Subject Area(s): Earth Science, Physics
Grade Level 6-9
Lesson Title
Convection Cell, Heavy Water, Magic Candles, Broken Mixer, Erin Brockovich
Lesson Summary
This lesson visually demonstrates the operation of a convection current, how
temperature and density affect the behavior of water, and the behavior of gas
(the air) as a fluid.
Time Required
20-40 minutes
Materials Required Convection Demo: Convection Apparatus, Touch Paper, Matches/Lighter, Candle.
Water Density: Demo Tank, electric tea pot, tap water, ice, red, and blue food
coloring, 2 large beakers (0.5 – 1L).
Air as a Fluid: 2Liter clear glass jar, 1 tea candle, 2 2” candles, 1 Tall candle.
Baking Soda, Vinegar, Matches/Lighter.
The convection apparatus consists of a metal box approximately 10” wide, 4” deep and 5” tall. The
front of the box includes a removable glass window to access the candle that is used inside during the
demonstration. On top of the box are two holes toward the sides of the box. On these holes are
placed two clear plastic “chimneys”.
The Demo Tank consists of 2 clear lexan sheets approximately 16” square with screws down the sides
and bottom to hold them together over 1/2” OD nylon tubing to form a very narrow see-through tank.
The tank is mounted with 2 blocks that have cutouts to accept the tank and hold it upright.
Lesson:
Warning:
The use of candles and touch paper in these demonstrations generates smoke that may irritate
students in your classroom. Use proper ventilation and seat students with breathing problems
away from the demonstration site.
Motivation:
This lesson will give students a physical understanding of what convection is, as well as its
causes and effects. Students will also visualize how water of different temperatures (densities)
behave, and realize that air is really a fluid. This lesson is mostly performed as a
demonstration, however it is possible to include students as first-hand 'observers'.
Convection Demonstration:
To perform the convection demonstration, the convection demo box should be elevated, where all
students have a clear view. It is a good idea to close window blinds to prevent glare from the
chimneys obscuring the results of the demonstration.
(1) Open the front window of the demo box, insert the tea candle directly underneath one
chimney, light it, and close the window.
(2) Hold the touch paper away from the apparatus and light it. Move the smoking touch paper
over the chimney opposite the candle. The smoke from the touch paper will be drawn down
rapidly into the chamber. A few moments later, faint smoke can be seen billowing out of the
chimney directly above the candle.
This is the bulk of the demonstration, and can be done as quickly as you have read this. It shows how
the heat from the candle causes now-hot air to rise from one chimney, forcing cool air to be pulled
down the opposite chimney.
Apart from the simple demonstration, this experiment provides the opportunity to guide students
through the process, predict what will happen and then explain why, which can extend the time to
complete this demonstration. Depending on the grade level, a simple extension to end this demo is to
cap off both chimneys, sealing the container (caps are included in the kit linked above). The container
will be sufficiently sealed that the candle will extinguish itself in a minute or two, providing time to
discuss with the class what will happen and why.
Water Density:
The next experiment, on water density, requires some setup beforehand. Near-boiling water
should be available (from an electric tea pot or similar source), and kept in one beaker with a small
amount of red food coloring. In another beaker, cold tap water, Ice, and a larger amount of blue food
coloring should be combined. The demonstration works best when the hot water is lightly colored
and the cold water is heavily colored. Again for this experiment it is a good idea to close window
blinds to reduce glare. In fact, it may be easier for students to see if the rear piece of lexan is made
opaque by taping a piece of paper to the exterior.
(1) To begin the experiment, the tank should be filled ¾ or greater with hot red-colored water.
(2) Carefully and slowly pour the blue-colored ice water into the tank.
(3) The ice water should sink to the bottom without mixing. This is another simple demonstration
that shows how the temperature of water affects its density, and in turn how the densities of
fluids affect their behavior.
Note: the 'teaching tank' linked above is shipped with cheap plastic screws that are difficult to keep
tight. If metal substitutes can be found, it will likely be easier to prevent leaks. Be careful as the lexan
will crack if too much pressure is applied in tightening the screws.
Air As A Fluid:
The next demonstration will hopefully amaze some students! The principle behind this
experiment is that CO 2 , being more dense than normal air, will fill a container as if it were a liquid.
It can also be 'poured' out of this container as if a liquid.
(Setup) 3 candles of varying height should be placed in a large 2L glass jar. A tea candle, a 2”
candle, and a cut down table candle work well. About 1/4” of baking soda should cover the
bottom of the jar as well. Place another 2” candle outside the Jar.
