Density, Buoyancy and Convection

The specific content learning objectives are...

1. describe how the density of a substance affects its buoyancy.
2. describe how the temperature of a substance affects its volume.
3. describe how the ratio between the mass and volume of a substance determines its density.
4. explain how, why and under what conditions convection occurs.
5. discuss how convection serves as an effective mechanism for transporting heat energy.

This activity has been extensively tested, revised and retested for more than 15 years in the Concepts in Earth and Space Science course for future teachers at California State University, Chico. We also use it in two additional courses, both of which are lower-division general education courses for non-science majors: Introduction to Earth's Environment and General Geology. Since convection is such a powerful concept in the earth and space sciences, we assign this lab early in the semester, allowing us to build on and refer back to it as we cover topics such as igneous processes, isostasy, the driving mechanism for plate tectonics, wind, cloud formation, global air circulation and oceanic circulation. Although we have used this activity almost exclusively in college-level courses for non-science majors, it should also work well -- with some modification -- in secondary school earth science courses as well as college-level courses for science majors.

• Student Handout (Microsoft Word 291kB Sep7 09) for the Lab Activity on Density, Buoyancy and Convection
• Other materials needed per 4-5 student lab station (in chronological order)
• Glitter lamp. We use the Lava Light brand, available from walmart.com among other websites. A video of a glitter lamp in action can be viewed at http://www.youtube.com/watch?v=Tv7Vw3XuwGA. (Warning: I find the music loud and obnoxious. I suggest muting the sound on your computer before viewing this video.)
• Sealed clear 12-oz. water bottle (label removed), filled with a mixture of 80% light corn syrup and 20% SAE 50 motor oil. I suggest sealing the lid with clear packing tape. These bottles can be reused for several years before the corn syrup starts to darken.
• Small (around 30 ml) tall clear glass or plastic vial filled with green-tinted water (use food coloring), capped with a one-hole rubber stopper. Ward's Natural Science supplies both glass vials and rubber stoppers (Item #15 V 8479).
• Clear glass or plastic eyedropper (rubber squeeze bulb not needed). The "glass dropping pipette" from Wards (Item #17 V 0230) works well.
• Black wet-erase overhead projection pen
• 2 large Pyrex beakers (or Pyrex measuring cups)
• Electric hot plate with variable settings.
• Set of two oven gloves (right and left)
• Ice cubes or crushed ice
• Large paper clip
• 2 clear plastic wide-mouth vials with plastic snap-top caps. Cut two 1/8" to 1/4" holes in each cap. These types of vials are available from SKS or Thornton Plastics. I recommend the 7 Dram (7/8 fluid ounce or 26 ml) size, which has a diameter of about one inch and a height of about 2 inches.
• Stirring stick or plastic spoon
• 2 small plastic bottles, filled with liquid food coloring (red and blue), diluted with water to half strength. We use the Leakproof Dispensing Bottles with Spout Cap (Item #18 V 3416) from Ward's Natural Science. Their plastic dropping bottles (item#18 V 2919) look like they would work a bit better because separate eye droppers would not be required.
• 2 eye droppers or pipettes (not needed if using plastic dropping bottles for the diluted food coloring)
• Thermometer designed to measure the temperature of a liquid.
• Electric immersion heater (the kind used to heat a cup of water)
• 4 blue colored pencils
• 4 red colored pencils
• Optional (for extension activities)
• Galileo thermometer
• Children's bathtub tints tablets (i.e. fizzy water coloring tablets)

General Comments: This lab activity can be completed in three hours. When introducing this activity, it is best to tell the students the structure of the activity: observation of the phenomenon, followed by the exploration of underlying concepts, culminating in the construction of a full understanding of the convection process.

Notes on Activity #2: Comparison of Motor Oil and Corn Syrup

Main point: Low-density substances will rise up through high-density fluids. High-density substances will sink down through low-density fluids.

Misconception that frequently comes up: Substances that are less dense have more air in them. I think students are often equating air with empty space. Or they may not be able to conceive of empty space with no matter in it at all. It might be helpful to remind them that even atoms are mostly empty space and that the universe has MUCH more empty space than space filled with matter.

Notes on Activity #3: Volume Change Caused by Temperature Change

Main point: Any substance will expand when it is heated and contract when it is cooled.

Misconception that frequently comes up: The molecules themselves are expanding. A fun exercise to do when students bring this up is to have the students in the class huddle together without moving (simulating cold temperatures) and then gradually start dancing around, which causes them to move apart.

In addition, students often have trouble with this concept because they know, from personal experience, that water expands when it freezes. When students bring this up, it is a great teachable moment to point out that (1) the phase change that occurs when substances crystallize adds an additional factor that, in the case of water, overrides the process of simple thermal expansion and compression, and (2) water is a very unusual substance; most substances shrink when they crystallize. It is helpful to have on hand diagrams of the crystalline structure of ice to show students how the geometry of the arrangements of the water molecules in ice causes the ice to take up more space than liquid water does.

Notes on Activity #4: Sinking and Floating Water

Main points: When fluids are heated, their density decreases and their buoyancy increases.

Notes on Activity #5: Comparison of Two Ways to Heat a Fluid

Main points:

• If you heat from below, you get convection and the temperature is quite evenly distributed.
• If you heat from above, you do not get convection and you get dramatic temperature gradients.
• During convection, hot fluid on the bottom rises; cold fluid on the top sinks; the fluid circulates.
• Convection is a very effective mechanism for transferring energy.

Notes on Activity #6: Cooling a Fluid from Above

Main point: Convection requires a temperature gradient in which temperatures increase downward. A heat source is not required. Mere cooling at the top can do the trick.

Constructing a Concept Map

This activity can be completed as a whole-class discussion or as homework. It is an excellent way to introduce students to concept maps.

At the end of this activity, I typically assess student learning by having student groups present their answers to the rest of the class. I divide the different parts of this activity among the student groups, assigning each group to prepare illustrations and orally present their part to the rest of the class. Each presentation is then followed by a whole-class discussion.

On exams, I ask students to observe a lava lamp in action and ask them to explain how it works. I also ask them to describe and explain any circulation of water (or lack thereof) in a lake that is frozen over during the winter.

A multiple choice question that can be used as an exam or clicker question:

To cause convection in a fluid, you need...

a. a heat source.
b. higher temperatures at the top of the fluid.
c. radiant heating from above.
d. a stirring mechanism.
e. an increase in temperature with depth.

Three GEMS (Great Explorations in Math and Science) teacher's guides provide additional hands-on guided-discovery activities for teaching these topics in middle school classrooms:

• Discovering Density
• Convection: A Current Event
• Ocean Currents

Go back to the Guided Discovery Problems examples page.