighs 1 gram, (i.e. the density of water is 1.00 g/ml). Ice floats in water because its density (0.92 g/ml) is slightly less than that of water. Students will discover this when they compare the mass and volume of liquid water with the mass and volume of the same cup of water after it is frozen. The ice cup will weigh the same as when it was in liquid form but the volume of ice in the cup will be larger than what it was in liquid form. This is because when ice crystallizes into its hexagonal structure the water molecules are farther apart than when they are in the liquid state.

The density of the ice is so close to that of water that when ice floats more than 90 percent of its volume is under the water. In contrast, a piece of wood—whose density is 0.5 g/ml—will float with half its volume under water since its density is half that of water. A piece of cork with a density of 0.2 g/ml will float with only 20 percent of its volume under water. While this activity demonstrates how much of an iceberg actually exists under water, the coffee cup-sized iceberg model differs from actual Antarctic icebergs in several major aspects: its composition (mud and water), its shape (cylindrical), its formation (the ice was not created under great pressure), and its temperature (O°C, not below negative 50°C as are actual icebergs).

Antarctica's ice sheets and ice shelf continuously discharge icebergs into the Antarctic sea. These icebergs often have unusual shapes due to the weathering effects. Some of these icebergs are almost as large as the state of Connecticut. Almost 90 percent of the iceberg is under water, which makes shipping and underwater exploration very hazardous (thus the expression "that's only the tip of the iceberg").

Procedure

The day before students do the floating iceberg activity have them make a mixture of water and mud or water and fine clay. Then have each group label the two paper cups and the glass measuring cup. Fill the cups with the following contents:
- cup 1 (paper cup): filled to the top with the water / fine clay or mud mixture
- cup 2 (paper cup): filled half full with the water / fine clay or mud mixture
- cup 3 (glass measuring cup): filled with pure water to the 275 milliliter mark
Once cup #3 has been filled, have students weigh it and record its mass and water volume.
Put the three cups into the freezer overnight.
The next day, have students weigh cup #3 again, record its mass, and record the volume of ice in it. What happened to the volume of the liquid water as it froze? What happened to the mass? (The density of a substance is defined as the mass of substance per unit volume.) Once students have compared the mass and volume of the liquid vs. frozen water, have them now consider a different scenario: If you take the same volume of both water and ice, say exactly 1 cup of each, which of the two has more "stuff," i.e., water molecules, in it?
Have students remove the ice from the cups so that cups 1, 2, and 3 now are just the frozen "icebergs." Put the "icebergs" into three separate beakers that are half filled with liquid water. Have students observe the three icebergs. Do they float in a similar manner? Are there differences? What are they? Where is most of the ice located, above or below the surface of the water? Have students estimate how much volume of ice is above the water. Is it approximately the same for all the three icebergs?
Have students discuss how their model iceberg is different from the icebergs found in Antarctica.
Sprinkle soot, graphite powder, or charcoal dust on iceberg #1. Put all 3 icebergs outside in the sun (or under a heat lamp). Observe the melting behavior of the three. Which melts the fastest? Does melting occur in the water or just in the air? Have students discuss the possible consequences of industrial pollution settling out on the polar ice caps.
Have students put the cork stopper on top of the water in the fourth beaker. How does its flotation differ from that of the "water" icebergs? Have students estimate how much volume of the cork stopper is above the water. Have students propose some possible reasons as to why when floating most of the cork is above the water while most of the iceberg is below the water.

Helpful Web Sites

Antarctric Meteorology Research Center (AMRC) Real-Time Data
http://uwamrc.ssec.wisc.edu/amrc/realtime.html

Includes real-time data for temperature, sea level, dew point, current weather, wind speed and direction, and precipitation.

USA Today—Ice shelves float on the sea
http://www.usatoday.com/weather/antarc/aiceshlf.htm

This article describes how the Antarctic's ice shelves are moving, and includes links to other articles about Antarctic ice.

Glacier (Rice University)
http://www.glacier.rice.edu/

This site, devoted to Antarctica and the role it plays in Earth systems, includes a section on ice and glaciers.

Assessment Recommendations

Students may be assessed through:

their participation in the activity.
the level of detail they provide in their observations. Very detailed work would include information about what fraction of the volume was above and below the level of the liquid, and about melting behavior in air versus in water. The information would be presented in a clear, organized, and concise manner.
being asked to draw and label a picture that diagrams the floating iceberg and another diagram that depicts the floating cork. Students should include a paragraph to describe how the flotation behavior is the same and how it is different. Students may be asked to propose an explanation.

Extensions/Adaptations

Experiment with different solutions. Students can put their iceberg into different liquids such as rubbing alcohol or nail polish remover. What happens? How can they explain this?
Determine the quantitative value for the density of liquid water and the density of ice. To do this the student must make two additional measurements beyond those described in procedure step #2. The student must weigh the empty coffee cup to determine its mass. To determine the volume of the cup fill it full of water and then pour the water into a graduated cylinder.

Now the density of the water can be calculated:
```
	(Mass of cup +water) - mass of cup = mass of water = density of water
	        volume of cup                         volume of water
```
In order to calculate the density of ice:
```
    (Mass of cup +ice) - mass of cup = mass of ice = density of ice
	         volume of cup                volume of ice
```
Determine the density of the cork by weighing it to get the mass and determining its volume. One way to determine the volume would be to calculate it according to its geometric shape and using the appropriate formula. At this point the student may see a relationship between the density and volume submerged when floating.
Investigate the types of icebergs and glaciers and how they are formed. Students can use a map to locate some of the major glaciers in the world. They can start their investigation of ices with Kingdom of Ice on this Web site. (Requires Macromedia Flash Player)