Flaming Vapor Ramp

What it Shows

Cyclohexane vapor catches on fire when it flows downwards to a burning candle at the bottom of a ramp.

How it Works

Cyclohexane is a clear, organic liquid with a formula of C₆H₁₂.

Because cyclohexane is a nonpolar molecule with only weak London dispersion forces, it is quite volatile with a vapor pressure of 78 mm Hg at 20°C. For comparison, the vapor pressure of water at 20°C is 17.5 mm Hg. In addition, with a mass of 84 g/mol, it is more dense than air, so it sinks when poured. When cyclohexane liquid is placed in a sealed flask, the vapor builds up so that it can be poured down a metal v-shaped ramp. When the vapor reaches the burning candle at the bottom of the ramp, a flame front travels from the bottom to the top of the ramp, combusting the cyclohexane vapor according to the equation: C₆H₁₂ (g) + 9O₂ (g) --> 6CO₂ (g) + 6H₂O (l).

This demonstration shows that gases are fluids that can be poured. In addition, you can discuss how intermolecular forces affect volatility, why some liquids vaporize more easily than others, as well as the dangers of working with flammable organic liquids. It also demonstrates the heat released by a hydrocarbon combustion reaction. 

Setting it Up

Safety

Wear goggles, gloves, and lab coat. You may prefer to wear heat resistant gloves. The demo should be done in a large room with good ventilation as the vapor can be irritating. After pouring the vapor, replace the cork to prevent extra vapors from escaping - this also allows for vapor to build up so that you can pour it again. Sometimes on a hot day, the cork will pop out of the flask on its own - this is normal. Do not substitute other volatile liquids for cyclohexane.

Prepping it

Pour about 2 mL of cyclohexane in a 1 L Erlenmeyer flask, just enough to cover the bottom with a very thin layer. Seal it with a rubber stopper. Set up the ramp using a clamp and ring stand and place the candle at the bottom, as shown in the picture below. Experiment with the angle of the ramp - around 30-40 degrees works. Have matches or a lighter ready nearby to light the candle.

Demonstrating it

Light the candle. Swirl the cyclohexane around, while holding the cork. Take out the cork and pour the vapors at the top of the ramp until they catch on fire at the bottom. As soon as you see the flame start, move the flask away and stopper it. Swirl and repeat, swirl and repeat.

An extension can be done using dry ice (CO₂) or CO₂ produced by reacting baking soda and vinegar. CO₂ is also denser than air, so it can be poured down the ramp to extinguish the candle flame.

Cleanup and Disposal

Try to use a small enough amount so that all the cyclohexane is vaporized and used up in the demo. Do not put down the drain. If there is some left, pour it into a container and keep it for next time.

Comments

Cyclohexane is used instead of hexane because it is safer. The reference from Flinn Scientific uses hexane vapor, which can be used as a substitute, but make sure to use in a well-ventilated area.

If you want to show that CO₂ puts out a fire, put some dry ice in a flask and pour the vapor down the ramp. Instead of dry ice, you can also produce CO₂ by reacting baking soda and vinegar, then carefully pouring.

References

This demonstration is attributed to Bob Becker for Flinn Scientfic. His book containing this demo is listed in the reference below.

Flaming Vapor Ramp: (2016). Flinn Scientific Safety ChemFax. https://www.flinnsci.com/api/library/Download/19b188e814f646f191c5035f2895be2a