Scientist's Pick: Minty-Fresh Chemistry

By Science Buddies on April 6, 2010 9:25 AM

Last month, Staff Scientist Dave opened a Science Buddies meeting with a small can of breath spray, a gas grill igniter, a film-canister, a homemade wooden apparatus to "hold" the canister in place, and the question: "Is it okay if I set this off in here?"

A few minty sprays into the canister, a few clicks of the lighter, and the canister was propelled across the room with a noisy "bang."

In subsequent demonstrations, Dave showed other ways to expand upon the initial experiment, even taking advantage of tire gauges and balloons, all the while pointing out the chemistry (and equations) behind the experiment.

Project: Getting a Bang Out of Breath Spray: Studying the Chemistry and Physics of a Small Explosion
Scientist: David Whyte
Science Buddies' Difficulty Level: 8

I chose the topic for Getting a Bang Out of Breath Spray because it involves making a great toy, a "Binaca bomb" that can be used to explore the chemistry and physics of a small explosion. To make the device, a small amount of ethanol (from Binaca breath spray) is spritzed into a film canister. The top is quickly placed on the canister, and the ethanol is ignited with a spark.

If all goes well, there is a loud "pop," and the canister flies through the air!

I like this project's vivid demonstration of how energy can convert into various forms: chemical energy from the combustion of the ethanol is converted into thermal energy (the heat that makes the gases expand in the canister), which in turn is transferred to kinetic energy in the form of the flying canister.

I also like that the project offers plenty of opportunity to explore questions about what is actually taking place during the explosion. There is more than just a "bang" and a "projectile" happening here. Some of the questions that can be asked include:

What is the pressure in the canister at the moment before it separates from the top?
What is the volume change that occurs when the gas is ignited?
What is the kinetic energy of the canister?

The project contains the science to determine the answers to all of these!

The project also has a nice mix of measurement and calculation. For example, the approximate temperature of the hot gas is calculated based on measurements of the pressure and volume.

Oh, and did I mention the project involves explosions?

~ Dave