Expt 009 -- Flame Tornado

A paper towel is soaked in lighter fluid, placed on an evaporating dish and ignited, producing a 15-20 cm high flame. When this is placed on a screened-in lazy susan, and spun slowly, the flame is spontaneously transformed into a roaring meter-high pillar of fire. Different ions make a remarkable flame test demonstration.

Chemical Concepts

1. Combustion is an exothermic process.
2. When gases are heated, they expand and become less dense.
3. Compared to their surroundings, less dense objects tend to rise upward; more dense objects tend to sink.
4. Inertia: An object in a straight line motion tends to continue in that straight line motion unless acted upon by an outside force. When forced to travel in a circular path, an object still tends to continue in a straight line motion, thus its momentum always acts to carry an object to the outer edge of its circular path. This gives rise to a fictitious phenomenon known as centrifugal force.
5. With all else equal, denser substances tend to have more inertia, and are thus more subject to this centrifugal "force." Hence, a centrifuge can act to separate out substances by their different densities.
6. Different ions emit different colors of light because each ion has different energy levels. The intensity of these colors is used routinely by chemists to detect metal ions and to determine the amount of each metal ion present.

Safety

Make sure a fire extinguisher is nearby ready for use.

Procedure

1. Place a wadded-up half-sheet of paper towel in the evaporating dish and add just enough charcoal lighter fluid to wet the paper. Place the evaporating dish on a nest of water-soaked paper towels in the center of a lazy susan. Light the paper towel in the dish and observe the flame.
2. Carefully center the screen cylinder on the lazy susan, and give the lazy susan a gentle spin. The flame suddenly grows to a tall, roaring vortex of fire! Stop the spinning, and the flame immediately resumes its original form.

!!!Click here to See Movie.

Optional Variation - Colored Flame Tornadoes

1. Instead of a paper towel and lighter fluid place about 1 g of lithium chloride and about 10 mL of methanol in the evaporating dish. Stir a bit. Light the methanol, place the screen on top, and spin.

   !!!Click here to See Movie.

2. Repeat with copper chloride. Compare and discuss the colors of the flames.

   !!!Click here to See Movie. Note that the colored flames appeared brighter to the eye than to the video camera.

   !!!Click here to See Picture. Note that the color film used here is more sensitive to the colored flames.

Questions

1. Suppose the formula for the burning liquid is C₆H₁₄. Write a balanced chemical equation for the burning (combustion) reaction.
2. Account for the colors in the flame when salts are added to the burning mixture.
3. Spinning makes the flame grow longer. Suggest experiments that you could perform to determine whether the overall rate of reaction is faster when the apparatus is spun.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 009 Flame Tornado

Watch the movies.

1. Describe the colors of the Li⁺ and Cu²⁺ flames.
   • Li⁺
   • Cu²⁺
2. Answer the questions.

Curriculum-

This demonstration may be used when discussing several different topics:

1. mixing and rates of reaction;
2. energy levels and analyses;
3. centrifuges and how they separate.

Activity-

Demonstration - Teacher Only

Demonstrate this one yourself. A serious fire danger exists in the wrong hands.

Safety-

• Make sure a fire extinguisher is nearby ready for use. You must take all precautions to prevent fires with the fuel in this demonstration. Bring only small amounts of fuel into the classroom.
• Perform in a well ventilated area.

Time-

Teacher Preparation: 10 minutes when all the materials are on hand. (Assembling the screen take about 10 minutes.)

Class Time: 10 -15 minutes depending on the number of ions.

Materials-

• 10 mL charcoal lighter fluid or kerosene
• a paper towels
• an evaporating dish
• a large lazy susan (40 cm diam.)
• aluminum screening: 80-100 cm wide 260-270 cm long
• 5-6 paper fasteners or straight pins

Optional:

• 1 g LiCl or another Li salt
• 1 g CuCl₂ or another Cu salt
• 10-20 mL methanol

See Lab Hints for instructions of assembling the screen and lazy susan.

Lab Hints-

1. Do NOT use plastic screening material. A plastic lazy susan (similar to the larger Rubbermaidª ones) is OK, but use care not to spill the lighter fluid on it. If you spill on the lazy susan, do not light until it has evaporated completely.
2. If the lazy susan has a lip, then the screen cylinder can be made to be seated just inside it. If there is no lip, make the cylinder a little narrower, leaving about a 2 cm border all the way around. Since the optimal spin is a rather slow one, there is no reason to attach the cylinder to the lazy susan.

   Alternative: If a large lazy susan is not available, one can be made by purchasing a lazy susan ball bearing coaster from a hardware store (the kind used in making pivoting bar stools). This may simply be attached to the center of a 2' x 2' square of plywood. On the other side, draw small circle to center the evaporating, and a large circle to center the screen cylinder.

Construction:

1. Roll the screen into a tube that fits on the lazy susan, resting just inside the lip. The screen length specified above should be enough to wrap twice around, with about 10-20 cm of extra (3-ply) overlap. To hold the screen in this shape, poke the paper fasteners through along the length of this overlap about every 15 cm; then spread the ends of the fasteners open. Straight pins may also be used for this purpose.

   !!!Click here to See Movie. Click |> or <| to step the slides forward or back.

2. Use a nest of water-soaked paper towels to hold the evaporating dish mouth-upward in the center of the lazy susan.

Observations-

• Not only does hot air rise, but, of course, cool air sinks! Since a flame is composed of hot gases, it naturally extends upward and begins to create convection currents around it. Above the flame, however, is relatively cooler (denser) air that is "trying" to sink straight downward, against the flow of the convection currents. Perhaps the flickering and dancing of a flame is due at least in part to these two opposing forces: the hot flame rising momentarily, then the colder air pushing back downward.
• This applies to a typical flame where the only force is gravity. If the flame and the cooler air above it are placed in a spinning frame of reference, the above mentioned "centrifugal force" would act more on the cooler, denser air on top, pulling it to the sides -- or pushing it? (...it's a fictitious force anyway, so what difference does it make!?) This outward flow of the air on top acts to enhance, rather than block the convection currents. Essentially, a partial vacuum is created directly above the flame, and so it grows upward, unimpeded. At the same time, the cold oxygen rich air is mixing rapidly with the fuel increasing the rate of combustion.

Answers-

Q1. Suppose the formula for the burning liquid is C₆H₁₄. Write a balanced chemical equation for the burning (combustion) reaction.

A1. 2 C₆H₁₄ + 19 O₂ --> 12 CO₂ + 14 H₂O

Q2. Account for the colors in the flame when salts are added to the burning mixture.

A2. Energy from the combustion reaction excites electrons in the added salts to higher energy levels. When these excited electrons give up their energy, light is emitted. Because the electrons reside in energy levels, the difference between the levels is fixed so that the energy emitted and therefore the color is fixed and characteristic of the added elements.

Q3. Spinning makes the flame grow longer. Suggest experiments that you could perform to determine whether the overall rate of reaction is faster when the apparatus is spun.

A3. Weighed amounts of solvent could be burned under conditions with and without spinning, and the total time, for which the flame burns, could be measured. If the reaction is faster when spun, the amount of time for burning should be lower, all other things being equal.

Makeup Ans.-

HQ1. Describe the colors of the Li⁺ and Cu²⁺ flames.

HA1. Li⁺ red

Cu²⁺ green

References-

I first witnessed this demonstration performed by Tik Liem at Chem Ed 91, in Osh Kosh, WI. Adding ions to the flame makes an impressive flame test. An original variation is that of coupling it with a second demonstration -- the water bottle race (Ron Perkins, personal communication).

Key Words 1-

centrifuge, vortex, density, flame test, energy level