This article incorporates, in modified form, material from Illustrated Guide to Home Chemistry Experiments: All Lab, No Lecture.

A sol is a solid phase dispersed in a liquid continuous phase. Ordinarily, a sol is a liquid, but it can be converted to a semi-solid gel by adding a gelling agent. In some cases, the solid phase itself may also serve as the gelling agent.

In this lab, we'll prepare a flammable gelled sol that comprises ordinary gasoline as the liquid continuous phase with polystyrene plastic serving as both the dispersed solid phase and the gelling agent. This gelled sol is a slightly modified version of the material the military calls Super Napalm B. (Actual Super napalm B uses low-octane gasoline rather than standard gasoline, and includes a small percentage of benzene, which we'll leave out because it's difficult to obtain.)

Required Equipment and Supplies

• goggles, gloves, and protective clothing
• balance and weighing boat
• beaker, 250 mL
• graduated cylinder, 100 mL
• stirring rod
• matches or lighter
• watch or other timing instrument
• gasoline (100 mL)
• rigid polystyrene foam (35 g)

All of the specialty lab equipment and chemicals needed for this and other lab sessions are available individually from Maker Shed or other laboratory supplies vendors. Maker Shed also offers customized laboratory kits at special prices, including the Basic Laboratory Equipment Kit, the Laboratory Hardware Kit, the Volumetric Glassware Kit, the Core Chemicals Kit, and the Supplemental Chemicals Kit.

CAUTION

To state the obvious, we're making napalm, or at least something very close to napalm. Napalm burns furiously, sticks to anything it touches, and is very difficult to extinguish. Use extreme caution when preparing the sol. Have a fire extinguisher ready, and make absolutely sure that there are no open flames, sparks, or other potential ignition sources nearby. Work outdoors if possible, or at least in an area with excellent ventilation. Do not ignite the product indoors. Wear splash goggles, heavy-duty gloves, and protective clothing.

Substitutions and Modifications

• You may substitute any glass container of similar size for the beaker.
• You may use any form of polystyrene, including Styrofoam or the polystyrene foam used in insulated drinking cups and foam packing peanuts.

Procedure

1. If you have not already done so, put on your splash goggles, gloves, and protective clothing. Verify that there are no open flames or other potential ignition sources nearby, and make sure you have a fire extinguisher handy.
2. Use the graduated cylinder to measure 50.0 mL of gasoline and transfer it to the 250 mL beaker.
3. Weigh 15.0 g of polystyrene. The density of polystyrene foam (Styrofoam or similar) is so low that you need a large weighing boat to contain a reasonable mass. I used a 1-quart plastic kitchen container that comfortably held 15 g of rigid Styrofoam packing material broken into small chunks.
4. Add a small (thumb-size) chunk of polystyrene to the beaker and observe the reaction. The foam fizzes and appears to dissolve in the gasoline, leaving a small amount of undissolved residue. In fact, what appears to be undissolved residue is the first appearance of the gelled sol.
5. Continue adding the first 15 g of polystyrene in small chunks, using the stirring rod to press the polystyrene down into the liquid. Note that the gelled sol continues to grow in volume. After you've added the first 15 g, the beaker appears to contain mostly gelled sol, but with a significant amount of liquid gasoline remaining.
6. Weigh out another 15 g of polystyrene, and continue adding it in small chunks to the beaker, with stirring. When you've added a total of 30 g of polystyrene, the sol appears to have "soaked up" nearly all of the liquid gasoline, as shown in Figure 18-3.

Figure 18-3. The gelled sol forms

7. Weigh out a final 5g of polystyrene foam, and continue adding it in chunks to the beaker, using the stirring rod to make sure the foam you add is incorporated in the gelled sol. After a total of 35 g of polystyrene foam has been added, the contents of the beaker appear to be completely gelled, with no liquid visible. At this point, the gelled sol is semi-rigid, enough so that it resists the force of gravity if the beaker is inverted, as shown in Figure 18-4. (Don't invert the beaker indoors, as the gelled sol may fall out unexpectedly.)

8. If you are not already outdoors, take the beaker to a safe outdoor location with a nonflammable surface such as dirt, gravel, or concrete. (Remember that asphalt, made from tar, is flammable!) Invert the beaker, and wait a few moments to see if the gelled sol separates from the beaker. If not, tap the beaker gently to release the gelled sol. When the gelled sol separates, note that it retains its form, as shown in Figure 18-5. There should be little or no liquid gasoline remaining. (The stain visible to the right of the gelled sol in Figure 18-5 is a small amount, probably less than 1mL, of liquid gasoline that was not incorporated in the sol.)

Figure 18-5. Although it is technically a liquid, the gelled sol maintains its form as though it were a solid

9. Make sure that you have a fire extinguisher handy, and verify that there are no children or pets in the vicinity (or indeed any other lifeforms who are unaware of what's going on). After you have verified that it is safe to do so, note the time and use the lighter or match to ignite the napalm. While the napalm burns, as shown in Figure 18-6, note your observations, including the appearance and intensity of the flame, whether the flame spreads or stays in one place, how long the flame continues, any unusual odor, and so on.

Paul Jones warns Be cautious with substances you haven't used before. We've had good luck taping a match to the end of a metal rod for igniting hydrogen balloons and methane bubbles. That might be a good idea here.

Figure 18-6. Napalm burning

10. After the napalm has finished burning, and you've allowed the surface to cool, repeat the burning test using 50 mL of liquid gasoline. (Use extreme care when igniting liquid gasoline, and don't do it in your glass beaker.) Once again, make sure that you have a fire extinguisher handy, and verify that there are no children or pets in the vicinity. After you have verified that it is safe to do so, note the time and use the lighter or match to ignite the gasoline. While the gasoline burns, note your observations, including the appearance and intensity of the flame, whether the flame spreads or stays in one place, how long the flame continues, any unusual odor, and so on.

Disposal

All of the products used in this lab are burned. Allow the beakers and stirring rod to sit outdoors until any gasoline residue has evaporated, and then wash them with soap and water.

Review Questions

Q1: What physical characteristic does a gelled sol possess that differentiates it from ordinary liquids?

Q2: What differences did you observe between burning napalm and burning gasoline?

Q3: Napalm was originally developed during WWII as an improved substitute for liquid gasoline for use in flame throwers, fire bombs, and other weapons. Based on your own observations, what advantages does napalm have as a weapon relative to liquid gasoline?