(1)
Light all of the candles.
(2)
Pours vinegar into the large jar. The reaction with baking soda releases CO 2 . As the
CO 2 fills up the beaker, the candles will “magically” extinguish. The students can gauge the
level of CO 2 in the container this way.
(3)
If desired, a lit match can be lowered into the jar to demonstrate the specific level of
CO 2 at any time.
(4)
Eventually, the last candle will extinguish.
(5)
To fully demonstrate the behavior of air as a fluid, the instructor can now carefully tip
the whole jar and 'pour' the CO2 onto the exterior lit candle. It will extinguish like “magic”!
“Broken Mixer”
The “Broken Mixer” demonstration is similar to both of the previous two demos. The principle is to
demonstrate the density of a fluid, specifically as a function of temperature.
(Setup) Two large (~2L) glass or other clear jars are filled with tap water. Two small (~150mL)
beakers are filled with water and food-coloring. Fill one with hot water, the other with ice
water. Cover both with tin foil, which should be secured by a rubber band below the lip of the
beaker.
(1)
Place the 'hot' beaker in one of the jars. The tinfoil should retain the colored water.
The instructor can then poke 4-5 holes in the tinfoil with a pencil or similar object. The hot
colored water should mix readily with the water in the large jar. This can be easily seen by the
students if enough food-coloring is used.
(2)
the same operation is done with the 'ice water' beaker. No mixing should occur. The
instructor may remark that it is 'broken' and try to 'fix' it. The instructor may 'accidentally'
knock the beaker on it's side, spilling the contents into the large container. The students will
observe that the colored water remains at the bottom of the jar and does not mix. This
demonstrates how fluids with different densities interact.
Closure
These activities are meant to be resources and inspirations for teachers of earth science, physics or
related classes. There is much room for extension and expansion of these demonstrations to fit the
grade level and course content of the class. They may be performed all in one class period, or
incorporated with other instruction. No one format is likely to be useful to all instructors, and how to
'close' these demonstrations or incorporate with curriculum is best left to the expertise and
imagination of the teacher.
Extension Activities
A. This activity is a good addition to an in-class viewing of “Erin Brockovich”, or as an independent
demo. It will demonstrate ground water movement and how water sources can be
contaminated.
1. Roll an 8.5 x 11 clear overhead transparency into a roll an inch or more in diameter.
2. Place the transparency against the side of a large clear jar (3L or more).
3. Fill the jar with small gravel and/or sand (leave at least an inch of the transparency roll
above the gravel, and allow some gravel underneath the transparency so it is not sealed
from the rest of the jar). The transparency 'well' should be visible through the jar.
4. Fill the jar with some water (pouring onto the 'soil' outside of the well). Students should be
able to see the groundwater fill the well. This is a good way to demonstrate the zone of
saturation, zone of aeration, the water table, and the basic idea behind well-sourced
domestic water.
5. Pour some heavily colored water onto the soil, representing 'contamination' from industry,
farmers, etc. It will be clear how the colored 'contamination' infiltrates the well, and thus
the water supply of those who depend on the well.
B. “Cloud in a bottle.” This simple demonstration is suitable as an extension into weather or
pollution topics. It simply requires a clear 2L plastic 'soda' bottle (label removed), a little water
and some matches.
1. Begin by putting a small amount ( a few tablespoons) of water into the bottle.
2. Light a match and swirl the water around in the bottle vigorously with the cap on.
Simultaneously extinguish the match and open the bottle, quickly putting the match inside
and re-closing the bottle. The goal is for the candle to smoke a bit inside the bottle.
3. Squeeze the bottle with both hands, the water inside should condense on particulates from
the match as you release pressure, and the bottle becomes somewhat opaque or cloudlike. It may take 2 squeezes to work the first time, but should then be repeatable many
times for 10 minutes or more.
4. Pass the botle around the room for direct observation by students. This demonstrates how
clouds form by water condensing on particulates, and also the effect of high pressure and
low pressure on condensation.
NOTES
This demonstration makes use of a custom demo box that can be purchased here:
http://sciencekit.com/gas-convection-apparatus/p/IG0023697/
Also, it is recommended to order the following 'touch paper', included smoke sticks work poorly:
http://sciencekit.com/touch-paper/p/IG0023698/
The “Demo Tank” can be purchased here:
http://sciencekit.com/teaching-tank/p/IG0031965/